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CHAPTER 2. Significant variables, their usefulness and data collection techniques

As proposed in the UWET document (Unified Wood Energy Terminology, FAO, 2001 Annex I), and considering that the planning approach used will be LEAP or an equivalent, the principal variables to be analysed are presented under three major thematic headings: demand, supply and provision. The main reasons for such grouping are that the methodologies for acquiring and processing information differ for each group and the sources of information are also different.

The present chapter lists the major variables2 to be taken into account in a survey on woodfuel use and, at the end, considers variables that are complementary to those described before. The guide provides a definition of each variable, an idea of its importance or usefulness, and comments on techniques and ease of acquirement – including equipment and materials required. At the end of the chapter, readers should list the variables they consider most important for their own work, assigning to each a perceived level of usefulness and indicating the sources of information they have access to.

Chapters 3, 4 and 5 describe the sampling design and explain methods of processing and analysing data and information. Readers will be able to compare these with those they envisaged using and/or have available to them.

2.1 Woodfuel demand

Demand for woodfuels embraces all wood products used as a source of energy by sectors of society. Demand is also differentiated to take into account the different types of users, their energy use pattern, combustion technology and consumption levels.

2.1.1 General Variables

The general variables are: classification of woodfuel end users by sector; size, geographical distribution and variation over time, together with other characteristics of a generic nature.

End users are: residential sector, agricultural sector, industrial sector, trade and services sector and institutional sector (details below). The sectors and branches into which end users fall need to be clearly identified by defining the types of users they include. This will require a review of the secondary data and the interviewing of qualified informants.

The size of a sector, branch or stratum is defined by the number of its component units (establishments, households), whether or not these are consumers of woodfuel. Size, as considered here, provides the basis for calculating total consumption. It is also an essential item in planning the survey, designing the sampling and analysing all the remaining variables. We therefore need to know, or to be able to estimate, this variable with the greatest precision possible, ideally resorting to and comparing all available sources of information.

The geographical distribution of a sector, branch or stratum (i.e. its location) may be very extensive or fairly restricted. It is always best to describe it in terms of the geographical units that are used in the National Census of Population and Housing, disaggregating the distribution as far as possible (e.g. into provinces or states, then municipalities, districts, etc.). This is very important to be able to relate the presence and size of consumer groups to resource availability and to plan data collection routes.

Certain other variables – specific variables – such as acquisition, availability of resources and form of provision usefully supplement this initial characterization of the survey universe and help the process of sample design. At this stage these variables will probably be described by estimates and in qualitative terms, relying on secondary sources or qualified informants. Subsequent refinement will follow once detailed, quantitative information comes to hand, whether from secondary or primary sources. The matter is dealt with in detail under “Sample Design” in Chapter 3.

End users of woodfuels

Residential users: People living in households using woodfuels exclusively for their own needs – cooking, boiling water, heating the home, laundering, preparing livestock feed. Households may also use woodfuel for commercial purposes, which must then be clearly differentiated from strictly household purposes, even where both take place under the same roof. An example would be preparing food for sale, brewing alcoholic beverages or laundering and ironing for third parties.

Residential users are normally classified into two groups, rural and urban, because their saturation and supply patterns generally differ. Examples of two extreme cases:

• in rural parts, where the people work in the fields and there is limited monetization of the household economy, it is common to find a high saturation of fuelwood (sometimes, charcoal) which is mostly self-provided;

• in urban areas, where most people work in industry, commerce or services, the family economy is monetized and other types of fuel are available. There is generally a lower woodfuel saturation and a higher charcoal consumption.

Collecting information

If the principal purpose of the survey is to determine the degree of saturation and forms of provision, and/or when the intention is to use LEAP, it is better to adopt the rural/urban classification. Where the survey is being conducted at national level, it is best to follow the localities/population groupings used in the National Census of Population and Housing, always citing the criteria used by the National Board of Statistics to define “rural” and “urban”.

In surveys conducted at the level of the province, municipality or micro-region the National Census groupings must be checked against expected saturation and provision. This is because the Census often looks exclusively at the number of inhabitants in a given locality to determine its classification, without considering local productive activities which, in some cases, leads to groupings that do not reflect the typical pattern of fuelwood saturation and provision.

Whether or not this classification is employed, the number of residential users must be recorded with the utmost precision. The first step here is to record the National Census data, which are generally disaggregated to the level of province (or state), municipality (or district), and medium-sized and large towns. Very rarely are they given at locality or community level, so such data must be sought from local registers, such school, health or municipal censuses, which are quite reliable.

It is very important to identify the year of the Census in order to update the size of population by means of a correction factor. This can be done by taking the rate of increase of the population in question in the period closest to that of the study. This rate is sometimes given in the National Census itself.

Otherwise, the following formula can be used:

r = (Pf/Pi)1/ tf - ti -1) • 100


r = rate of population increase (%)

Pi = initial population

Pf = final population

t i = initial time

t f = final time

If no records of size of population are available for a period close to that of the survey, the present size can be determined by a count of households. This is a simple process in the case of small localities, while sampling is needed for larger ones (see Chapter 3).

Sampling design for the residential sector is usually by stratification, where differences in supply, saturation and consumption patterns might be thought to exist. In this case, it is essential to know the size of the stratum, otherwise it will be impossible to determine the size of the sample, to calculate the effort needed to collect field data, or to make any inference as regards the universe (see definition in Chapter 3).

Agricultural producers: establishments producing crop and livestock commodities, without processing, who thus belong to the primary sector. Examples include farms using charcoal or fuelwood for heating greenhouses and poultry houses. Data for this sector are obtained in the same way as that described for the industrial sector (See Annex II).

Industrial users: establishments processing raw materials and using woodfuels among their energy inputs. These establishments make up the secondary sector and the sampling unit is the single establishment. In most censuses, industry is classified in terms of subsectors, branches and sub-branches of activity, depending on the kind of inputs used and the commodities produced. Examples of industrial activity based on woodfuels are charcoal plants, brick and tile works, coffee drying and roasting, bakeries and sugar mills. The electricity industry is a special case, considered a distinct sector in certain classifications (e.g. that of the Latin American Energy Organization - OLADE). It is recommended that the characteristics of the sector or branch be clearly specified when using one or other of these classifications, in the interests of compatibility with other classifications (See Annex II).

Within each branch or sub-branch, it is usual to stratify the establishments, according to size, workforce or processing technology, into artisanal workshops or micro-industries, and small, medium-sized and large industries. Stratifying in this way is very useful because energy use patterns usually differ considerably within a given branch when the establishments differ in size.

Collecting information

It is important to note that many establishments, and even entire sub-branches made up of small, artisanal enterprises, go unrecorded in censuses of economic activities, so their existence and significance have to be detected by other means.

Such information is usually acquired through sequenced approximation: first questioning people with a general knowledge of the areas under survey; then reviewing documentary evidence such as economic censuses, reports, case studies, municipal tax records, membership lists of chambers of industry and commerce; then visiting areas where these industries are located and interviewing owners, managers, workers and associated traders (ironmongers, building material dealers, markets, workshops); lastly, carrying out rapid field counts. Data from different sources must constantly be compared and the final register is established when data from more than two sources coincide.

A table of the essential information required needs to be established from the very start, so that no item is overlooked. Important variables in this sector are:

Commercial users: establishments engaged in buying and selling goods or providing services, such as bakeries, hotels, restaurants and laundries, which belong to the tertiary sector. The sampling unit is the individual establishment and there are several classifications by branch of activity (See Annex II).

Collecting information

Commercial establishments are usually well recorded at municipal level, except for the very small ones that often fall within the residential sector but that sometimes assume importance by reason of number and level of energy consumption (e.g. selling food from the home, washing and ironing for others). Here it can prove difficult to obtain a census or listing of establishments or even to recognize them as they may be hidden from street view. The number of such commercial users can however be estimated if the study covers the residential sector.

Collection of general data on medium or large commercial establishments is done in the same way as for industry (see above).

Institutional users: these belong to the public sector and include educational establishments, hospitals and police and military establishments that use fuel and energy for their activities. They are always registered and their stratification is relatively easy.

2.1.2 Specific variables

These are numerical or categorical variables that can be directly estimated, observed or measured for the purpose of characterizing woodfuel consumption patterns, intensity and sources of provision. Among the most important are:

Source of provision

This is a categorical variable for which several classifications are possible. This guide adopts the UWET proposal (Annex I, FAO, in preparation): direct, indirect and recovered woodfuels. The three sources are dealt with in greater detail in subchapter 2.2.1.

This classification is used in analysing the data and presenting the findings in order to correctly depict physical flows, in particular to avoid double counting, and also to evaluate the environmental impact of woodfuel use. But it should NOT be applied at the data collection stage, when local names, types or definitions must be used.

It is also important that the classification be clear, detailed and exclusive, i.e. that the same product does not appear under two or more categories.

Collecting information

At the time of collecting information, the names used for the woodfuels must correspond to those given locally in the survey area or documents consulted, with clear descriptions to determine their equivalence with the classification used in the analysis stage (UWET, LEAP, OLADE, etc.).

Examples: using the field and analysis terminology (UWET):

• in a rapid appraisal based on energy balance, woodfuels are fuelwood (direct) and charcoal (indirect);

• in a detailed survey of lime production, fuels are sawdust (indirect), oak fuelwood (direct), pine offcuts (indirect) and salvaged building timber (recovered).

The best information on types of woodfuel used is obtained by directly asking the users. Secondary sources are not reliable because their sole bases of information are usually the far-from-complete logging, trade or transport records that are kept by the Forest Service’s statistics unit.

Depending on the sector under study, rapid appraisals should consult and compare the following data sources:

• Residential sector: population and household censuses, household surveys, statistical yearbooks, energy balances;

• Industrial Sector: economic censuses, statistical yearbooks, energy balances.

In detailed surveys, information on types of fuel is obtained by interviewing users. The most reliable tool is the direct question incorporated into a structured questionnaire used in surveys or censuses, together with first-hand observation or measurement. How to devise questionnaires is dealt with in Chapter 4.

Information can also be sought on fuels other than woodfuels, such as LPG, kerosene, electricity, diesel oil, natural gas and agricultural waste. This is helpful if the intention is to use LEAP as a planning tool or to carry out multiple-use and energy substitution analyses (see Annex IV).

In the residential sector, where the simultaneous use of different fuels is common, the questioning must be thorough, naming the different fuels that might be used in the area and recording ALL the replies. This prevents respondents overlooking fuels they may consider of lesser importance or “prestige”, as sometimes happens with fuelwood and charcoal.

Saturation or penetration

Saturation (or penetration) is the fraction or percentage of a given branch or economic sector making use of woodfuels or other fuels among its energy inputs. It can also be expressed in absolute numbers by referring to the aggregate size of the branch or sector in question. For example, a locality may have an 80 percent penetration (saturation) for fuelwood (i.e. 160 out of 200 families using fuelwood) and 20 percent for LPG (40 out of 200 families).

Information on penetration is crucial in calculating the total consumption of the sector or branch. It is also a dynamic variable that changes over time according to changes in technology, economic conditions or availability of energy sources.

Collecting Information

In rapid appraisals at national, provincial and municipal scale, data on residential penetration are found in the population and household censuses, although these do not usually record multiple users (see definition below).

A census taken more than ten years earlier should not be used to estimate the current level of penetration. In such cases, assessment can be made by questioning qualified informants (minimum three) and getting them to estimate the penetration of the fuel(s) under investigation (e.g. 0-25%, 26-50%, 51-75%, 76-100%). The operating scale will depend on the objective of the study. If feasible, some data collection in the field should be carried out for this variable, by means of sampling, in order to check that the estimated classification corresponds to the field data.

Information on saturation in the remaining sectors is difficult to find in records, but an estimate can be made using a procedure similar to that for the residential sector. Electricity companies or fuel suppliers sometimes keep registers with data on the use of other energy sources, which can be useful for a rapid estimate of saturation.

Where a detailed study is being conducted, saturation is estimated by sampling or a census of all sectors. The information needed concerns the number of users of each fuel elicited by the question “What kind of fuel do you use?”, as explained in the previous section, and the total number of units making up the sector, as indicated earlier under “General Variables”.

When a stratified sample is used (see Chapter 3), it is important to ensure that the saturation of the entire universe is calculated by weighting the saturation of each stratum in proportion to its relative size in the universe.

The following formula is used to represent saturation in percentage terms,:

Sx = (nx/N) .100


Sx = saturation for fuel x

nx = number of users of fuel x

N = total number of units making up the branch or sector (universe).

Multiple fuel use

This heading covers situations where families or establishments make alternative or contemporaneous use of two or more energy sources. This category variable is important since it is associated with saturation and indicates the possibility of substitution, supplementation or alternation in the use of woodfuels.

Collecting information

Secondary information on multiple fuel use is usually unavailable, though it can sometimes be found, with no great precision, in population and housing concensus. It is useful in such cases to elicit the information with the question “what fuels do you use?” where a multiple reply is expected. In some small branches or sectors this information can be acquired from qualified informants.


Substitution refers to situations where other energy sources are used instead of woodfuels and is often associated with multiple fuel use.

Substitution trend is understood as the speed at which the changeover in fuel takes place, and is expressed by the formula:


ST = Substitution Trend

Sx = Sx at t1 – Sx at to

t = t1 – t0

The substitution ratio refers to the amount of woodfuel replaced by a unit of alternative fuel. This is important in analysing processes of change in use of energy sources, which trigger changes in saturation and multiple use.

Its estimation is complex because theoretical substitution ratios – based on energy equivalents – are heavily influenced by the efficiency of the facilities used, which in practice vary considerably and bear little relation to theoretical efficiency measured in the laboratory. Also, substitution depends on many other factors, such as the relative prices of energy sources and appliances, user cash liquidity and financial capacity, physical availability of energy sources, and cultural patterns (Arias, in the press; Masera et al., 2000; FAO, 1997a). It is also important to distinguish between substitution and supplementation, a process by which other fuels are used in addition to woodfuels to address specific energy needs.

Collecting information

In rapid surveys of the residential sector, a very general evaluation of substitution can be conducted by analysing historic variations in saturation. Such information is obtainable from population and housing censuses. For other sectors it is most difficult to obtain information from secondary sources, though the energy balances can sometimes be useful.

For detailed surveys, information on substitution can only be obtained from specific case studies, involving user interviews.

In order to estimate the substitution trend, the questionnaire for users must include questions on fuels currently used and those used in a given year or in the year in which they changed fuel. Questions must also cover the reasons for using woodfuels, for using other fuels and for changing from one fuel to another. Reasons commonly cited for using woodfuels are the flavour given to food, the use of the smoke to control pests and availability. Reasons given for using other fuels include convenience, social status (hard to identify) and safety.

Estimating substitution and supplementation ratios requires analysis of specific consumption by single- and multiple-fuel users. Measurement procedures are described in another part of this chapter. Analysis should focus on similarities and differences in consumption by users of different fuels, including multiple-fuel users. In the case of the latter, where woodfuel is essential for a specific activity, consumption must be measured for each activity.

Other variables having to do with substitution are the degree of monetization of the household economy, family or industrial level of income, liquidity and financial capacity, and expenditure for each fuel. Questions on all these items need to figure in the questionnaire.

End uses

This is a categorical variable. It indicates the needs that the user satisfies by using woodfuels and/or other energy sources.

In the residential sector, the most common end uses are cooking, heating the home and heating water. Other uses not readily springing to mind include the drying or smoking of grain or meat, protection against insects and lighting. Some end uses may be combined or inextricably associated (e.g. cooking and heating the home in cold regions). These must be indicated clearly so that the effective margin for possible substitutions can be assessed.

Collecting information

In rapid surveys, information can only be obtained by interviewing qualified informants, since secondary sources of information do not usually record end uses.

In detailed surveys, the information can be obtained from user statements or direct observation. Where a given end use is significant in overall consumption, it should be measured or estimated and differentiated from other end uses.

For a list of non-residential end uses see Annex II.


These are the major unitary operations that can be differentiated or characterize an end use. Thus, the "cooking food" end use may differ from one country or region to another. In rural Mexico, cooking tortillas and cooking beans are the main cooking activities.

Major activities in industry include heating water, generating steam for a process or mechanical energy, heating air for drying, firing ceramics, calcination and roasting coffee.

Activities must be clearly characterized in order to estimate substitution possibilities and their economic, ecological and social impact. Their relative importance depends on the proportion of energy consumed, their efficiency and their role in the end use process. For example, cooking beans in Central American homes is the activity that uses most of the energy demanded for food preparation and has a priority role as beans represent the staple diet. In the dairy industry, milk pasteurisation is a very important activity as it underpins all other milk processing operations.

Collecting Information

In rapid surveys, there is generally no information to be found on tasks, except in case studies.

In detailed surveys of the residential sector, approximate information can be obtained on activities by asking users specific questions (e.g. What activities consume most fuel or take most time?), but replies are usually vague or imprecise. If there is a particular interest, consumption for each activity must be measured directly. The technique is described below.

In the industrial and commercial sectors, activity analysis calls for a specialist energy audit.

Fuel-burning means

These are installations or appliances (stove, hearth, oven, boiler, lamp) where fuel combustion takes place and energy is transferred to the process or product.

In general studies it is useful to understand the different types of fuel burning means that exist in a given sector or branch in order to analyse other related variables, such as specific consumption and work environment (smoke, heat, length of working day). A detailed description is important to assess the efficiency of use and the possibilities of substitution of means or fuels.

Collecting information

In rapid surveys, it is not possible to obtain information on means of fuel-burning from official statistics, except for boilers which have to be registered with industrial or labour safety departments. Case studies may sometimes mention these means and include a detailed description.

In detailed surveys there are two levels of approach:

The first level is to identify the different models and types, provide a general description, and estimate the number of users of each type. In the course of the preliminary field survey, the different types of means are identified and described, so that the records in the final survey only show the existence of this or that type in each home or establishment visited. With such a type classification it is possible to compare specific fuel consumption or the opinions of users on respective work environments (See Box).

Noteworthy differences emerging from such analyses may encourage deeper examination, leading to a second level of detail to evaluate the efficiency of use and possibilities of substitution of means or fuels.

The detailed description of fuel-burning means is an essentially technical task: a mere description of shape or materials made from is not sufficient to provide a clear idea of performance and efficiency. Combustion characteristics must be based on direct observation, with the help of specialists and/or qualified informants.

With domestic and other types of stove used for food preparation (in restaurants, snack bars, etc.) the main characteristics to be included in a second-level analysis are:

- height above floor;

- size of hearth; height, width, length ( volume);

- evacuation of fumes: unobstructed on several sides; unobstructed on one side, via flue with or without induced draft;

- ash removal; discontinuous (no grate), continuous (with grate and ash chamber);

- construction materials: three stone; tripod, clay and sand, bricks, concrete blocks, metal plates, car wheels; etc.;

- heat transfer sector : shape (circular, rectangular), area (diameter or length and breadth), materials (metal sheet or cast iron, grate, earthenware slab, none of these), and heat transfer surface (flat, concave, enveloping, irregular).

In the industrial and commercial sectors it is difficult to evaluate the above characteristics because of the considerable diversity of equipment and technical performance within each branch. For this reason it is advisable to consult a thermal energy expert.


Consumption is the total amount of fuel used by the consumer, production unit, or universe.

Unit or specific consumption is the amount of fuel consumed per consumer, unit of time, activity, unit of product obtained or unit of raw material processed. It is necessary to know unit consumption in order to estimate overall consumption by a given universe, as well as for other purposes such as analyses of efficiency, cost or substitution.

Consumption by universe is the chief numerical variable determining the scale and intensity of woodfuel use. The usefulness of surveys will be governed, in large measure, by the accuracy of its estimate, and saturation is a basic variable that needs to be known in this regard.

Collecting information

Unit consumption is usually estimated through user sampling, applying various measurement techniques. These measurements or estimates are complicated due to: (i) enormous diversity among traditional or local units of measurement applied by woodfuel users; (ii)  great variability in specific weight, moisture content and caloric value among different types of woodfuel; (iii) the need to characterize and at the same time gauge the consumption, activity or end use, means used, and the resulting product or service, if the measurement is to have any meaning.

The problem of having different local units can be overcome by obtaining their equivalences to the International System. This is dealt with in detail under the “Supplementary Variables” section of this chapter.

Estimates or measurements of consumption must be made in terms of units of weight and always expressed on material with zero percent moisture (“bone dry weight”). Dry weight is used because it is the basis for calculating energy content. LEAP and energy balances require that consumption be expressed in units of energy. Volumetric estimates or measurements, which have often been used, are not recommended because they contain serious errors due to the normally irregular shape of woodfuels. They are also more laborious.

In order to compare such consumption measurements with availability measurements or those of other sources, they need to be converted to units of volume or traditional units (such as the stere). This is done by using specific weight values and IS equivalents of local units, as detailed below.

For rapid surveys, estimates of total consumption of the residential sector can be made from saturation data obtained from population and housing censuses and from consumption estimates contained in case studies or from direct data collection, applying a simple technique (see below). Forest sector consumption statistics should not be used because these have been found to cover only 10-72% of the consumption determined in case studies (FAO, 1985; Riegelhaupt, 1997; Díaz, 2000; FAO 2001a).

Since consumption is a fundamental variable in any survey, it must be estimated or measured as accurately as possible. The techniques recommended for detailed surveys and for certain rapid appraisals are: estimation from user statements, measuring average day and direct measurement.

Estimation from user statements

Estimation here is based on three assumptions: that the user knows exactly how much he or she consumes; that he or she is prepared to give complete and true information; and that the interviewer can record the information correctly.


• The user (woman in the home; boilerman; stoker, etc.) is asked about how much fuel he/she uses. It is left to the respondent to decide on the units of time, volume and weight.

• Whenever the reply is expressed in units other than IS units, equivalences are determined as set out in the “local units” section of this chapter.

• The fundamental variables to be recorded at the same time as the consumption figures are: number of days per month on which woodfuel is used; number of persons regularly taking their meals in the home or units of product obtained (e.g. thousand bricks) or raw material processed (e.g. kilograms of flour, hundredweight of coffee), and moisture content of the wood. Where woodfuel is used for drying material, the record should indicate the quantity of dry material produced per unit of wet material. If possible, specimens are taken for laboratory determinations of moisture.


• This technique is very quick and cheap, so many samples can be taken.


• The three assumptions are not always met, leading to major errors of estimation.3 For example, users who provide for their own needs have no clear idea how much they consume.

• Where there are very different local units, the estimation is complex and more exposed to error.

When to use this estimation technique

• In wide-ranging, low-cost surveys.

• Where volumes of fuel are very great and direct measurement is very difficult (e.g. in major brick making units).

• Where supply is commercial.

When not to use this technique

• In the case of clandestine users or those otherwise in an irregular position (e.g. non-authorised charcoal makers).

• In studies on energy efficiency.

• In cases of self-supply.

Equipment: the wherewithal to convert local units to IS units.

Measuring average day

This technique combines user statement and direct measurement. The same assumptions are used as under “estimation from user statements” but it also meets the third assumption (that the interviewer will record the replies correctly).


• When measuring the “average day”, the person operating the stove, furnace, boiler, etc., is asked to gather all the fuel used in a normal day’s work. He is then asked to check that this quantity is actually used.

• In industry and commerce where activity is discontinuous, the unit will not be the average day but, for example, one session of brick firing, the drying of one barn of tobacco, the raising of one batch of chickens.

• The material is weighed on scales, taking care not to load too much at a time as the person doing the measuring must not be in an awkward position as this usually leads to error. Small-sized fuels (charcoal, sawdust, sawmill waste, etc.) are best placed in a sack, and fuelwood tied in bundles.

• For laboratory determinations of moisture content or where there is only one hygrometer for use by several persons, fuel specimens are taken and given a reference number (number of questionnaire, number of home, etc.). The specimens are wrapped and processed as indicated in the “moisture content” section of this chapter. One hygrometer for a five-man team operating within a short radius makes for efficient use.

• Data are recorded for the same supplementary variables as in the “user statement” technique.


• Since this technique is fast it can be widely applied, with more questions asked or measurements taken.

• A high level of accuracy is attainable, even if there is a tendency towards over-estimation.4

• It is not necessary to repeat visits to the same user.


• There will be occasions when the user does not have all the fuel needed for a day or unit of processing.

• It is more laborious and time-consuming than the “user statement” technique.

When to use the technique

• For medium-sized samples.

• For collecting a lot of additional information in each home or establishment.

• For quantities of fuel that can be measured by the enumerator.

When not to use the technique

• When a high degree of accuracy is required.

• When the amounts of fuel consumed are very large (e.g. in industry).

Equipment: 1 x 25 or 50 kg spring balance per enumerator

Direct measurement

• To take “direct” measurements, the procedure is to identify and mark with a crayon the fuelwood or charcoal container declared by the respondent as ready for use. The quantity of fuel must be greater than the amount estimated to be used between first and second measurement.

• The moisture of the wood is measured as indicated under “measuring the average day”.

• The user is asked to consume only fuel from the marked heap and not add more. The enumerators must be sure that these instructions are complied with, so should fully explain the purpose of the measurement to the fuel user(s) and the supplier(s).

• Once the set period of measurement (one, two, three, four days or a week) is over, the remaining fuel is weighed at the same time of day as the first measurement. If there is any unmarked fuelwood on the heap the interviewee must be asked if he has added any; if he has, the measurement is annulled. In the case of appliances for cooking food, the measuring must cover at least one week to allow for variations in consumption. In sectors or branches where activity is discontinuous, the measurement must be by unit of process.

• As the consumption is measured, so the data on the supplementary variables must also be recorded, as indicated under “user statements”.



When to use the technique

When not to use the technique

Equipment: the same as for “measuring the average day” plus one crayon.

2.2 Woodfuel supply

Woodfuel supply is the total energy-producing biomass available to consumers. It is important to differentiate between actual supply (i.e. woodfuels effectively available) and potential supply, which is what could be made available with integrated and sustainable resource use. A further distinction needs to be made as regards effective availability, discussed under section 2.3.

As a general rule, actual supply equals actual consumption, since woodfuels are not normally stored for a long time. But sometimes actual supply exceeds effective consumption and resources are underutilized, as when most of the wood from forest clearance for farming or logging is burnt or when crop residues are burnt.

Assuming actual supply is equal to consumption means its estimated volume can be based on estimates of actual consumption.

Determining potential supply is more complicated since this calls for identification and measurement of all woodfuel sources, and a correct estimate of productivity under sustainable management regimes.

2.2.1 General variables

General variables are those answering the questions; 'what are?', 'where are?', 'who owns' the woodfuel sources. They indicate the distribution of resources from which woodfuels are or could be obtained.

Woodfuel sources

These are quantitative or numerical variables. Obtaining the information differs according to whether the source is direct, indirect or recovered.

Direct sources

Direct sources are trees, bushes and other plants growing on different lands and making up distinct plant communities and forms or types of natural vegetation such as rainforest, woodland, shrubland and scrub. They also include cultivated or man-made plant communities such as annual crops, pasture, forest fallow, hedges, forest plantations, orchards, gardens and vegetable plots. The latter, also known as “trees outside forests” are usually very important as sources of wood for rural households and even for urban and industrial markets.

Defining the magnitude of direct woodfuel sources is usually based on their territorial extension, or area expressed in hectares or square kilometres. This is the principal general variable needed to define the quantity of primary resources. Even so, it is insufficient for estimating stocks and productivity, a subject that will be taken up later.

There are major differences between estimates of stocks and productivity of wood resources at national level and at local level. This is due to the lack of sensitivity of non detailed forest inventories that fail to appreciate the availability of sizeable and productive resources at the local level, such as trees outside forests. Alarm has thus often been hastily raised about the supposedly imminent exhaustion of a country’s or region’s resources, to the embarrassment of woodfuel policy-makers and analysts when their catastrophic predictions eventually prove groundless. Regions are often found where, according to the national forest inventory, there are no forest resources worth mentioning, only to discover that the local population regularly meets its woodfuel needs, to the great surprise of the authors of the inventory.

Reconciling the results from different scales for the purpose of estimating the size and location of woodfuel sources is a difficult task. If detailed local surveys are available, it is possible to apply “correction factors” for availability of resources in respective types of soil cover (or at least the most important) in specific regions. Where no such surveys exist and there is a well-founded concern over the availability of resources, it is worth making the effort to obtain detailed information from field visits.

Collecting information

In rapid surveys, with little detail and at national level, the classification of direct sources will generally be that used by the national inventory of natural or forest resources or national land use surveys.

The location and dimensions of direct sources are noted in these inventories (whether nationwide or more restricted) of natural or forest resources, and also in agricultural censuses. The scale (on the basis of which area and stock calculations are made) is normally from 1:250 000 to 1:1 000 000. On scales such as these, vegetation surface areas of less than 1.6 km2 (160 ha) to 25 km2 (2500 ha) cannot be represented, and this information is lost.

It is very common to find that forest inventories record only areas of “closed” forest communities and make no allowance for availability and location of “open" formations and other forms of more or less man-made land cover, even though these may contain sizable wood resources. Another frequent shortcoming in these forest inventories is that they generally take into account only roundwood stocks (trunks, boles) of commercial species, disregarding: (i) other parts of the tree (branches, tops, stumps) which are widely used as fuel; (ii) non-commercial or non-timber-yielding species such as those of the underbrush; (iii) dead wood; and (iv) trees of minor girth (with a DBH of less than 0.10m). For these and other reasons, only an approximation of the volume of direct sources can be made on the basis of forest inventories, as there is a fairly wide margin of error or uncertainty and nearly always a tendency to underestimate. However, equations exist to estimate total volumes of wood or biomass from the value of commercial volumes.

Where no formal inventory of forest resources exists, reference can be made to information on land use and cover in agricultural censuses, although such information is obtained from statements and may be heavily distorted. A further limitation of the censuses is that land use classification is highly generic and poorly disaggregated. For example, the natural pasture category includes scrub and bush communities and even open woodland.

In semi-detailed surveys, the inventory of resources is conducted at the level of province, state or region, where a mapping scale of 1:50 000 to 1:100 000 is usual so making it possible to represent units of 0.06 km2 (6 ha) to 0.25 km2 (25 ha) in size. Here one can differentiate between a larger number of types of vegetation or include combinations, e.g. grazing land with isolated trees, orchards, shifting cultivation, etc.

In detailed surveys, at the level of the locality, the inventory can be disaggregated to a much greater extent with scales of 1:5 000 to 1:10 000, where it will be possible to differentiate between areas measuring less than one hectare. Classification and quantification can be very precise in such cases, and differences can be discerned between plant communities or forms of land use that are not noticeable on other scales. It is also possible to recognize and determine important variations in resource productivity and accessibility, associated with local forms or patterns of use and ownership.

At local level it is also possible to draw maps of land use and resource availability on the basis of information from residents. These maps – a sort of rapid inventory – are usually quite accurate and relatively easy to obtain. They can be supplemented by information from remote sensing, including aerial photography or satellite images.

Indirect sources

Indirect sources of supply are the establishments that process woody plants to obtain their main product or by-products: charcoal makers; sawmills; cellulose plants; furniture factories; tannin, resin and vegetal oil extraction plants. Some consume a large portion of these fuels to produce their own mechanical, electric or thermal energy and may generate surplus energy or surplus fuels that can be used by other sectors.

Collecting information

In the case of rapid surveys, identification and location of indirect sources is acquired from industrial censuses, municipal registers of industry and registers of chambers or associations of the various branches of industry. However, micro-industries or artisanal workshops are often not registered so their share can only be estimated by qualified informants.

For detailed surveys, the identification and location of these establishments entails field work or reliance on qualified enumerators. In such cases, a preliminary sample needs to be taken to estimate the size of each sector or branch, before deciding whether to conduct a special survey of the most important.

Sources of recovered woodfuels

Sources of recovered woodfuels are the final consumers of forest products that generate residues or products that have come to the end of their useful life but still have energy use: discarded building timber, paper, cardboard or wood packaging, municipal waste, etc.

Some of these sources generate important amounts of recovered woodfuel, but tend to be overlooked and are thus underestimated in the overall supply.

Their identification is very difficult, because of their geographical dispersion and irregular production. The case of municipal waste may be simpler although such residues are rarely classified or treated in such a way that their contribution to energy supply can be estimated.

Collecting information

In rapid surveys, the volume can be estimated from production statistics (paper, cardboard, sawn timber for construction or wood for furniture making) by applying specific coefficients of recovered materials estimated by qualified informants.

For detailed surveys, the estimation of sources of recovered woodfuels is usually through studies of demand, identifying types of demand and quantities by asking the question "what fuels are used?".

2.2.2 Specific variables


This numerical variable represents the total woodfuels from direct sources present in a unit area of a given resource at a given moment. It is not used for indirect sources or recovered woodfuels.

A knowledge of average stocks for each type or category of resource (and its probable error) is necessary in order to calculate availability, although this is only spot availability as real availability depends on productivity. A supply and demand balance based on spot stocks produces unreal conclusions on woodfuel availability and impact of use on forests. Spot availability may appear very high against demand when, in reality, productivity is below annual consumption. For example, a recent estimate conducted in Ghana indicated a total wood stock of 813 Mt with accessible stock of 401 Mt, against an annual consumption of about 20 Mt. The situation did not seem to be cause for alarm until annual growth was calculated at some 18 Mt, indicating an annual shortfall of 2 Mt (FAO, 2001c).

In the IS, stocks are expressed in units of volume (m3 solid wood) or weight (tons of dry matter) per unit area (hectare). The UWET uses the solid cubic metre (CUM) as the unit of reference. This terminology document is recommended for a list of commonly used unity units and their equivalents.

Collecting information

Direct woodfuels

In rapid surveys, secondary information on stocks of forest fuels can be found in inventories and forest studies or stand management plans, but in most cases only for dense forest stands and high-value commercial species. There are few studies or management plans for sparse formations, open forests, secondary forests, forest fallow, hedges or "trees outside forests".

Rapid surveys at local level can use data from qualified informants (woodcutters, managers of forest enterprises or charcoal burners), who are well aware of stocks in each type of woodland, and their estimates are usually close to the findings obtained by forest experts from more complex and detailed measurements.

In detailed surveys, direct measurement of stocks is necessary, a relatively simple but laborious process. A very simple variant is to measure diameters, calculate basal areas (BA = D2 x p / 4) and measure dominant tree or bush heights (Hd) in a group of sample plots to calculate the cylindrical volume (CV = BA x Hd). Multiplying CV by an average shape factor (sf), one can estimate the real volume (RV) or solid volume in cubic metres per hectare per species. The result needs to be converted to dry weight by taking into account the specific weight of each species (see Annex VI, “Inventory of woodfuel resources”).

A rapid, simple and low-cost method for assessing the “total” shape factor is to measure diameter (D) and dominant height (dH) in sample plots that are then cut and the timber stacked to measure the apparent volume (AV) or, better still, the green weight (GW) of the wood material of each tree.

We can thus calculate a regression between CV: AV or CV: GW. If we also measure moisture content, when measuring or weighing the wood, we can calculate dry weight, which is the most suitable variable for expressing stocks, because it determines their heating value or energy content.

With indirect and recovered woodfuels, it is not possible to use the term “stocks” in the strict sense, because these fuels are generated as the raw material is processed or the product comes to the end of its useful life. For this reason they will be considered in the next section on “Productivity”.


Direct woodfuels

The wood productivity of a direct resource is equal to its net annual wood growth, i.e. the increase of woody biomass in live trees and bushes. It is expressed in units of volume or weight per unit area and unit time: m3/ha/year; t/ha/year; CUM/ha/year.

Productivity is the most important variable in defining the potential contribution of a direct source of supply, and also in assessing the risk of its degradation or overexploitation as against a given rate of extraction. It is therefore important to allocate sufficient resources and time to the work of measuring or estimating, unless one has complete and reliable secondary information to hand. Estimation requires knowledge of growth rates (gross and net) of the resource (forest, shrubland, hedge, crop).

In the specific case of estimation of forest resources used for the production of charcoal, it is important to directly estimate the productivity of the forests in order to calculate the reduction of the resources used for their transformation into charcoal.

Collecting information

For rapid surveys, we can estimate productivity from mean annual increment (MAI) figures which are found in the management plans for commercial stands. These usually suffer from the limitations described above (they refer only to the volume of trunks or commercial logs) but are generally the only available sources of data. It is recommended that they be supplemented with more detailed studies, where possible.

In detailed surveys, productivity must be determined in the field. The greatest accuracy is gained by establishing permanent plots and repeating measurements over several years. There are also less precise techniques based on non-permanent plots, where less time and cost are involved (See Annex VI). For tree crops and secondary stands of known age, net productivity can be rapidly estimated by measuring the stock at a given time and dividing it by age. Thus, if a ten-year plantation or secondary forest has a stock of 80 tDM/ha/year, its mean net productivity or MAI will be 8 tDM/ha/year.

Indirect woodfuels

In the case of rapid surveys, and if production statistics are available for wood processing industries, it is possible to calculate supply by applying coefficients of by-product generation. Charcoal presents a special problem because the few wood processing industries that are also charcoal makers are for the most part small, scattered, unstable and not registered. It is therefore inadvisable to attempt estimating output by means of rapid surveys.

In detailed surveys, supply from indirect sources is estimated by sampling the establishments. Charcoal estimates are made from saturation and consumption studies.

Recovered woodfuels

In rapid surveys, this supply is estimated on the basis of production statistics for wood derivatives, applying empirical or estimated recovery coefficients, e.g. sawnwood production x 0.6; cardboard packaging production x 0.3.

In detailed surveys, recovered woodfuel supply is estimated from studies of demand and on the basis of saturation and consumption data.


Biomass availability for wood energy in a given area is the sum of all amounts provided by all sources (direct, indirect or recovered) for the production of fuelwood or charcoal in that area per unit time (usually one year). It can be seen as equivalent to the productivity of all the resources under sustainable management regime, plus the production of indirect and recovered woodfuels, plus production in areas where land use is changed.

Collecting information

For direct sources, we have to compute the productivity of accessible areas plus the yearly supply of biomass arising from changes in land use (e.g. forest clearance for agriculture, pasture or shifting agriculture). It may be difficult to acquire reliable information on such changes without repeated land cover surveys or, better, proper land cover monitoring studies. Unless specific information on land cover change is available at national level, some indication of the processes of change taking place in tropical regions may be obtained from the remote sensing survey carried out by FAO as part of its Forest Resources Assessment of 1990 and 2000 (Drigo 1999, FAO 1996, FAO 2002).

For indirect sources, it is necessary to estimate the amount of by-products generated each year by all related activities. The procedure for obtaining the information is described under “Productivity”.

For recovered woodfuel sources, it is possible to estimate the proportion of wood products whose useful life ends during a year and calculate the recoverable proportion of, for example, cardboard packaging and packing wood from production or consumption records.


This variable, which is generally treated as a qualitative or categorical variable, defines the degree to which a given source is effectively accessible for use. The usual categories (‘not accessible’, ‘poorly accessible’ ‘fairly accessible’, ‘fully accessible’) can be treated as quantities, if expressed as coefficients ranging from 0 to 1. Accessibility is relative and can differ for the same resource depending on location and technical means available to the group using it.

Accessibility is constrained by several types of limits: physical (lack of roads or other means of access, sloped terrain, etc.); legal (areas protected for resource conservation); tenure (land owners or occupiers prevent or condition access for extracting woodfuel); or economic (when the resource has other possible uses offering greater economic value; transport costs are greater than the market -or use- value of the fuels; or the interested group lacks the necessary transport or financial means).

Collecting information

Information on accessibility varies in quality and accuracy depending on level and scale.

In rapid surveys, large areas can be identified as not accessible due to legal restrictions, to total lack of infrastructure, or to great distances from source to centres of demand, provided that maps containing this type of information exist. Smaller areas with other access limitations are usually not recognisable. At the local level, it is possible to compile accessibility maps by means of rapid participative appraisals.

In detailed surveys, inaccessible areas can be recognised by topography or distance to consumers. At the local level, many other areas can be identified as inaccessible for reasons of land tenure. Related information can be obtained from cadastral maps, farmers’ maps or user interviews.

It is difficult therefore to produce overall estimates of accessibility. Nevertheless, it is possible to improve on descriptive classifications and obtain quantitative indicators or coefficients of accessibility constructed from travel time, transport costs or other numerical variables that give some idea of the effort needed to reach the resource. With good maps and a geographic information system (GIS), we can obtain a quite precise estimate of accessibility.

2.3 Woodfuel provision

In this guide “woodfuel provision” refers to the totality of processes and activities whereby woodfuels move from place of origin and to end user. These processes and activities always imply a certain amount of physical and human work. If the work is done by the users themselves, it is referred to as self-provision. Otherwise, if paid third parties enter into the process, the provision is commercial.

2.3.1 General variables

General variables characterize the number and location of actors entering the woodfuel production, transport and marketing processes.

Type, size and distribution of woodfuel

Woodfuel producers

Woodfuel producers are individuals or undertakings that harvest or recover woodfuel from their direct or indirect sources – loggers, farmers, charcoal burners, wood-based industries, wood recovery operators. Knowing their quantity, type (self-provision, commercial) and location is necessary to assess the economic and social dimensions of their activity.

Collecting information

For rapid surveys, no secondary source information exists on self-provision or on small-time commercial producers operating in the informal economy (with the possible exception of some household surveys noting sources of household income). In the case of formally established enterprises, it is more likely that secondary information may be available, but its relevance in the overall provision situation is largely insignificant.

In detailed surveys, it is possible to estimate the number of those engaging in self-provision and of small or medium-sized commercial producers by referring to qualified informants and user interviews with questions on their form of provision and transport, and the quantities or units purchased. This information can then be consolidated by interviewing producers on their production or sales capacity, and thus stratifying them in the light of this variable.

The number of commercial producers is calculated from the total consumption of the sector or the universe of consumers, the commercial proportion of consumption per type of provider (self-provision, commercial) and the average production capacity of the respective types of commercial producer.

Transport operators

Transport operators are individuals or firms that use any mode of transport (human, animal, mechanical) to take woodfuels from producers to traders and/or final consumers. It is important to differentiate between those that are paid for this activity (“commercial transporters”) and those engaging in it for their own provision (“self-providers”). For the latter, we need to know the time spent, the means of transport used and the distance covered, information obtained by directly interviewing the self-providers. For “commercial transporters” we need to know the means of transport, distances travelled and time taken, their transport capacity, transport costs and prices, and whether they are also producers. Information here is obtained from interviews.

Collecting information

In rapid surveys, it is not possible to obtain secondary information regarding this group.

In detailed surveys, the relevant data are acquired by combining information supplied by qualified informants and specific interviews with the transporters (questions on transport capacity, frequency, seasonal variations), and by calculating on the basis of the proportion of demand met in this way, along the lines used for estimating the number of producers.

Commercial suppliers

These are individuals or firms engaged partly or exclusively in buying and selling woodfuels. They may be at the same time producers and transporters. It is important to know their number, their location, their size, and whether or not their activity is seasonal, since these data are indicators of the degree of woodfuel ”commoditization”5 and play an important role in determining market prices.

Collecting information

In rapid surveys with national coverage, it is virtually impossible to obtain information on commercial suppliers.

In detailed surveys at local level, the information is available from qualified informants (producers, transport operators, users) and through a representative sample of traders, with questions on volume, frequency and seasonality of sales. The final calculation of the number of commercial suppliers is arrived at by using a similar procedure to that followed for producers and transport operators.

2.3.2 Specific variables

Type of provision

This is a qualitative variable describing the manner in which consumers obtain their woodfuels. There are two types of provision: self-provision and commercial provision.


Here woodfuel is procured by the consumers themselves directly and with no payment involved. Cases where the user has to make some form of payment to access the source are considered “commercial provision", as described below.

Information on self-provision can only be acquired through sampling or qualified informants.

Self-provision is frequently combined with or supplemented by commercial supply, in which case the respective proportions need to be quantified.

Collecting information

For rapid surveys, it is only possible to obtain data on this variable at local, municipal or micro-regional level from qualified informants. At the provincial or national levels it is not possible to obtain information.

In the case of detailed surveys the number of self-providers is acquired through sampling and user interviews. In cases of self-provision and commercial provision combined, the proportion of each is obtained by enquiring about the quantities of fuel gathered and the woodfuel purchased over a given period (see below “Periodicity of provision”).

Commercial provision

Commercial provision exists where woodfuel is obtained against payment in cash, kind or work.

The term is also applied to situations where, to gain access to the woodfuel, the user must pay in cash, kind or work, irrespectively of the form of land tenure (private, state, domain, public, communal, ejido, etc.) or type of source (direct, indirect, recovered). For example, where a family with no land has to do unpaid work for the owner of the land from which it obtains firewood, or where brick makers cut and transport the wood they use, but pay the forest owner; or where people are allowed to remove sawdust from a carpentry work in return for cleaning the place.

The existence of commercial provision can be an indicator of limitations in availability, access to energy resources or degree of monetization of the economy. Data on woodfuel consumption and prices indicate the magnitude of economic flow.

Collecting information

See under “self-provision”.

Periodicity of provision

This numerical variable represents the time elapsing between gatherings or purchases of woodfuel. An alternative variable here is frequency – the number of times that a product is acquired within a given unit of time (day, week, month, year). This variable can be used to estimate consumption, provided the units of measurement are known and constant as in the case of commercial supply, but is not suitable in the case of self-provision (see under “Consumption” in this chapter). It is also useful as an indication of cash liquidity in the case of commercial provision. In the case of self-provision, periodicity helps to estimate the time taken to procure fuel.

Collecting information

For rapid surveys, it is not possible to obtain information on periodicity or frequency (though this may sometimes be included in household surveys).

For detailed surveys, the information is obtained directly from users. By associating periodicity with quantity procured, we can estimate the proportion of fuels obtained by self-provision commercially. For example:

      How often do you buy (or fetch) fuelwood?

      And how much do you buy (or fetch) each time?

Cost of woodfuels

This is a numerical variable indicating how much work or money the user must expend to obtain fuel. If it is a question of personal labour (in the case of self-provision), there is no direct money cost. With purchases or barter there is a monetary, or quasi-monetary, cost. When payment is made in the form of labour to acquire the right of access to the resource or for the provision of the fuel itself, here again a quasi-monetary cost is entailed.

This variable is fundamental to evaluate the impact of woodfuel use in the household, regional or national economy. With self-provision, the cost is equal to the work involved, though there is the problem of attributing an economic value or price to the work. In the case of commercial provision, the variable refers to the money paid or the monetary of equivalent of the goods bartered or work done in exchange.

Collecting information

In rapid surveys on a national scale, a rough approximation can be made if the degree of saturation, the commercial proportion of consumption and the prevailing prices are known – information that can be obtained from qualified informants, as described under saturation and commercial or physical flow (see Chapter 5). At the micro-regional and local levels, the information can be obtained from qualified informants.

For detailed surveys, information on saturation, consumption, provision and prices is obtained by interviewing end users, so enabling calculation of the costs of procurement.

Market network

Individuals and firms intervening in the commercial provision of woodfuels constitute the market network. This is a qualitative or discrete variable that needs to be constructed from knowledge of the respective stages and actors in the marketing process. The degree of complexity is an indicator of the amount of value added to the raw material in the marketing process and the existence of constraints to access (the more complex the network, the more constrained the access).

Collecting information

Different instruments need to be used, such as questionnaires for end users and interviews for producers, traders and qualified informants, as indicated above for producers, transport operators and commercial providers.

Price setting for woodfuels

This is a process rather than a variable and refers to price mark-ups during the passage from producer to end user via market supply chain or network. Its analysis helps evaluate the size of economic flow and the share of respective actors and activities in the gains.

Collecting information

The relevant information for rapid surveys of price setting is obtained from qualified informants. For detailed surveys, it is obtained by interviewing suppliers in the market chain and end users.

There may be a wide variation in price, according to quality and quantities of woodfuel marketed. The utmost caution must therefore be taken in extrapolating sampling results, relying closely on measurements of local units to express prices of quantities of woodfuel in IS units.

Where fuel is acquired by barter, the “price” is determined by the opportunity cost of the work performed or by the shadow price of the bartered product.

Woodfuel values

Determining the economic value of woodfuel is not simple. For a start, it is essential to distinguish between its exchange value, which more or less equals the market price, and its use value and existence value, which may be greater or lesser than the exchange value.

The exchange value, as reflected by market prices, is useful to assess the advantages and possibilities of fuel substitution and also impact on the user (and national) economy.

Collecting information

Both for rapid surveys and detailed surveys, this variable can only be analysed at local and micro-regional level, in accordance with the specific procedures for each of the variables needed to estimate the value of the woodfuels.

In order to estimate the exchange value, it is sufficient to know the market prices (or, where these are not available, the barter rates of woodfuels and other goods or services) and the physical flow of each type of fuel (see Chapter 5). Estimating use and existence values is more complicated, and goes beyond the scope of this guide.

2.4 Supplementary variables

There are four supplementary variables needed to express in International System (IS) units certain highly important variables such as consumption, prices, stocks, productivity, and physical and economic flows. These variables are the IS equivalents of local units, the specific weight of the wood, its moisture content, and its heating value. They are presented here in a separate sub-chapter as they relate to many of the previously mentioned variables, but this does not imply that they are secondary. Indeed, their determination is needed in all studies of this type if comparable results are to be obtained.

2.4.1 Local units and their IS equivalents

Local units are the conventional or commonly used woodfuel units in a specific area that differ from IS units; in the case of fuelwood: head load, bicycle load, donkey load, back load, maze, billet, stick, cord, as well as the stere cubic metre; in the case of charcoal: bag or sack, can, net; sawmill waste: slivers, sacks, cartload, truckload. There is an enormous diversity of local units and the same term can often denote different quantities in different locations or regions.

An understanding of the local IS unit equivalence is essential in estimating consumption when this derives from peoples’ statements and cannot otherwise be measured, in standardizing woodfuel prices and in calculating economic flows (cf. Taylor, 1995). Where equivalents are not firmly established or require further checking, the principal steps and procedures to be followed are described below.

Collecting information

As a general rule, there is little secondary information on local units, and one must resort to their direct measurement by sampling (see chart in Annex V). It is always the weight that is measured, expressed as dry weight (see “moisture content”, below). The complexity of these measurements varies and depends mainly on the dimensions and diversity of local units. Many local units are volumetric (e.g. stere, cord, back load for fuelwood; sack and can for charcoal), but these refer to apparent volume and not to solid or real volume, a point that needs to be noted carefully whenever weight is being determined, in order to avoid confusion with volumetric units of forest products which are statutorily expressed in solid or real volume.

2.4.2 Specific weight

Specific weight is the weight of the material contained within a set volume. For present purposes, specific weight needs to be expressed in dry weight and in IS units (kg/m3, t/m3). This is very useful for converting woodfuel consumption and flows from weight to volume and vice versa, since units for measuring demand are traditionally in terms of weight while those for measuring supply are in volume.

Collecting information

Information on specific weight is readily available if one has sufficiently precise measuring instruments: 105oC oven; scales to one gram precision, and measuring flasks accurate to 1 cm3. The formula used is:

SW = W / V


SW = specific weight

W = weight of dry wood

V = volume of wood

The wood sample with bark is weighed wet. Volume is determined by measuring the displacement of water when the sample is immersed in the volumetric flask. The sample is then dried in the oven until a constant weight is obtained.

The specific weight of the wood is noted in dry weight and fresh weight, including in this case the moisture content (the fresh weight divided by the volume is the fresh specific weight). For each species no fewer than ten samples from different specimens and all parts of the tree used as fuelwood must be taken.

If no volumetric flasks are available, volume determination can be done by immersing each sample in a receptacle that is full of water and that has first been weighed to the nearest gram, allowing the water to spill over. The difference in weight (in grams) after the sample is removed is equal to the volume of water displaced (in cm3). If the measuring equipment is not available, it is better not to attempt the measurement as there is normally considerable margin of error.

Under international standards, specific weights are taken on samples of seasoned wood, without bark. For this reason, these values are not comparable with values for wood "in natural state", which normally includes bark and sapwood. One must therefore determine the specific weight of fuelwood as it is used, i.e. on integral samples that include bark and perhaps some rotted parts.

2.4.3 Moisture content

Moisture content is the proportion of water physically in the woodfuels. Its determination is essential if one is to calculate dry weight, which must be the basic unit of measurement for all woodfuels, and also estimate its heating value “as burned”, i.e. its effective energy content.

Collecting information

All woodfuels contain a certain amount of moisture, which varies according to origin, composition and drying process (natural or artificial). Moisture content is determined by drying samples at 105 °C until constant weight is obtained, and applying the following formulas:


MWB = moisture wet basis

MDB = moisture dry basis

wW = wet weight

dW = dry weight

A rapid though less precise way for determining moisture content is by using a resistance hygrometer. This small and cheap apparatus permits many readings at very low cost, thereby reducing error due to size of sample.

Moisture content can be expressed as a coefficient, always lesser than one unit, or as a percentage. To calculate dry weight, one multiplies wet weight by (1 - MWB) or wet weight by (100 - % moisture) /100.

2.4.4 Heating Value

The heating value (HV), also named calorific value, is the amount of energy released by each unit of mass (or volume) burnt. It is expressed in kcal/g or Joules/g and is determined in the laboratory by means of calorimeters. It is an essential variable for calculating the energy contained in woodfuel, and considerable time and effort is devoted to its determination.

Heating value can be expressed in either of two ways: Higher Heating Value (HHV) and Lower Heating Value (LHV), the former determined by calorimeter when the water formed or evaporated during combustion condenses and transfers its condensation heat to the apparatus. LHV is estimated from the HHV, knowing the quantity of water and hydrogen present in the fuel and subtracting the sensible and latent heat contained in the water vapour lost together with the combustion gases.

Collecting information

The calorific power of dry biomass is fairly constant: for cellulose materials, it is of the order of 4.6 kcal/g, and for lignocellulosic materials averages 4.7 kcal/g on a dry basis. The most important variations derive from the moisture content and, secondly, from ash content. In the case of wood, values are usually from 4.3 to 5.0 kcal/g on dry basis, representing a range of approximately 6% around the mean. Since its determination is expensive and not always easy, it is not recommended for rapid surveys (see Severns et al, 1961).

In detailed surveys, there may be some justification for calorific power determinations in the case of species with high regional importance or value.

2 A variable is a characteristic of a given item or phenomenon that is not constant. The different values this characteristic may take are called data. A quantitative or numerical variable is one that can be expressed in terms of numerical values (e.g. weight = 30 kg), while a qualitative, categorical or nominal variable is one that is expressed in terms of a characteristic (e.g., who fetches the fuelwood = man, woman or child)
3 Differences of 50 percent or more were found in detailed surveys comparing this technique with direct measurement, (FAO, 1997b).
4 In case studies in Mexico estimates by “average day” differed from direct measurements by: 0% in Chiapas (Purata, 1999 and Arias et al., 2000a), +4% in Veracruz (Purata, 1998; Arias et al., 2000b), +7% in Michoacán, and +19% in Guerrero (Masera, et al., 1997b).
5 “Commoditization” is the process or practice whereby a good is traded subject to certain quality standards irrespective of its origin or provenance. This term is used in connection with merchandise in general use that enters into extensive markets and whose prices are determined by aggregate supply and demand.

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