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Applications of land evaluation
Choice in rainfed agriculture
What price irrigation?
What
future for extensive grazing?
Choosing land for forestry
New land for settlers
'Recent droughts and famines, notably in the Sahel, are a reminder of how precarious our dependence on rainfall can be.'
Arable land and permanent crops cover about 1500 million hectares of the Earth. Only about 15 percent of this area is irrigated, the remainder depends on rainfall. Recent droughts and famines, notably in the Sahel, are a reminder of how precarious this dependence on rainfall can be.
Reduced production due to moisture stress, however, is not confined to arid areas: it can occur wherever rainfall fails. Moisture availability is thus a land quality of special importance in land evaluation. It is a complex quality, reflecting the interaction of many characteristics of climate, soils, landform and hydrology. The balance between rainfall and evaporation is, of course, the primary determinant, but effects on plants are modified by the capacity of the soil to accept, store and release excess moisture to the roots. Topographical position and the presence of groundwater may modify the plants water supply.
Changes in any of these factors can alter moisture availability and, therefore, the suitability of land for a particular rainfed crop or farming system.
These factors change not only from place to place but also with the seasons and from year to year. Furthermore, different plants, and even different stages of growth of a single plant, respond to moisture stress in unique ways.
The FAO approach to land evaluation emphasizes the possibility of raising the suitability of a land unit for a particular use by good management. For example, even if irrigation water is not economically available, it may still be possible to improve moisture availability by water harvesting, through minimum tillage, better weed control, mulching and planting wind breaks to reduce evaporation. These techniques can raise productivity to make a previously unsuitable farming system suitable.
Many other land qualities are amenable to improvement by good management practice. Nutrient availability, for example, can be improved by the judicious use of mineral fertilizers, animal manure or garden compost. Erosion and flood hazards, soil workability and susceptibility to pests and diseases can also be controlled by improved management.
During the process of matching land and use, all these constraints are examined and the possibility of making improvements is considered. The selected management techniques are written into the definition of the Land Utilization Type (LUT), which is then evaluated on the assumption that these techniques are used.
If a new technique such as irrigation or terracing involves major investment or will induce major changes in the nature of the land, the evaluation can be approached in two ways: by recognizing and comparing the land use with and without the investment as two separate LUTs; or by defining a present suitability classification (without change) and a potential suitability classification (with change) for uses with the same end-product. In either case the advantage or otherwise of any major investment can be readily assessed. The choice is one of convenience in presentation (see box opposite) and illustrates the flexibility of the FAO approach.
Qualities to be examined when choosing land for rainfed agriculture
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Land uses and units within a region form a complex land system. Choosing which systems are worth evaluating is complicated by the range of crops that can be grown separately, in sequence or in mixed or intercropped systems.
Mauritius: Mapping agricultural suitability
The first comprehensive use of land evaluation as advocated in the FAO Guidelines was in a study of agricultural suitability on the island of Mauritius in the early 1970s. The main island of Mauritius, situated in the southwest Indian Ocean, has an area of 186,000 ha. At the time of the study, sugar and tea production accounted for about 95 percent of GNP. Land and water resources were under pressure from population growth and a better understanding of these resources was needed to form the basis of planning strategy. The evaluation study was seen as a first step in this direction.
The land units of the island were depicted in colour on two map sheets at 1:50,000 scale. A key indicated the actual and potential suitability of each land unit for each of seven land utilization types. The types examined were sugar cane, food crops (large mechanized holdings), mixed cropping (smallholdings), paddy rice, tea, productive forestry (or livestock grazing), and conservation forestry/wildlife/recreation.
In some parts of the island, major improvements - particularly for the first three LUTs - could be achieved by irrigation (1), removing large rocks (R), terracing (T) or by a combination of these actions. Possible improvements were predicted by the difference between the potential and actual suitability class (with and without the improvement respectively) for the land use on a given land unit. This information was presented in one box of the tabular key as shown above right.
Numerous suitability sub-classes were defined, and lower case subscripts were used to reflect local constraints, and combinations of constraints, on individual land units (the 'c' of S3C in the example above indicates a climatic constraint - shortage of rainfall). Several 'conditionally suitable' classes were also defined to reflect situations where suitability was conditional on provision of adequate erosion/flood control measures or on the choice of certain crops and varieties adapted to special conditions.
Land suitability was assessed for each LUT in terms of five land qualities: availability of water; limiting super-humid climate; availability of plant nutrients; suitability of land for cultivation and erosion susceptibility. Tables of specifications relating these qualities to measurable land characteristics and to the requirements of the LUTs were prepared. The study yielded a concise but unusually comprehensive summary of the agricultural potential of the island.
Figure 1
The amount of irrigated land in developing countries has increased by more than 50 million hectares over the past 25 years, and there is scope for a similar increase in the next quarter century. Where rainfall is inadequate or uncertain, notably in much of Africa, only irrigation can provide enough water for reliable higher crop yields. Careful land evaluation is especially important in planning irrigation, as without it irrigation can be costly, disruptive of life-styles and environmentally damaging.
Irrigated land utilization types are diverse and a wide range of crops can be grown, even including dry land and wetland crops within the same year. There are two main ways of dealing with the variations in water availability that occur: separate LUTs can be defined for specific cropping sequences and water requirements; or water availability can be used as a factor to determine the suitability class of a smaller number of LUTs.
Separate land utilization types may need to be recognized to distinguish alternative methods of irrigation. Surface irrigation, for example, often involves spending a great deal on land levelling and drainage. These preparations are less crucial for sprinkler irrigation but the costs of pipes, sprinklers and pumping equipment may be high.
Because of the investment involved, land evaluation for irrigation should always be carried out, so that the scheme's economic and financial feasibility can be analyzed. Estimates of capital investment and land development costs, for example, need to be defined as class-determining factors in evaluating the suitability of different methods of irrigation.
Quantified evaluation calls for detailed examination of the physical environment. Irrigation water is an erosion hazard even on gentle slopes, and water that cannot drain away will cause waterlogging and other detrimental effects. Thus the factors that bear directly on these hazards - topography, the physical characteristics of the soils and local hydrology - are especially important in relation to irrigation.
In the early stages of an investigation, the amount of water available for irrigation and the exact areas that can be supplied are likely to be uncertain. If this is the case, a suitability classification identifying 'provisionally irrigable land' can be carried out for the whole project area. This will help in the formulation of a project plan and will indicate the areas that are physically unsuitable for irrigation and can therefore be excluded from further investigation.
Recent droughts in Ethiopia have led the government to give high priority to irrigation as a means of providing an assured food supply for resettled people. To this end, FAO was asked in 1985 to investigate the feasibility of controlling flow on the Kesem River, a tributary of the River Awash in the northern part of the Ethiopian Rift Valley. The aim was to provide irrigation in the Kesem area and generate hydroelectricity.
FAO commissioned consultants to investigate soil and land suitability for a variety of irrigated uses in an area of 22000 ha that might be commanded by such a dam. After a preliminary reconnaissance, intensive field work was carried out in the spring and summer of 1986. Subsurface characteristics were carefully investigated with 1900 auger borings to a depth of 3 metres or more. Sixty deep soil pits were dug and described, and some 1700 soil samples were analyzed - mainly to check for salinity and alkalinity. Soil infiltration and permeability measurements were also made in the field. The suitability of the land for a number of alternative irrigated land uses was then assessed. Finally, the information was described in a detailed report and portrayed on maps of soils, irrigation suitability, vegetation and land use at 1:20000 scale.
The findings of the study, however, were disappointing. Although most of the area was at least marginally suitable for some form of irrigated agriculture, the soil pattern was found to be complex, and soils wholly unsuited to irrigation because of salinity, alkalinity or poor internal drainage were widely scattered amongst the better soils. Large patches of uniform good quality soils that could be irrigated efficiently and economically were rare. Thus, although data appears promising on the availability of soils suited to uses other than irrigated agriculture in the area (see illustration), problems associated with efficient water delivery, drainage and management make it unlikely that the irrigation project can proceed as originally conceived.
Suitability of soils in the Kesem area for non-irrigated agricultural uses
The next stage - classifying the 'irrigable land' - involves more detailed investigation of the promising areas, and elimination of those that would be too costly to serve or drain, that would provide only marginal returns or that should be eliminated for other social or economic reasons.
Assessment of land suitability classification can also proceed in stages. The separate classes (S1, S2 and so on) in a simple qualitative comparison of land can be quantified in terms of a 'land productivity index' that reflects expected production - the best S1 land normally being given an index of 100, with lower land classes having smaller indices.
Economic considerations can be introduced next by relating the classes to 'net farm income' - values obtained by deducting all direct farm costs from the value of production on each class of land. This measure is appropriate for the classification of 'provisionally irrigable land'.
A more precise economic index - 'net incremental irrigation benefit' (NIIB) - is desirable for the final 'irrigable land' suitability classification. This is arrived at by estimating the net benefit on a unit area of land with and without the project, taking account not only of all the ordinary farm investment and operating costs and returns but also project investment, operating and maintenance costs. NIIB is therefore a measure, expressed in economic terms, of the potential increase in productivity on a specific class of land under the project plan.
The possibility of environmental degradation as a result of the introduction of irrigation is very real. Historical evidence of the collapse of civilizations from this cause underlines the need to look beyond mere economic yardsticks in predicting the suitability of land for irrigation. Apart from damage to soils, which may be difficult to reverse, the risk of spreading vector-borne diseases such as malaria, bilharzia and river blindness must also be considered. So, too, must a wide variety of socio-economic issues ranging from traditional attitudes, laws and customs to problems of labour supply and food preferences. Any of these can prejudice the success of new irrigation schemes.
What future for extensive grazing?
Rangelands occupy more than one-third of the Earth's land surface and, although generally ill-suited to arable farming because of insufficient or unreliable rainfall, short growing season or excessively rugged terrain, they provide a livelihood for more than 25 million people engaged in two of mankind's oldest activities - livestock grazing and hunting. Demand for animal products continues to increase and there is a tendency for farmers to increase livestock numbers, sometimes beyond the viable carrying capacity of the land. In the drier range-lands the risk of desertification through overgrazing is increasing and in the wetter areas pressure from expanding human populations is leading to competition between pastoralist and arable farmer.
Well-planned livestock grazing and arable farming can be mutually supportive, and such an arrangement provides plant feed for the herds in critical seasons in exchange for animal proteins, manures and tractive power. In contrast, straight competition can be disastrous, leading only to erosion and the loss of farms and pastures. Such competition for land calls for careful planning and creates an important demand for land evaluation.
The financial return from extensive grazing is low. This fact has two important consequences for land evaluation. Firstly, it makes it difficult to justify large expenditures on data collection and analysis. Secondly, it implies that land improvements requiring significant expenditure are unlikely to be feasible - particularly if the expenditure would be recurrent, as in repeated applications of fertilizer. These considerations greatly reduce the predictive value of land evaluation for extensive grazing as land must be mainly evaluated in its current condition, using the status of the existing vegetation as a general yardstick. The range of investigations should be sufficient, nevertheless, to detect changes already in progress or immediately foreseeable, and assess their likely effects.
Suitability of land for extensive grazing is normally measured in terms of the number of animals that can wisely be allowed to graze a land unit for all or part of a year the 'carrying capacity'. This depends mainly on the amount of feed and water that will be available to the animals on that land in all but exceptional years. Other factors that may enter the suitability analysis include biological hazards (toxic plants, pests and diseases), climatic hazards and practical issues such as access to grazing areas, ease of fencing and location.
'Grazing capacity' - a measure of the ability of land to meet just the feed requirements of a particular species during a particular part of the year - can be used in evaluation as a kind of 'super' land quality embracing the interacting effects of all the big-climatic factors in producing feedstuff. Since grazing capacity can be assessed by sampling existing vegetation in conjunction mainly with climatic data, the need for other aspects of basic data collection is reduced.
Distances which herds are moved in search of fresh grazing and water and in response to the seasons vary greatly with the kind of livestock husbandry being practiced. In communal grazing, possibly combined with agriculture, movement is limited and may be negligible. In ranching, movement is greater but still systematic between fenced paddocks. Nomadism extends erratically, as need dictates, over considerable distances while transhumant herdsmen may move their livestock hundreds of kilometres over traditional routes in pursuit of seasonally favourable conditions. These, together with hunting and recreation, represent the major kinds of grazing land use, but can be sub-divided for evaluation purposes in terms of kinds of animals and animal produce and by criteria such as labour intensity, capital input, knowledge and attitudes.
The mobility of livestock presents a special problem in land evaluation because it implies that the acceptability of a single land utilization type may depend on its suitability on each of several successive land units during specific periods of the year. An acceptable sequence and timing for the use of different land units may have to be recognized and assessed - creating problems that have to be faced anew in each location. A suitable grazing use will be one that provides a herd of desired size with adequate feed and water throughout most years, without placing any part of the land under excessive grazing pressure. Defining the desired size of herd is itself a complex socio-economic issue and the solution must take into account the needs and attitudes of local people.
Kenya: Land suitability for nomadic grazing
FAO is still developing definitive procedures for evaluating grazing land and research has been going on for some years. One study was undertaken on behalf of Unesco's Man and the Biosphere (MAB) programme in the Mount Kulal/Marsabit area of northern Kenya.
The study area, which covers 15630 km2, is mainly populated by nomadic pastoralists. The Rendille tribe uses most of the area to keep camels, sheep and goats whilst smaller numbers of nomadic cattle-herders occupy discrete areas in the mountainous fringes. Annual rainfall is only 225mm over most of the plains, rising to 700mm on Mount Marsabit. The study, which followed a soil survey, aimed to contribute to the solution of environmental problems associated with desert encroachment and ecological degradation of arid lands.
Nomadic and semi-nomadic, small-scale, extensive grazing and browsing was recognized as a single land utilization type, although the differing requirements of camels, sheep, goats and cattle were recognized in parts of the analysis and in the final presentation. The land qualities invoked in assessing relative suitability for this land use were accessibility, resistance to erosion, moisture availability and salinity/sodicity of the soils.
Accessibility has special significance in an area that is partly mountainous, stony and subject to flooding, and where the single through-road and most tracks are likely to become impassable in wet periods. Susceptibility of land to erosion was judged to be important where herds gathered near villages and waterholes. Moisture availability and salinity both influence the essential production of vegetation for grazing.
A method was developed for rating each of these land qualities in terms of relevant land characteristics and then of combining their mutual effect to determine the suitability of different land areas for the nomadic grazing of camels, sheep and goats, or cattle (see table). The method involves rating the constraints imposed by different land characteristics as a series of indices of decreasing severity (1-5) and developing a conversion table in which the separate indices are combined to give a suitability classification.
The results of this investigation have been integrated into an area management plan aimed at improving the lifestyle of the pastoralists and protecting the natural environment. Only the Chalbi Desert was assessed unsuitable for all animal species. It was, however, found that the pastoralists sometimes use the areas assessed as unsuitable and that some highly suitable areas are avoided for social, religious and inter-tribal security reasons.
Land suitability for extensive grazing in part of the Mount Kulal/Marsabit area
Every year more than 25 million hectares of forest land are converted to other uses, even though fuelwood is in critically short supply in many parts of the world. Land evaluation has an essential role to play in deciding which areas should be preserved for production and conservation, where forest clearance is justified and where new forests should be planted or allowed to develop.
In the past, forestry was often assigned to the poorest classes of rocky, stony or steeply sloping land. While forestry may well be the best and even the only effective use of such land, future demand for fuelwood and timber will be met only if the specific growing requirements of the trees are taken into account. Furthermore, in deciding whether better quality land should be allocated to forestry, all the benefits of forestry must be considered.
The major benefits include the production of commercial timber, pulp and household items (including fuelwood and products as diverse as dyes, medicines, fibre, glues, mushrooms and honey), the protection of land and water resources, and the creation of resources for tourism. These major forestry land uses apply to natural forests and plantations.
These categories can then be subdivided into land utilization types defined by the nature of the products and expected benefits, and by the forms of management and investment planned. In detailed studies, examples of criteria that can be used to distinguish different forest land utilization types include the intended levels of labour, capital investment and technology for use in silviculture, harvesting and conservation (notably fire fighting).
Because forests nearly always serve more than one purpose, the multiple land utilization type is the norm in forestry. Mutually supportive combinations of forestry and agricultural cropping - known as agroforestry - often deserve consideration, especially in cases where forests are being reestablished or where risks of wind or water erosion are severe. Soil and water conservation are nearly always important objectives of forestry, and there is also growing appreciation of the importance of the preservation of genetic resources, both flora and fauna. This diversity of potential uses calls for wide-ranging consultation with users, policy makers and technical experts when formulating evaluation objectives and land utilization types.
The process of suitability assessment is essentially the same as that used for rainfed cropping. The data collection stage, however, may need to include established procedures of forest inventory, volume measurement and yield prediction to provide knowledge of existing forest stands and growth potential. Moreover, the evaluation must take account of the long time-scale involved in bringing a forest to maturity and harvest. This does not involve any change in principle because even arable cropping should be evaluated on a sustained basis, but it may be of relevance when assessing other pressures on the land over the long term.
Land planning in the Philippines
The Marakina Watershed Reservation in the Philippines occupies some 28000 ha and is the major source of water for Metro Manila, the national capital. The Reservation is drained by two major rivers, one of acceptable water quality, the other highly polluted. Due to its proximity to Manila, the area is subject to land speculation and a national highway passing through the watershed invites migration and re-settlement. Today, less than 30 percent of the Reservation is forested and there is tree regeneration in a further 11 percent. More than half of the Reservation was formerly forested, but is now cleared and has reverted to grassland.
In 1985, concern over the long-term future of the area as a water source for Manila led to it being chosen for a pilot study to test land-use planning activities destined for nation-wide use. An FAO Technical Cooperation team working with the Philippine Bureau of Forest Development undertook the study.
Types of data collected
big-physical
• precipitation
• bedrock; kind and depth
• mineral and coal deposits
• land form, slope, aspect, elevation
• soil depth, pH, drainage, texture, fertility, erosion evidence
• current land cover and land use
• timber volume by vegetation class
• herbage production by vegetation class
• wildlife; kind and distribution
• water table location
• water; quality and availability
• flooding hazardeconomic
• access by kind
• timber haulage distance
• internal markets
• external markets
• distance to markets
• products
• product disposition
• disposable income
• total incomesocio-cultural
• total population
• population structure
• tribal/cultural communities
• land ownership
• employment
• community services
The activities undertaken resembled a more detailed version of the flow chart on page 6. In establishing study objectives, 11 potential land uses were identified for investigation. These included protective forest, production forest and agroforestry, agricultural, residential and industrial use, settlement, fish farming, mineral exploitation, recreation and grazing.
The land uses were defined, and their characteristics and requirements specified. Land units were identified and mapped by overlaying maps of landform, soil and vegetation. The characteristics of each land unit, defined in biophysical, economic and socio-cultural terms (see below) were then matched with the requirements of the separate uses to develop suitability classifications for each land use. Three classes of diminishing land suitability (S1, S2 and S3) were recognized, and unsuitable land was categorized (N1 and N2) depending on whether or not constraints were likely to prove permanent.
Future land-use patterns in the Marakina watershed
The approach was found to be particularly satisfactory for guiding land-use planning in politically sensitive areas such as the Marakina Watershed. Future land use suggested by the suitability classification is illustrated above.
The difficulties related principally to defining appropriate economic and socio-cultural criteria, and securing the confidence and cooperation of local people in divulging information of this kind. A need to develop simple. yet comprehensive operational procedures for collecting field data was recognized. Subject to suitable adjustments in the light of this experience, the procedures were deemed suitable for nationwide application.
Overcrowding in fertile areas of the developing world can be solved either by using existing land more intensively or by resettling people on new land. In both cases, careful land evaluation is required.
Unfortunately, many of the still unsettled areas are agriculturally poor and cannot support sustained arable cropping. This is obvious in places that are arid, cold or steeply sloping but less obvious in the tropical rain forest. However, lush rainforest vegetation is no guarantee of high soil fertility - more often the soil contains a limited volume of nutrients originating mainly in the vegetation itself. This supply of nutrients is irretrievably lost if the forest is felled. Usually less than 20 percent of this kind of land can sustain the unsophisticated arable cropping with limited fallow that typifies re-settlement.
In the past, much money and effort have been wasted by planning and even implementing resettlement schemes without first establishing the suitability of a chosen site. The distress suffered by unsuccessful settlers on these sites has been even more painful. To rely on chance when selecting a site is to accept odds of at least five to one against success.
The first task is to use land evaluation to identify suitable areas for resettlement. Very large areas - perhaps a whole nation - may have to be examined; however, if no national overview exists, the study can be planned to serve many purposes besides re-settlement. Many countries could benefit from such an overview, which can unify much scattered, uncorrelated information. Modern technology, especially satellite imagery, radar scanning (so useful in the cloud-covered humid tropics) and computerized data storage is on hand to assist. In the context of resettlement the aims of the overview should not be ambitious. Small-scale mapping (say 1:250000) will not yield more than broad indications of the likely quality of the soils but it will help eliminate huge areas by revealing unfavourable aspects of topography, existing land use, swamps and other factors. The areas remaining are worth further examination.
Two more stages of land evaluation are often desirable before detailed planning can begin. The first confirms the general suitability of the soils, examines alternative land-use possibilities and indicates where each alternative should be considered. The final step is a detailed land evaluation, particularly if the topography and soils are complex, so that optimum land-use patterns can be determined.
Land evaluation for resettlement concerns people who have moved from a familiar environment. If new conditions arc very different, their situation is correspondingly more difficult. They may lack experience of how their new land should be used or of the vagaries of local climate and soils - an unusual and precarious situation for farming families. Sometimes farmers used to growing paddy rice have been resettled in areas where only dry land cropping is possible. In such circumstances, land evaluators have a special responsibility to anticipate all the problems that may arise in relation to the land utilization types that they put forward.
