After identifying the major problems of the watershed, detailed biophysical surveys should be designed on a problem-solving basis. Healthy parts of a watershed should be put on routine care while special attention and urgent treatment must be given to critical areas or problem sub-watersheds.
For instance, if the main purpose of the effort is to reduce sedimentation of a reservoir, the survey work should be concentrated on identifying erosion or sediment source areas. Survey of forest areas should be concentrated on identification of cut-over areas, bare areas, reforestation needs, as well as cover types, densities and hydrological conditions of the land, rather than on volume or value of timber. Detailed surveys will also need to be carried out on disturbed areas such as cultivated fields, road slopes, streambanks, mined-out areas and landslides, etc.
If the main objective is for watershed or rural development, then, the survey should concentrate on resources inventory, distribution, uses, establishment and land productivity, etc.
Collection of too much or too little data can only result in a waste of time and money. The way to avoid this is to have a careful design at the beginning of the survey. Attention should be given to what data are really needed for future management and whether they are relevant to the main objectives.
Since watershed conditions change over time, future biophysical surveys will be needed every ten years or so. These periodic surveys are also used to evaluate management effects. For this reason, the initial surveys should be considered as benchmarks, and all results kept and stored for future monitoring use (see 11.3).
Although the design of the survey will vary depending on objectives and actual needs, some general rules should be observed:
- the data to be collected should be accurate and useful for the final analysis;
- survey forms, tables and guidelines to be used in the field should be easily understood. They should be field-tested before extensive use. The forms and tables must produce objective and not subjective results;
- the survey should be designed to identify problems and their location, extent, and areas which will be useful for deciding on treatment and control measures;
- all the field surveys should be so arranged that they can be carried out orderly within the allowable time period. A network analysis or a flow chart is sometimes needed to indicate a step-by-step approach;
- to facilitate future surveys, all measurement units, mapping and photo scales, survey forms and analysis procedures and records should be kept in a standard format.
Many types of surveys do not need to cover the whole area or the whole population. However, the difficult question is how big a sample size or how many samples are considered sufficient. For practical purposes, if the known population is large, a sample of 3 to 5 percent may be adequate; if the population is small, a sample of 10 to 15 percent may be more appropriate. The sample must represent the population and allow the work to be done within the time frame and financial limits.
Many sampling techniques can be used in the biophysical survey of a watershed. The general ones are briefly described below. For those who are interested in theories and detailed techniques, standard textbooks should be consulted.
Random sampling. This technique requires a knowledge of the population to be sampled. This sampling method can be used to check large quantities of gullies, landslides, streambank cutting where total numbers can be identified (for example, from air photos) but specifications of each cannot be measured except by field sampling. This technique can also be used to investigate, for example, hydrologic soil conditions of forest lands or of the entire watershed. After sampling size is determined, a random starting point should be selected on a map. From there, the subsequent points can be decided upon by use of a predetermined distance or grid. All the chosen points should then be visited and investigated in the field. Sometimes, a random number table is used to select the samples of gullies and slides.
Stratified sampling. A population is divided into sub-populations of known size, and then random samples are taken from each stratum. For instance, survey and estimation of sheet erosion can be done by major soil types and by major land uses. In each category, random samples can further be determined for field investigation. Such strata can also be established in different elevation groups, slope categories, cover types and farm sizes to be used for various kinds of sampling surveys.
Cluster sampling. In this technique random clusters are selected and then the entire sample in the clusters is surveyed. This method is usually employed to check survival rates of tree plantations (by small parcels or rows); to investigate fuel consumption of villagers (by village), or to estimate land use patterns in different parts of a watershed.
Whatever method is used, it needs to be done faithfully and as accurately as possible. Sampling saves much time and effort compared with surveys of the entire population, but if not properly carried out, can be useless or misleading.
Basic maps need to be prepared at one convenient scale. If the country's base maps are at 1:10 000 or 1:12 500 scale, the watershed maps should have the same scale in order to facilitate the transfer of information or the production of subsequent maps by superimposing one on another.
To transfer images from air photos to a map, a "Sketchmaster" can be employed. A Sketchmaster is a simple and inexpensive mapping device used extensively in many developing countries. If the map and photo scales are the same or similar, the work can be done much more quickly. For more precise mapping work, a desk type of "Map-o-graph" can be equipped to do the job. With some training, a draftsman can enlarge, reduce and transfer information from map to map.
For using computers for mapping, please see Appendix 3.
To obtain area statistics from maps, the dot counting method is usually employed. However, if a map contains hundreds of small parcels, the work can become tedious and time-consuming. A much simpler and quicker method which can be used by non-technical personnel has been devised recently. It is called the "cutting and weighing" method, in that parcels are coloured, cut, grouped and weighed by a balance against the weight of a known area. Various categories of areas can easily be obtained using a simple ratio calculation. The area figures thus acquired are quite accurate provided that the thickness of the paper used is constant.
When computers are used for mapping, area statistics are presented automatically and graphs are in the form of histograms or pie charts.
The kind of biophysical data needed for survey and planning depends on watershed problems and management objectives. Only brief descriptions are given in the following sections. Survey techniques and samples are given in Chapter 7.
General data on a watershed should include watershed name, location, boundaries, size, elevation, the presence of streams, tributaries, etc. The watershed may need to be divided into many sub-watersheds and each should be assigned a number for easy identification. Further data will be needed on a sub-watershed basis. General information such as population, administrative districts, accessibility and roads, etc. is also useful.
Data on geology and soils can usually be obtained from existing reports. Nonetheless, field checking is often necessary to verify or supplement the existing information. Geomorphological data such as drainage patterns, stream density and order, channel profiles, etc. can be obtained and/or quantified by using a good topographic map.
Data on soil hydrologic conditions are sometimes required in forest, rangeland, and cultivated lands (section 7.3 and examples 18 and 19 explain some of the methodology for reference).
Land slope information must be obtained and analysed in order to determine land capabilities of a watershed and appropriate conservation or treatment needs. With a topographic map of appropriate scales and a corresponding circle sheet, a simple and practical method called the "circle interception" can be used. The method is explained in section 7.1.
Using air photos and a stereoscope and with a slope scale model or a parallel wedge it would also be possible obtain slope values, but this requires a highly trained photogrammetrist for satisfactory results.
Climatic data such as precipitation, wind, evaporation, temperature, humidity, etc. can normally be obtained from climatic stations in the watershed or nearby. For watershed management, especially for run-off estimation and erosion control, rainfall intensities are required. However, they are often lacking in the upland or mountain watersheds. In this case, some supplementary automatic rain gauges may need to be placed in the watershed area.
Unless there is a big engineering project under way, data on streamflows and sedimentation are often not available in upstream watersheds in developing countries. Many times, the investigator can only collect data from stations at lower reaches of the same stream or from neighbouring watersheds.
For information on flood damage, drought and other hydrologic problems, the usual techniques are visiting damaged areas, interviewing people and discussing the matter with knowledgeable institutions in the area. The water balance or water budget of the watershed should be estimated. Water use problems regarding quality or quantity need also to be collected.
This category of data usually includes present land use, land use history and future trends, land capability or suitability and a number of vegetation surveys. For present land use, a new survey is often required in order to identify forest and range cover, cropland, plantations, recreation and wildlife reservations, urban and water areas, etc. The needed data and survey criteria for each major land use will depend on management objectives and individual conditions.
Land use history is needed to reveal past lessons and experience. This kind of data can be obtained from reports, records, or from knowledgeable persons in the local community. Future trends in land use are very important to planners. Trend prediction requires estimating population growth, forecasting migration and development, and surveying farmers' intentions for changes.
Land capability or suitability data are usually required to show the limit, extent, and proper use of each piece of land in the watershed. Although criteria may vary, they are essentially based on soils, slope, land capability classification which has been used in many hill watersheds in developing countries is given in Example 4 in 7.1. Appendix 2 shows the classification scheme in summary form. Also in section 7.1, an explanation of land suitability classification framework is given.
Special biological, vegetation and multiple use surveys are sometimes needed. They may cover forest resources, protection forest, range and grassland, wildlife reservations and recreation areas. These surveys are discussed in more detail in section 7.3.
Since erosion is a major concern in most watersheds, the collection of erosion data becomes a very important part of the overall surveys. The causes of erosion should first be identified. They may include many activities of human beings such as cultivating, grazing, logging, mining, road building, housing, fire, recreation activities. Nature also causes erosion in the watershed in the form of landslides, stream cutting, wild-fire, etc. Some of the erosion may be caused by a combination of man and nature. In Chapter 7, survey techniques are introduced and examples provided regarding the collection of data on sheet and gully erosion, road erosion, landslides, stream erosion, torrents, etc. Survey of geology and land forms or geomorphology, both of which are related to erosion, are also explained, in Chapter 7.
Determination of proper land use based on land capability or suitability is always the first step toward the protection and development of a watershed. A land use adjustment map can be produced by superimposition of land use and capability maps. Land showing serious over-use should receive urgent attention. On the other hand, land which is presently under-used can be used more intensively. In case of public lands, those under-used can be designated for resettlement of farmers who are cultivating steep slopes or encroaching upon forest lands. The map will not only show the sites, extent and seriousness of the problem areas, but will also provide the basis for rationalization of use of watershed lands. Land presently being used within capability but needing soil conservation treatments will also be shown on the map, and can be used for planning soil conservation activities. Details of such survey and planning can be seen in sections 7.1 and 8.3.
From the basic data collected, an analysis should be made of streamflows including annual and seasonal, maximum and minimum, and qualities such as turbidity, types and sources of pollutants, etc. The timing and frequency of flood and drought should also be studied. Any water use problems, including questions regarding rates of use and problems of quantity and quality, should also be addressed. Section 7.3 shows some examples of analysis. For more details, a water resources and a hydrology book should be consulted.
The various sources and damages of erosion and sedimentation should be identified and analysed, and potential hazards should be pointed out. The latter is very important since most watershed rehabilitation or protection is centred on the minimization of potential hazards. Special efforts should be made to analyse the data collected on erosion and sedimentation. A general methodology may include the following:
- compiling data from the field surveys, observations or from interviewing people;
- analysing soil loss and run-off plot data in the area;
- using erosion models or soil loss prediction equations to estimate quantities;
- analysing storm frequencies, sediment delivery ratios and yields, etc., from the existing hydro-meteorological data;
- compiling reservoir or water storage sedimentation data;
- using geology and geomorphology data for estimating landslide hazards;
- estimating results from all the above data.
The cost of erosion and sedimentation, treatment needs and the benefit of minimizing or controlling them should eventually be estimated.
Maps and statistics are a major product of this type of biophysical survey. The kinds of maps and statistics produced depend on watersheds and management objectives. The fundamental ones may include the following:
- base map, showing boundary, sub-watersheds, villages, roads, etc;
- topographic map, showing contours, elevations, land forms, streams, etc;
- soil map, showing soil types and boundaries, depths and soil limiting properties;
- climatic map, showing mainly rainfall, but statistics may include temperature, evapotranspiration, etc.;
- geology map, showing rock types, structures, displacement, morphology, etc.;
- slope map, showing different slope classes or exposures/aspects;
- present land use map, showing major land uses and cover types;
- land capability or land suitability map, showing different land capability classes; or land suitability classes:
- land use adjustment map, showing land being over-used or under-used and adjustment needs;
- erosion or sediment source maps, showing sites of various types of erosion and sediment potential areas;
- hydro-meteorological network map, showing the location of climatic and stream gauging stations;
- water resource map, showing surface and underground sources.
Many other maps and statistics of a detailed nature can also be added according to the needs. Some examples are forest and vegetation; landslide potentials; slope stability; stream profiles; land ownership; demographic and population distribution. Many maps can also be combined together.
To produce these maps and statistics on schedule, a flow chart is sometimes needed to set the sequence for collecting data and developing the needed maps.
Brief and essential interpretations are needed for the data collected and analysed. The interpretations should be relevant to watershed problems or management objectives.
Based on survey data and the results of analysis, the team or teams responsible for biophysical surveys should make draft plans (including treatment, costs, etc.) and management recommendations for further discussion. Planning approaches, techniques and management recommendation details will be discussed in Chapters 8 to 11.