Setting up a soil conservation project comes within the framework of an over-all strategy which involves numerous disciplines and follows a logical process in decision making. The first phase of the process is the analysis of the situation from the physical, human and economic point of view. This should provide the necessary factors for deciding on the value of the project, its limitations and advantages, and the economic fundamentals from which it is possible to evaluate the significance of any measures that might be envisaged.
Preliminary studies are the first stage to help in the decision-making process aimed at defining the aims of coherent and well-founded development, and the means to be employed in achieving these aims.
This analysis will cover successively: the physical and hydraulic characteristics of the soil degradation and the impact of this degradation on the socio-economic picture in the region. The analysis should comprise a review of all the available data in order to arrive at a first estimation of the measures that might be envisaged.
188.8.131.52. Data collection
The data may be obtained following consultation with the relevant national services. The type of the basic data to be collected and the services that may supply this information are as follows:
- topographic maps. The most widely used scale is 1/200,000 or 1/100,000.
Certain regions may be covered by maps using a scale of 1/50,000 or 1/20,000 In the case of numerous countries (North Africa, West Africa, Madagascar, etc.), maps may be obtained in France from the National Geographic Institute (Institut gographique national);
Fig. D.1: Over-all flow chart for drawing up a soil conservation project
- aerial photographs. Many countries have been covered by aerial photograph: on black and white panchromatic film at a scale of approximately 1/50,000;
- satellite photographs (LANDSAT pictures). These may provide interesting information at the regional level.
Geological maps at a scale 1/1,000,000, 1/500,000 or 1/200,000. These are usually available from the national geological services (BRGM in France), universities, etc.
General pedological maps, where they exist, often have a small scale (1/1,000,000). For certain projects, pedological maps may be available with larger scales, and they can be obtained from geological or agricultural services.
Usually, all countries have a precipitation observation network which measures daily precipitation.
Climatological stations make more detailed data collection covering rainfall intensity, temperature, evaporation, hygrometry, wind speed and direction, etc.
These data can be obtained from meteorological services, airports and agricultural departments.
Data about the type and distribution of natural vegetation and the density and main species that can be used for afforestation purposes can be obtained from agricultural, water, forest and animal breeding services.
Characteristics of the hydrographic network and the hydrological regime. Type of hydrometric observations carried out.
184.108.40.206. Data analysis
During the preliminary study, utilisation of the data should make it possible to determine the magnitude of soil degradation phenomena by highlighting the incidence and intensity of the factors behind the degradation.
Soils may be classified according to their erodibility while specifying the type of erosion to which they are subject and the incidence and intensity of the damage caused.
220.127.116.11. Data collection
The main data to be collected deal with:
(a) demography: population in the zone in question, agricultural population, number of working people, trends;
(b) farming: type of farming (family, industrial, etc.), areas farmed, production (type, yield, costs), agricultural income;
(c) soil utilisation: agriculture, animal breeding, forest, industrial or urban zones;
(d) animal breeding;
(e) agricultural policy, development plans, current legislative measures.
18.104.22.168. Data analysis
An over-all review should be made of agricultural activity and soil utilisation in order to specify all sectors which might be affected by soil degradation.
Items which may be damaged or disrupted may be classified under three headings:
- permanent assets such as land, agricultural infrastructure (buildings, irrigation networks), the infrastructure of economic activity (roads, etc.);
- seasonal assets such as crops which may be damaged to different degrees depending on the intensity and period of occurrence of the phenomenon (flooding, crop destruction, etc.);
- economic activity which may be perturbed, due for example to the destruction of communication routes, water run-off or by wind-borne materials which may make cultivated land sterile or seriously compromise a regions industry.
Probable economic growth rates should be estimated in order to determine the growth trend in the value of these assets over coming years.
22.214.171.124. Collecting data about damage
Damage that may occur in the absence of soil conservation measures may be classified into two categories:
(a) Capital losses, the main one being the loss of land capital under the same heading as other infrastructural assets such as farm buildings, irrigation networks, etc. In estimating capital losses, it is possible to use as a basis estimates of damage that occurred in the past. These should be re-evaluated to bring them in line with current monetary conditions.
(b) Production losses which may result from reduced soil fertility, flooding, deposition of wind or river-borne sterile soil. Production loss may be a variable phenomenon related to the intensity of the destructed phenomena, the main ones being, precipitation intensity in the case of rain erosion and wind force in the case of wind erosion. In this case, the updated cost of annual damage should be used for calculation.
126.96.36.199. Mean annual damage
The methods that may be used for determining mean annual damage are numerous and related to available statistics:
- where a number of values exist for annual damage, the mean damage should be calculated from the arithmetical mean of damage data;
- when only a single value for annual damage is available and the incidence of this damage is not known, it may be hypothesised that damage is proportional to the intensity of the phenomena which cause it. With a knowledge of the relationship between the phenomenon behind the damage and the incidence of this phenomenon, it is possible to deduce the cost of damage for various incidences and from this assess the mean annual damage cost;
- where a number of damage incidence combinations are available, it is possible to draw a damage incidence distribution curve (see fig. D.2).
Mean annual damage is the area bounded by the co-ordinate axes.
188.8.131.52. Future damage
Future damage will vary depending on the forecast economic growth rate for the region. The calculation for mean annual damage should therefore be weighted by the forecast economic growth rate.
The amount of future damage at current prices is the sum of envisaged investment. The sum invested may, however, be higher where the planned improvements are expected to result in an increase in the value of agricultural land which, after conservation work, will have higher crop yield. The level of land value increase may be assessed from the amount of increased revenue, at current prices, indexed by the economic growth rate for this type of income.
Fig. D.2: Damage incidence curve
Once the draft study has made it possible to assess the value of a soil conservation project and the envisageable investment, the draft project should define the means to be employed to achieve the set aims.
The draft project should define the various possible improvements from both the technical and economic point of view, to permit decision making as to which project will best meet the final objectives. These objectives are often contained in an over-all land improvement policy for the development of a catchment basin. They may be economic, social, political, ecological, etc.
This analysis should cover all the data relevant to the establishment of the draft project. The basic data collected should usually be supplemented by field studies and surveys and then subjected to detailed analysis so as to form the basis for assessing possible improvements. The analytical methods that can be used are outside the framework of this book. We will, therefore, consider only the main data required in analysing a soil conservation project.
- monthly and annual precipitation: mean values and statistical analysis;
- exceptional daily precipitation with statistical analysis of maximum annual precipitation;
- rainfall studies and determination of relationships between rainfall and rainfall duration for various probabilities.
Morphology of the catchment basin:
- gradient characteristics: classes of slope;
- erosion forms: erodibility classification of various sectors of the basin.
- surface area of catchment basins - boundaries of sub-basins;
- estimation of run-off coefficients;
- determination of flow rates from man-made structures;
- determination of flow rates from collectors.
- general reconnoitring of terrain;
- auger soil samples with sample density depending on land variations and the scale of the project (usually one sample per 5-10 ha);
- collection of soil samples for analysis of physical and chemical properties and permeability;
- determination and mapping of crop suitability categories;
- evaluation of earthworks.
- drawing up of plans suitable for the project (1/10,000 to 1/20,000) by direct surveying or by photogrammetry;
- survey of land profiles along the main emissaries with preparation of cross-sections for the main features.
Agriculture - animal husbandry:
- types of crop;
- crop rotation;
- animal density, type of grazing, use of passage routes.
- industrial structures;
- agricultural income;
- market and rentable value of land.
When the basic data have been assessed, the project leader may present one or more hypotheses for improvements that are both technically and financially acceptable giving justification for the principle behind the project and the main technical arrangements.
This hypothesis for improvements should be accompanied by the following documents:
- location plan at a scale of 1/20,000 or 1/10,000 from which it is possible to locate the siting of the main structures;
- the main technical arrangements together with the relevant standard plans, and:
- characteristic flow rates,
- cross-section and spacing of defence and drainage networks,
- layout of main and secondary collectors,
- emissary improvements,
- standard plans for the main structures,
- spacing and type of plants,
- layout of forest roads,
- requirements for and location of nurseries, etc.;
- over-all assessment of improvement costs with an indication of cost per improved hectare.
For each draft project it is necessary to draw up an income and expenditure account, in current prices, for each of the planned conservation measures.
The expenditures include:
- investment: preliminary studies, cost of improvements, land purchases, eviction compensation, etc.;
- maintenance and operating expenses.
These expenditures should be discounted so that they can be compared on a valid economic footing.
- capital appreciation, the main component in a soil conservation project being the anticipated additional discounted income;
- reduction of damage. As a result of the protection measures, damage will be reduced for a given phenomenon recurrence time, i.e. totally eliminated in the case of small recurrent periods. In the damage incidence distribution curve shown in fig. D.3, this will be seen as a reduction in damage costs for given recurrence time. By comparing this curve with the damage curve as it was prior to the improvements, the reduction in the mean annual damage is the area between the two curves. These curves may have a different shape depending on the type of improvement, and their comparison is essential for selecting between different types of improvement. For example, we have shown in fig. D.3 two types of curve that followed improvements. The curve C1 shows damage distribution incidence following afforestation work. Curve C2 applies, for example, to an absorption network in an area where there are heavy, low-incidence rainfalls, embankments may be broken with damage which would be greater than the initial damage.
Fig. D.3: Damage incidence distribution
- Damage C0 before defence measures
- Damage C1 after defence measures
Income R = C0 - C1
Mean annual return
The balance sheet of income and expenditure makes it possible to define viability and selection criteria.
The main criteria used are:
- Discounted profit, which is the difference between income and expenditure. The improvements are viable if there is a profit at current prices.
- Internal profitability rate, which is the value of the actualisation rate for which the discounted profit is zero. If the improvement is financed by a loan, the internal profitability rate should not be greater than the loan interest rate.
- The relative gain at discounted prices, which is equal to the quotient of receipts. This makes it possible to choose between various independent projects.
- Cost benefit ratio.
The draft projects make it possible to define the alternative improvements intended to achieve a stipulated objective and which are technically and economically feasible. They also provide the factors for making a choice; however, this will not always be guided by the economic factor alone but may also take into account other criteria, e.g. social, political, etc.
Once the draft project has permitted the selection of a possible type of improvement, the project itself covers the drawing up of all the necessary items for the implementation.
The project covers additional field studies and the preparation of documents and drawings.
- preparation of plans at a scale of 1/2,000 or 1/5,000 with contour lines at intervals of 0.25 m for slight gradients and up to 0.5-1 m for steeper gradients;
- longitudinal profiles of natural water outlets and main collectors at a scale of 1/2,000 for the horizontals and 1/100 or 1/50 for the verticals;
- cross-sections of outlets and all salient points at a scale of 1/100 or 1/50.
The technique and depth of sample borings and the analyses to be carried out on them will vary depending on the type of work envisaged.
For plantation work, it is necessary to determine soil type, humidity and the difficulty of digging planting holes, by carrying out a sample boring of 1.00 m in depth every 10 ha or so.
For cut-and-fill earthwork, trenching difficulty should be assessed.
For drainage works, it is necessary to determine soil type and in particular permeability. Soil analyses are carried out on samples taken with an auger at a depth greater than the maximum drain depth and at a rate of one sample every 10 ha; it is also necessary to dig a trench every 50 ha to make a descriptive soil survey.
For the construction of small dams, it is necessary to dig one or more trenches along the axis of the structure to a depth equal to the height of the finished structure in order to determine soil types and permeability.
These comprise a description of the project and assessment of expenditure. They comprise:
(a) an explanatory memorandum describing the project principle and the main technical arrangements;
(b) a descriptive estimate which enumerates and describes the work to be carried out and the origin, quality and preparation of materials;
(c) specifications for the work to be carried out by a commercial firm;
(d) an analysis and breakdown of prices;
(e) a quantity estimate which calculates the expenses for each item of work;
(f) a location map at a scale of 1/20,000;
(g) a map of the defence network at a scale of 1/2,000;
(h) a longitudinal profile of the collectors and the emissaries;
(i) drawings of the structures at a suitable scale (1/20 or 1/50).