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Shaping of land to receive irrigation can either be a minor operation, as where fields are already leveled and bunded for rainfed paddy, or a major operation, where topography is irregular or sloping and where there has been no prior land preparation. The form of initial land shaping depends to some extent on who is to carry out the work, the cultivator himself, or a contractor or other agency. The principal concern of a cultivator, with only his animal-drawn cultivation equipment, is the volume of earth to be moved. His capacity is very limited, and he may find it necessary to extend land shaping over several years, beginning either with very small bunded basins or graded furrows and progressively improving the system year by year. In the initial years, efficiency of water distribution at the field, and labor requirements for irrigation are likely to be secondary considerations to the cultivator.
On the other hand, land shaping with mechanical equipment usually aims at bringing fields to their final gross shape in a single operation. Hence longer-term factors such as irrigation efficiency, labor required for water management, volume of earth to be moved, convenience of shape and size of field for cultivation, and cost are taken into account in designing the field shaping system.
Consolidation of holdings provides larger units for land shaping and facilitates the use of mechanical equipment for the operation. However, as noted earlier, with smallholdings land consolidation is the exception rather then the rule in areas of irregular topography, and unless landowners agree otherwise land shaping must be carried out within the boundaries of the holding, more specifically of the parcel. This commonly limits the type of equipment which can be used. A further key factor in the design of land shaping, particularly with mechanical equipment, is the depth of topsoil and the nature of the material underlying it. For example, in contour terracing along a 3% side-slope with 15 cm of top-soil underlain by granular material, a 20 m wide terrace, even with balanced cut and fill, would require 30 cm depth of cut. This would already be 15 cm into the infertile subsoil at the upper boundary of the terrace. Even a 10 m wide terrace would still involve a cut extending down to the top of the subsoil. In the usual case of rolling topography the situation is aggravated by increased depth of cut when rounding each spur. The problem can be remedied, nominally, by stripping and stockpiling the top-soil before shaping, and subsequently re-spreading, but this is a costly operation and is seldom practiced. Part of the problem of land shaping in shallow soils with heavy equipment lies with the equipment operator himself In a situation which may call for a delicate touch, the approach of a dozer operator is usually more heavy-handed, favoring deeper cut and "full-blade". In a soil situation similar to that described an elderly farmer stood despairing as he watched a machine terracing across his holding, cutting down through the fragile layer of topsoil into the sterile material beneath. The farmer had spent years working up his land into small bunded plots, carefully building up fertility. He had subsequently become a reluctant beneficiary of a communal land development project.
As in all matters to do with irrigation, there are arguments on both sides of the question. There are circumstances in which use of mechanical equipment in land shaping of small holdings is desirable, particularly where the difficulty of the work or the size of the holding puts it beyond the capacity of the cultivator. Clearing of forested lands for conversion to irrigated agriculture is an example. At least initial rough terracing and stumping may best be carried out mechanically. Also where holdings are five to ten hectares in area, rather than the more usual two or three, there is a case for use of mechanical equipment for land shaping on the grounds that only by such means can a project area be brought to full production in reasonable time.
However, there may be other constraints. Consider the case of a project area in gently rolling topography with deep soils. Under the sparse rainfall of the area a holding of six to eight hectares permitted only subsistence-level agriculture. The cultivators were land rich but cashpoor. With the prospect of canal irrigation, a farmer was faced with two problems, the cost of land shaping to receive water and the cost of cultivation and other inputs needed to bring his holding into full irrigated production. Both would involve amounts well beyond his limited experience. Credit could be provided for mechanized land shaping, but the risk of default would be considerable in view of the limited likely returns from the initial years of operation. Credit could also be provided for cultivation, fertilizers, seeds, etc. But such credit has to be repaid each season or it is not renewed for the next. For the farmer, default is not a practical solution for crop credit.
The alternative course available to the cultivator is the minimum input approach. He prepares only a portion of his holding to receive water in the first year of irrigation. It is, in any case, as much as he can cultivate and plant with his limited initial resources. Each succeeding year as his resources increase, he extends the area under irrigation possibly engaging a small local contractor with farm tractor and blade for limited land shaping and cultivation. Eventually he graduates from subsistence agriculture, becoming the substantial proprietor of eight hectares of land fully under irrigated crops.
This is not a particularly satisfying alternative from the viewpoint of project economics, but it may well be the course chosen by the cultivator. The form of assistance most needed by the cultivator in this latter case, in addition to a limited amount of credit, is agricultural extension relating to irrigated crops and advice on progressive land shaping and water distribution. The last two subjects unfortunately fall in a gray zone between irrigation engineering and agricultural extension, and competent advice has not generally been available to the cultivator in these areas in the past.
As implied in the above discussion there is little advantage in carrying land shaping ahead of the capability of the farmer to put the area fully under irrigation. There are situations in which it is, in fact, very undesirable to do so. An example is in dune sand areas, as in the Rajasthan desert. The area has relatively level interdunal flats, winding between generally low dunes. The size of holding is 6 has. Eventually most of the area of each holding will be under irrigated crops (dune sands can be surprisingly fertile). However, there is an interim problem of wind-blown sand and dune formation. In areas leveled but left fallow dunes can re-form overnight, in a single sandstorm. It is essential to keep an area under irrigated crop (or crop residue) if dune formation is to be avoided. As most of the incoming cultivators (settlers) had few resources and little experience it proved desirable to limit initial land-shaping to that portion of the holding (usually the interdunal areas) which the cultivator could keep under cultivation and to extend land shaping and area under cultivation in successive years. Large scale mechanized land shaping operations originally planned for this area were subsequently dropped.
The form of land shaping for irrigation in smallholder agriculture generally differs from that in large scale cultivation. Where wetland paddy is grown, the bunded, level field is used in either case, the only difference being in size of field or plot. In large scale cultivation convenience in use of mechanical equipment for cultivation and harvesting is an important factor, influencing minimum size and shape of field. In most smallholder situations, however, cultivation is either by animal-drawn equipment or small single-axle cultivator. In either case size, or shape of plot is not an item of priority. Initially, plots can be very small, being progressively combined year by year, for example in the case of hill-slopes eventually becoming graded terraces each consisting of a series of plots stepped around the contour. Where multiple-cropping is to be practiced, alternating wet-land paddy with non-paddy crops, the bunded level plot is again the unit. Where wetland paddy is not to be grown, however, there are several options in land shaping. It has been the practice in Western countries to use either sprinkler or gravity irrigation using long graded furrows or graded strips. Hence, much attention has been given in the literature to appropriate rates of inflow versus slope of the graded furrow or strip and soil infiltration rates. Recently there has been some return to large level fields with gravity application, in view of the increasing cost of energy for sprinkler operation. A feature of large scale gravity irrigation has been the use of laser beam guided equipment for generating graded or level fields with high precision.
The situation of the smallholder is substantially different. The smallholder does not have available the means to grade a sloping field precisely, or if presented with such a field he does not have the means of maintaining it in that condition. Where land shaping involves varying depths of cut and fill, there is inevitably differential settlement on subsequent irrigation. Even precisely-graded fields require subsequent correction. The smallholder can, however, form and maintain a level field, as distinct from a graded field, because ponding of water rapidly demonstrates whether or not the field is level and the areas where correction is required. Further, the smallholder is not as concerned with labor cost for water application as is the large scale Western farmer, and the long uniformly graded field is not as attractive to him on that count, even if the irregular geometry of his holding permitted such an arrangement. Finally, in some clayey soils encountered in tropical climates, the infiltration rate varies widely with moisture content. The soils may be self-mulching, shrinkage cracking forming small pea-size units resulting in a crumbly structure of high infiltration rate when dry, rapidly changing to a low infiltration rate when expansion occurs on re-wetting. This situation would call for considerable judgement in irrigating a long graded strip.
In fact, the majority of smallholder irrigation, whether in paddy areas or not, is by level basin, or by strips or furrows within a basin. If presented with a naturally-occurring or man-made graded slope, the small farmer will usually convert it, for water-management purposes, into a stepped series of small level basins, or level furrows extending at right angles to the basic slope, supplied by a down-slope field channel with earthen checks. The basins may be permanent, or if the grade is small, they may be formed, after cultivation, each season by temporary ridges or bunds. The arrangement allows full control of water application, but requires constant attendance during irrigation, as the basins are small and the irrigation stream has to be changed frequently from one to another. Clearly, this is a disadvantage as farmers have an aversion to night irrigation.
Where the grade is slight and can be maintained uniform, the alternative of down-slope furrows each served by a siphon-tube supplied from a contour field channel would appear to be attractive, but is not widely practiced in the South Asia.
A particular soil condition encountered in some areas facilitates irrigation by down-slope furrows, even with non-uniform grade. The top-soil with moderate to high infiltration rate is underlain at shallow depth by a relatively low infiltration rate sub-soil (a lateritic sub-structure in one particular case). In such soil the intake of water during irrigation is self-limiting. After the top-soil profile is saturated there is little further infiltration, making water management relatively simple. However, even in this situation the cultivators preferred to use small basins stepped down the slope. A soil condition in which basin, or furrow-in-a-basin, irrigation is mandatory is in some very low infiltration rate clay soils, where water must be left ponded for hours to ensure sufficient intake. In contrast are dune sands with very high sustained infiltration rates. These are very effectively handled with high water-application efficiency in north-western India, by dividing a level basin (50 m x 50 m) into 2 m wide strips by temporary ridges. Each strip takes the full flow of around 2 ft³/sec for a period of minutes only, producing a uniform depth of impondment before appreciable infiltration has occurred. A plastic sheet is placed temporarily where the discharge from the field channel enters the strip, to prevent erosion at that point.
An important question in the operation of irrigation systems under conditions of limited availability of water is the minimum practical amount which can be applied in a single irrigation. As discussed below, some crops respond very well to "sub-optimal" irrigation, ea. mustard, pulses, or millets. In a water scarce situation, the economic amount of water per irrigation may be considerably less than the conventionally estimated demand. While the equivalent of 10 cm depth is often considered to be the minimum which can be applied with reasonable assurance of uniformity, application of half that amount may be desirable on grounds of special plant needs. The custom of referring to the irrigation of non-paddy crops as the application of the equivalent of a uniform depth of water is, in fact, inappropriate. Many such crops are row crops and only the furrows receive water. The "equivalent" of 5 cm, or less, is regularly being applied by experienced cultivators either via furrows, corrugations, or using a micro-distribution within the field. Such limited water application, however, requires precision in landshaping and a fine filth in cultivation.
Precision in land shaping is a prerequisite to efficient irrigation, from the viewpoints of both application of water to and drainage of water from the field. The quality of land preparation actually encountered varies from excellent to very poor. In formulation of new projects in smallholder areas not already levelled and bunded for rainfed paddy, land shaping in the past has frequently been made a project component, usually involving communal mechanized operations, via credit. However, because of the poor repayment record this practice has largely been discontinued with some exceptions, and land preparation is left to the cultivator's initiative. The missing element in this situation is technical assistance to the cultivator in the design and layout of his progressive land shaping operation.
Improvement in land shaping in areas already under irrigation but where land shaping is notably deficient, would nominally be a desirable subject for the attention of development agencies. However, the assistance actually needed is again the provision of field-level technical staff rather than funding (other than to credit institutions). As back-up to such land development extension efforts, provision of audio-visual demonstration and training material, and the development and demonstration of improved animal-drawn or farm tractor drawn land shaping equipment could be effective areas of assistance.
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