Rainfed agriculture in a semi-arid tropical climate : aspects of land- and watermanagement for red soils in India

Rainfed agriculture is defined as the production of field crops that completely depend on the local precipitation for their water supply. Although in the semi-arid tropics the mean annual precipitation might seem to be sufficient to grow (adapted) crops, its variability over the years and its erratic distribution over the season pose problems. During relatively dry periods, the crop might suffer from moisture stress, at other times excessive rainfall occurs, causing water logging and erosion. This creates specific problems for crop production. The red soils, as a general indication of a group of mainly sandy loam soils, including Alfisols, have a low profile water storage capacity, often aggravated by their shallowness. Therefore, they generally lack sufficient buffer capacity to transfer water from a rainy period to a subsequent period of insufficient rainfall. Thereby, red soils have a poorly developed structure and the aggregates of the topsoil are easily dispersed upon wetting, resulting in a surface sealing. Raindrop impact causes a further compaction of the top layer. Under these adverse conditions, the infiltrability of the red soils will be strongly reduced and frequently surface runoff occurs well before the profile is saturated, even early in the rainy season.Production levels under such water-limited conditions are bound to be low. Yet, millions of people in the semi-arid tropics depend on them. In tropical India alone, the area of red soils that is yearly cropped can be estimated at as much as 50 million hectares. Common food-crops are mostly local varieties of sorghum, millets and grams, with average yield levels well below 1 t/ha. Important cash crops include groundnut, castor and sesame, with similarly low yields. Expansion of agricultural fields, in the case of red soils mainly under the pressure of population growth has been bringing less suitable areas under permanent cultivation and worsening crop rotation over the last 50 years or so. This leads to a further impoverishment of the soils.Shortage of sufficient water at the right time has always been a problem the farmers in the drier regions of the world have had to face. Depending on the land conditions and climate, different systems have been developed in order to tackle this problem. Some techniques are briefly described (chapter 4). Unfortunately', most techniques are based on the availability of a high retention capacity of the profile to store water, which makes such system unsuitable for red soil areas.In national and international agricultural research, attention is in the first place focussed on the development and introduction of modern crop varieties in combination with the use of synthetic fertilizer. Suitable varieties are the ones that have a higher yield potential and relatively good properties in respect to drought resistance or - avoidance as well as a minimum susceptibility to pests and diseases. Additional attention is given to beneficial crop combinations in respect to efficient water and nutrient use.More than the traditional cultivars, improved varieties require uniformly good growth conditions for optimal production. Moreover, higher demands are set for accurate and timely soil- and crop management, including seed bed preparation, seeding, fertilizer placement and mechanical weed control. As the technology that is traditional for the rainfed areas of India can not fulfill all these requirements at the proper level. introduction of improved implements and land management appears necessary. In combination with this there is the assumption that observed problems on excessive runoff, local water stagnation and high erosion could be dealt with much better in a bedded field than in the traditionally flat cultivated fields.At ICRISAT (International Crops Research Institute for the Semi-Arid Tropics), Hyderabad, India, these assumptions have lead to the introduction on experimental scale of such field lay-outs together with bullock-drawn wheeled tool carriers. For the sandy Alfisols the system of ridges was abandoned soon after introduction, because they proved unstable and difficult to handle with most field operations. The system of a bed-and-furrow configuration, however, continued to be used and seemed workable in combination with the improved equipment. However, in contrast to its performance on Vertisols, the system when used on the Alfisols appeared to result in higher runoff and soil loss compared to flat cultivated fields.In this study for which experimental work was done at the ICRISAT research station, a number of observations are reported that helped to validate assumptions on the runoff characteristics of red soils and to understand the reasons for the differences between surface treatments in this respect. As the infiltrability of red soils is easily reduced to low values through surface sealing, the surface depression storage proves to become an important parameter that influences the cumulative infiltration, as it most effectively prolonges the time of residence of the water in the field and therewith the time available for infiltration.Micro-depression storage (or surface retention) is related to the surface roughness and can be appreciable directly after cultivation. Under the influence of (heavy) rain , however, a rapid and almost complete decline of it can be observed ( section 6.1.1.2 ), mainly due to the low stability of the top soil. Only shallow depressions will be left, which on top of a more or less crowned shaped bed have little or no storage capacity, while they do have at least some in a flat cultivated field.Mini-depressions, as formed by unintended marks and irregularities, are much more stable but have a low storage capacity. Mini-depressions could also be created purposely, for example by damming furrows at intervals, which is only feasible on bedded fields. Under the conditions of Hyderabad, they do not prove effective.Macro-depress ions, as far as they are formed by topografic undulations within the field, also pose a difference between flat cultivated and bedded fields, as in the latter the stagnation of the water is restricted to the furrows. This can be a major advantage of the use of beds, as prolonged stagnation of water, resulting from continuing rain, will adversely affect most crops in the waterlogged areas.Mainly by the differences in depression storage between flat and bedded fields, their runoff performance is also different; actual differences thereby depend on size and intensity of individual storms ( section 6.2.2. ).At the same time, approaches to reduce runoff and erosion from bedded fields were searched for. In this respect a much more intense system of primary tillage, as compared to the usual way of ploughing beds at ICRISAT, earlier proposed by Klay (1983) proved to increase infiltration ( section 6.1.1. ). A significant difference in bulk density of the top soil was measured even by the end of the growing season.The influence of the shape of the bed-and-furrow on runoff behaviour was observed and hydraulic roughness co-efficients of the furrows calculated ( section 6.2.3. ). Again, as with the difference between flat and bedded fields, the pros and cons of a certain shape and size of the furrow also depend on the expected storm sizes. For Hyderabad, however, preference goes to narrow furrows along with a level bed. This is also the shape that is easiest to handle with bullock-drawn implements.Observation on erosion and soil loss ( chapter 7 ) stressed the need to differentiate between the local loss of soil within a field and the ultimate sediment yield at a measuring point. The necessity to include the composition of transported material in comparison to that of in situ material is made clear and its difference expressed as "dispersion coefficient". Measurements on the texture of eroded material over the season, showed the occasionally high values of this dispersion co- efficient. The high contribution of suspended material, particularly for red soils, in total soil-loss was obvious from the experiments.Although a more intense system of tillage, both in respect to depth and frequency, might well be able to decrease runoff, the maximum storage capacity of red soil profiles may often become a limiting factor. Most red soils have a profile retention capacity below 150 mm of crop available water, frequently even below 100 mm. In many years, this will prove to be too low a reserve to adequately support a standing crop during the droughty periods that can be expected to occur in the semi- arid tropics. observations at ICRISAT are referred to ( section 8. 3. ), where small amounts of water, applied as supplementary irrigation during periods of stress, resulted in considerable yield increases. But small water gifts that supported the growth of an additional post-rainy season crop also proved to be very effective. As far as no other source of water is available but the local precipitation, water for supplementary irrigation has to be drawn from earlier rainfall excess that has lead to surface runoff and has been collected in (excavated) reservoirs. Chapter 8 describes two alternatives for such an approach. Firstly, a runoff collection and water re-utilization system could be based on the collection of all season's expected runoff leaving the choice open, depending on the season, to use this water to break dry spells or to support a subsequent crop, possibly on a reduced area. Such a system would be based on reservoirs, with a storage capacity of say 5,000 - 6,000 m 3 for a 5 ha area (100 - 120 mm on area basis). The second system would be based on a field-scale water collection. Here, the envisaged storage capacity would amount to a much lower value of, say, 40 mm on area basis, or 200 m 3 for a 0.5 ha field. in this latter approach collected runoff mainly serves as a source for supplementary irrigation during dry spells.A choice between the two systems is complicated and, among others, depends on local precipitation, soil depth, grown or envisaged crops and available technology. Yet, a number of small reservoirs might well have some distinct advantages over a single larger one. These relate to a higher water collection efficiency, increasing the probability of a filled reservoir at the time the water is needed and to the possibility for small farm units (0.5 ha) to use the water at the right moment, using simple (and cheap) means for water lifting and transport. The relatively higher seepage from small reservoirs, c.q. the relatively higher costs for lining them, might be made good by a higher frequency of use, as the reservoir will mostly be filled up again after water extraction.In watershed development in red soil areas in the semi-arid tropics attention is generally directed to both resource protection and increase of productivity. only the former could possibly be considered as a single objective. In the combined objective, the land users have to drasticly change their systems of farming, as traditional technology already uses the environment to its optimum. Introduction of modern crop varieties and fertilization, costly inputs, should go along with an optimum management of the land, the soil and the water. Watershed development in this context is only possible if farmers are able and willing to spend knowledge, labour, capital and co-operation at the required level.For reaching this goal, relevant groups of farmers should be organised to enable co-operation in necessary land consolidation and construction work. For longer term maintenance work and for the organisation of machine pooling etc., separate bodies are required