21 low, as such it cannot be economically developed. Therefore, proper evaluation of the MCWH and soil trap systems in the arid region is necessary for optimal sustained production.

Study Area

The area is situated in western region of , . The surface land scape is of dunes and interdunal plains. The dunes are generally 15-40 metres high. They are usually longitudinal and oriented SW- Abstract NE. The plains are flat to gently undulating, and lack surface drainage. The With little rainfall and growing population pressure, Western Thar desert desert plains contain occasional dry rivers which disappear in the sandy is very short of water. However, in adapting to the harsh and variable plains. physical environment the Thar agro-pastoralists have developed principles The soils of the area are generally deep calcareous wind blown fine- and strategies for managing the land, water and natural vegetation sandy to loamy-fine sands. The dunes have wind blown sand deposits. The resources. Micro-catchment rainwater harvesting is an interesting sandy plain soils are aeolian and loessal sands. They have a high infiltration alternative to arid zone reclamation by irrigation and drainage. The paper rate and high wind erosion hazard. They are generally deep but are impeded discusses the benefits of micro-catchment water harvesting (MCWH) and by a lime concretion barrier in the sub-soils. The desert plain soils are soil trap techniques in the loessial soils of Western Thar desert with generally shallow to deep calcareous fine sand to fine sandyloam with much higher available soil moisture and nutrients and reduced soil underlying geological barrier of massive calcium carbonate or gypsum or clay salinity resulting in 40 to 150 percent higher production. or rocky substratum (Soil Survey, 1972). The area has rich desert vegetation wealth. The dominating are Introduction khejri (prosopis cinerarea), khejra (prosopis spicigera linn.), babul (acacia arabica), jal ( oleoides) and Rohida (tecomella-undulata seem), the Sustainable development in western Thar desert requires a decentralised but dominating shrubs are phog (calligonium polgoinedes linn.), and kheep coherent effort for the delivery of goods and services aimed at promoting (leptadeniaspartium wight) and the dominating grasses are seven (lasiurus traditional environment supportive . The diversified agricultural sindicus), and ganthil (eleusine compressa). production under MCWH system increases yields and income and reduces The drainage of the area lies towards the Sutlej and rivers. The risk. surface drainage of the plains is restricted by the dunes. The desert and The erratic and spatially variable rainfall in western Thar desert limits sandy plain having geological barriers at moderate to shallow depth have agricultural production. For sustaining levels of production conservation of impeded vertical drainage. The region has an arid climate with hot and dry soil, water and natural vegetation resource base, through total catchment summer and cool and dry winter. The annual rainfall is extremely variable management which involves the coordinated use and management of land, and ranges from 100mm to 200mm (Soil Survey 1972). water, vegetation and other physical resources in a catchment is required to ensure minimal degradation and erosion of soils and minimal impact on Data Collection water yield and quality. Ecological agriculture under the MCWH system in the arid desert region is not only an alternative but also a necessity imposed Present day subsistence farming in western Thar desert region is prone to by the need for self-sufficiency and sustainability. an alarming degree of land degradation and desertification. The earlier However, intensive irrigation in the region results in the depletion of diversified agricultural production by the nomadic agro-pastoralists was a natural grasses and other vegetation, water logging and soil salinity. The strategy for increasing yields and reducing risks. A comprehensive socio- ground water is generally brackish and the ground water potential is fairly economic plus traditional agro-technical approach was and is the only realistic frame of reference in which the interactive and interlocked 22 problems of depleting soil moisture and nutrients and increasing soil salinity can be appreciated effectively. Development with environmental protection can be achieved with rational management of nature, energy and resource conservation, application of ameliorative and non-waste technologies, thrifty use of the ecological resource base, and reversal of environmentally ill-conceived human activities. Intensive irrigated agriculture may be undesirable in the arid zones from an environmental point of view or may not be the best solution because of the cultural background of the local population. Already an area of about 1900 km2 is water logged in Project stage I Area and about 3500 km2 is potentially vulnerable to water logging in stage II Area, (RGWD, 1991; Bithu, 1981). Rain water harvesting can therefore be a cost effective and sustainable alternative to arid land reclamation by irrigation and drainage (Boers et al, 1986). The advantages of rain water harvesting have been well documented by Issar, who states that water sheds of up to 10 hectares produce about 5 to 10 m3/ha of annual runoff annually (Issar, 1977). Although low intensity and short duration rainfall of the loessal soils of western Thar desert region does not result in big runoff on large catchments, such soils are effective producers of run off on micro-catchments. Under the micro-catchment water harvesting systems, overall surface runoff is reduced as a consequence of maintaining infiltration rates and surface storage. Run off velocity is reduced due to increased flow tortuosity created by the bushes and grasses and their litter in contact with the soil (Fairburn et al, 1986). The roots of the bushes and grasses enhance percolation of rain water which in wet years recharges the ground water. The infiltrated rain water in the basin area is stored in the soil and surface evaporation is minimised due to capillary break in the dry loose top sands. The natural trees, shrubs, and grasses of the MCWH system in the region serve as nature's groundwater provisioning mechanism through the root system and herefore enhance groundwater recharge and soil productivity (Bithu 1989). Table 1 shows the effect of the natural MCWH system on groundwater recharge and land productivity. Table 2 shows effect of the MCWH system on two desert crop yields.

Soil moisture analysis Root zone soil moisture analyses have been conducted from the bare loessial soils and from the sand plains with the natural MCWH system (rich growth of Lasiurus sindicus and Calligonium polgonoides at an average spacing of 5 metres). The root zone soil moisture and salinity data from the bare and vegetated (MCWH) land are given in Table 3. 24 The vegetated land under MCWH system has increased infiltration particularly in the basin area and consequent effective leaching of the salts. The bare and devegegated land has crust formation, reduced infiltration, higher run off and consequent ineffective salt leaching. The higher root zone soil moisture and reduced soil salinity contributed to higher crop yields under the MCWH system. Figure 1 shows a typical micro-catchment water harvesting system with catchment area A and basin area B.

Such micro-catchment water harvesting or rain water capture systems were well developed in western Thar desert region in the beginning of the century but now stand redundant due to continuing abuse and neglect. The author has seen two such systems in the region where excellent trees, shrubs and grasses grow, and where wells for cattle watering had been developed. The loess soils of Western Thar desert are situated on geological barriers. In such areas the accumulation of water behind a small dam of clay or any other comparatively impermeable material causes infiltration into the soil and formation of a perched lens of ground water on the underlying impermeable layer. In such soil trap dams, the runoff water is stored in specially built or natural soil or sand reservoir instead of an open body of water (Issar, 1977). The drainage from the soil reservoir is restricted by the dam and the underlying geological barrier, and evaporation is minimised by the sand mulching action of the top dry loose sand. The stored water can be used by the grasses, shrubs, trees and field crops. Excellent seven (Lasiurus sindicus) grass lands have developed in the region on the moisture conserved, through the rain water harvesting and the natural soil trap systems (Bithu, 1989). Because of the availability of the soil moisture the grasses have survived droughts and the pastures tend to become non brittle. On intensive irrigation the grasses and shrubs slowly 26 27 disappear followed by high perched water table and soil salinity. The References complex geological drainage situation of the region makes post-irrigation drainage very difficult and costly. The economic evaluation of irrigated Bithu, B.D. et al. (1981) 'Drainage of agricultural lands in Rajasthan Canal agriculture, rainfed agriculture under MCWH system, and pasture dry land Project' Stage II Area Report 1981. farming with supplemental protective irrigation in drought years is given in Bithu B.D. (1989) 'Planning of Water Resources in Arid Area of Western Table 4. The table shows the desirability of combating the drought through Thar Desert' Seventh Afro-Asian regional conference. Tokyo, 1989. protective supplemental irrigation making every year a good rainfall year, Bithu, B.D. (1989) 'Irrigating sevan grass-land in Western Rajasthan' Trans. and environmental and economic superiority of rainfed farming under the Indian Society of Desert Technology 14: 49, 57. MCWH system (Bithu, 1993). Thus dry land and live stock farming under Bithu, B.D. 1993 'Irrigation and Drainage in Western Thar Desert, India', MCWH systems, aided by supplemental protective irrigation if and when 15th ICID conference, the Netherlands, September 1993. needed, is the appropriate drought coping and environmentally sustainable Boers, T.M.; K. Zonderven and J. Ben Asher (1986) 'Micro-catchment strategy in the Western Thar desert region. Water Harvesting (MCWH) for Arid Zone Development', Agriculture Water Management Vol. 12, No. 1-2, October 1986. Figure 2 shows the subterranean dam or soil trap system in the Western Fairburn, D.M.; C.H. Wockner and D.M. Silburn (1986) 'Effects of Thar desert region. Catchment Management on Run off Water Quality and Yield Potential from Vertisols', Agricultural Water Management Vol. 12, No. 1-2 October 1986. Issar, Arie. A. (1977) 'Water in the Desert', KIDMA. Rajasthan Ground Water Development Report (1991). Soil Survey and Land Classification (1972) 'Reconnaissance Soil Survey of the Phase II Area of Rajasthan Canal Project', Technical Report No. 1 AGL-SF/IWD 24 UNDP/FAO Rome 1972.

Conclusions

Root zone soil moisture and shallow ground water recharge is higher under the micro catchment water harvesting system than under bare soil, under the MCWH system the crop yields increase by 40-50% due mainly to availability of higher soil moisture and nutrients: the salt-free rain water leaching under the MCWH system is more effective than on the bare soil. Rainfed agriculture and dryland farming or livestock farming in pastures with supplemental protective irrigation aided by the MCWH system are environmentally and economically sustainable and are more remunerative than the conventional (canal) irrigated agriculture in the region. The ODI Irrigation Management Network is sponsored by:

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ISSN: 0951 1881