In Pakistan, about 51 per cent of land resources are affected by desertification, which can be made productive, if the available water resources are managed properly, both qualitatively and quantitatively.

Cholistan is one of the four major deserts of Pakistan, which because of extreme aridity lacks fresh water resources to meet the needs of the local inhabitants and their livestock. Since the groundwater is either very deep or highly saline in Cholistan, therefore, the only source of fresh water is rainfall.

The occurrence of rainfall is erratic with high spatial and temporal variation. One main feature of the deserts is sand or sand dunes. Due to sandy soils and high temperature (generally more than 40C during summer), most of the rainwater is lost through wide spread runoff and evaporation.

Water conservation is an important component of water management for deserts, where water losses must be minimised and the available water used as efficiently as possible. Water management in deserts could be done by: (i) rainwater harvesting (ii) soil moisture retention (iii) controlling seepage and evaporation losses from water storage facilities (iv) improving water use efficiency of the irrigation system (v) pre-treatment of saline groundwater and (vi) saline agriculture.

Rainfall: The average annual rainfall in the Cholistan desert is about 166 mm. Depending on the soil types there are many locations in the desert where runoff in large quantities is available during the normal rainfall events.

Rainwater so available could be collected by adopting rainwater-harvesting techniques and by using appropriate interventions to store and utilize water. On the average, there is 423 million cubic meter (MCM) rainwater available for storage. Amount of water available, besides fulfilling domestic and livestock needs, could be used for developing small-scale agriculture with high efficiency irrigation systems such as sprinkler and trickle.

Storage practices: While the government, on its part, has been making efforts to find good quality groundwater and create rainwater storages in different locations, water scarcity continues to persist. The local people have directed their efforts towards harvesting and storing rainwater in locally constructed ponds (tobas) in which runoff water from higher elevation areas (catchments) is collected in natural depressions for domestic and animal use.

With these efforts, more than 1500 tobas have been constructed in the Cholistan area (Akram et al., 1999). These tobas however, are not scientifically designed and constructed.Due to high infiltration rates of sandy soils and high temperatures during summer season, most of the rainwater is lost as seepage and evaporation. As a result, a small amount of water is collected and stored which usually supply water for only three to four months. Moreover, due to siltation problems and ordinary designs of these tobas, the original storage capacity of these ponds get seriously reduced. The recent prolonged and extended drought, which started in 1997, aggravated the situation of water availability which forced the local population to migrate causing huge financial losses and high mortality rates of their livestock.

Another technique of storing harvested water is called kund, which is capable of storing rainwater for longer periods. Kunds are constructed by using brick and cement in the shape of well above the level of groundwater. Kund has concrete floor and roof with ventilators and is capable of storing water for long period. However, its storage capacity is small as compared to its cost. Further, the water stored becomes smelly and contaminated, and provides favourable environment for mosquito breading. There are presently more than two hundred kunds in the Cholistan desert.

Seepage and evaporation: Seepage and evaporation losses are the major concerns in desert areas. A pond of 3,701 m3 storage capacity monitored by PCRWS showed that about 60 per cent of total stored water was lost through seepage and evaporation. Therefore, most of the rainwater is lost due to evaporation and seepage. Lining the ponds, by using polyethylene sheets or by using bentonite clay etc. reduce seepage from bottom and sides of the ponds.

Techniques: The rainwater harvesting systems consist of (i) a treated catchment area with a storage facility for storing runoff according to the shape and slope of the land (ii) guiding the runoff to flow to the pond through a series of interconnected macro and micro channels (iii) using improved pond design with less surface area to reduce evaporation and plastic lining at bed for seepage reduction. The area of the catchment and the capacity of the storage pond should be matched so that neither is over-or under-designed.Typically, such systems are used to provide water for domestic and livestock uses.

Alive to the problems faced by the population in Cholistan, Pakistan Council of Research in Water Resources (PCRWR) initiated a project aimed at mitigating drought in Cholistan by rainwater harvesting to improve freshwater availability at a total cost of Rs152 million. The project was a direct outcome of the very elaborate field research efforts by PCRWR to establish that these tobas can play a major role in the uplift of the desert. The project includes construction of 70 storage reservoirs, 20 tubewells and four reverse osmosis water desalinization plants to provide sustainable supply of drinking water to the local residents and their livestock. Each of these ponds is designed to store about 18,200 m3 of water. The Council has so far completed construction of 39 ponds with a total storage capacity of 0.581 MCM.

Apart from the construction of new Tobas, PCRWR desilted 33 old tobas. These ponds have been designed to collect maximum rainwater with the minimum surface area and thereby minimum evaporation losses. The depth of these ponds varies between four to six meters. Each pond is connected with the catchment by a main channel and network of ditches connecting all lowest points in the catchment through a small pond to settle silt before reaching the main pond. Seepage loss from the bed of the pond was minimized by spreading polyethylene, covered with 15 cm thick dense impervious clay layer and clay mud coating on the sides. Reducing the surface area and increasing the depth of ponds as well as by planting trees around the banks had minimized the evaporation. At each pond, besides provision of drinking water, five hectares (ha) of land is being developed with high efficiency irrigation system using the rainwater and saline water conjunctively.

The 50 mm rainfall of February 2003 in Cholistan supplied the newly constructed tobas with an estimated amount of 0.581 MCM of water. This rain also filled the 33 desilted tobas providing 0.12 MCM of water. The stored water in the PCWR tobas is expected to meet the drinking water requirement of the local population and their livestock for at lest six months or till the arrival of monsoon season, even if no rains occur during this period. The additional ponds that the Council is constructing combined with the tubewells and Reverse Osmosis Units will solve the freshwater availability problem for domestic and livestock uses.

Evaporation: Evaporation from open water surface can be reduced by covering them with monomolecular films. However, more success has been obtained with floating objects such as, Styrofoam blocks, or empty bottles. For tanks or other small reservoirs, floating sheets of foam rubber have been used successfully. Evaporation reduction close to 100 per cent has been obtained with such covers (Bouwer, 1990). Reducing the area of the water surface can also reduce evaporation from open water surfaces. In Cholistan deeper ponds with less surface area were constructed. These ponds store same amount of water with much less surface area and evaporation loss.

High efficiency: The stored water could be used to develop small scale agriculture by using harvested rainwater and saline groundwater conjunctively. The traditional methods of irrigation such as basin/flooding result in poor application and distribution efficiencies. Typically these systems have efficiency less than 40 per cent. Surface irrigation therefore, is not possible in these areas and the scarce water should be utilised most judiciously and efficiently.

Small-scale sprinkler and drip irrigation systems are particularly well suited to the water scarce desert areas where evaporation rate is comparatively high. The higher cost of these systems is justified by producing high value vegetables and orchards. Application efficiencies can be in the order of 75 to 90 per cent, permitting almost full use of the scarce available water supplies (Leeds-Harrison and Rickson, 1992). An additional advantage as compared with surface irrigation is that efficient irrigation can be carried on areas with undulated topography and soil is of lighter texture.

Raingun sprinkler and drip irrigation systems have been locally developed which is comparatively cheaper (Figures 3 and 4). PCRWR is developing 30 ha fruit orchard with drip irrigation system and 10 ha with sprinkler irrigation system for evaluation and demonstration of the system to the local community.