Where groundwater or surface water is not available for supplemental irrigation, water harvesting can be used to provide the required amounts during the rain season. The system includes surface or subsurface storage facilities ranging from an on-farm pond or tank to a small dam constructed across the flow of a wadi with an ephemeral stream. It is highly recommended when inter-seasonal rainfall distribution and/or variability are so high that crop water requirements cannot be reasonably met. In this case, the collected runoff is stored for later use as supplemental irrigation (Oweis et al., 1999). Important factors include storage capacity, location, and safety of storage structures. Two major problems associated with storing water for agriculture are evaporation and seepage losses. Following are management options proven to be feasible in this regard (Oweis and Taimeh, 2001):
1. Harvested water should be transferred from the reservoir to be stored in the soil as soon as possible after collection. Storing water in the soil profile for direct use by crops in the cooler season saves substantial evaporation losses that normally occur during the high evaporative demand period. Extending the use of the collected water to the hot season reduces its productivity because of higher evaporation and seepage losses.
2. Emptying the reservoir early in the winter provides more capacity for following runoff events. Large areas can be cultivated with reasonable risk.
3. Spillways with sufficient capacity are vital for small earth dams constructed across the stream.
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