ABSTRACT
We start with a community of farmers in a river basin, each of whom has a given hectarage of land with a known cropping pattern. In output terms the farmer seeks tonnage of the crop, product quality, and crop composition and timing of the harvest appropriate to the demands of the market. On the demand side, a fundamental agronomic fact is that crop growth depends on water availability to each plant's rootzone. The soil water generated by precipitation may source this. But in those cases where soil water is known to be insufficient for crop needs between planting and harvesting, then irrigation is required. The same holds true where precipitation and soil water availability cannot be predicted with reasonable accuracy; in this case irrigation sources are necessary to make up for any deficiency that does occur, for example during a drought period. Where rainfall follows a seasonal cycle, as in the case study of the Lower Bhavani Project (see section 1.6), the demand for irrigation water is counter-cyclical.
