| Design and operation of smallholder irrigation in South Asia |
|Chapter 5 - Cropping patterns in irrigation design|
Cropping patterns may, or may not, be dictated by government authority. Government policy may be to steer the selection of crops in a direction believed to be in the best public interest and the supply of project water may be made conditional on a cultivator accepting this direction. Alternatively, cultivators may be left free to follow their own inclinations and the forces of the market. However, even in the latter case there are likely to be unavoidable technical constraints on the supply of water, as few surface systems can be made entirely demand-responsive. A delivery schedule (down to the tertiary level) may be worked out to suit the supply situation and the water needs of the principal crops likely to be grown in the area. Cultivators are then left free to work out their individual cropping patterns around this pre-ordained schedule, deliveries within the tertiary command being subject to any exchange arrangements which may be set up between neighbors. The primary delivery schedule may be varied seasonally, or from year to year, in accordance with the supply situation or the anticipated pattern of demand.
Restrictions may be placed on cultivation of particular crops, where there is special reason for doing so. For instance, the proportion of a holding under sugarcane may be limited by decree, to avoid waterlogging or salinization in an area with restricted internal drainage. In a situation more generally encountered, some portions of a command are suited (due to soils or other reasons) to irrigation of monsoon season crops, while others are better suited to dry season crops. One course is to divide the command into areas of the two categories, each with different irrigation delivery schedules. This virtually imposes a restriction on the class of crop which may be grown in each area. An alternative course is to leave the choice of crops to the cultivator, within limits as to total water requirements, and to work out a delivery schedule which meets the summation of these demands in each minor or distributary command from month to month. This is a more complicated arrangement operationally, and illustrates the generalization that the greater the degree of freedom left to the cultivator to choose his cropping pattern and delivery schedule, the greater the operational complexity of the delivery system (and the greater the likelihood of its break-down or mismanagement).
The problem discussed does not occur, to any extent, in areas of homogeneous soils and topography (such as the Gangetic and Indus Basins), but there are other areas in which the soil situation unavoidably ranges from shallow upland to heavy wet-land all within the small area of a minor canal command. A solution which would largely avoid the problem and leave cultivators free to choose their cropping patterns and irrigation schedules is to provide pondages at the minor canal level. This would permit re-regulation between supply from the main canal system and demand within the minor canal command. Unfortunately, there are few sites for such pondages in which flow from the canal to the pond, and from the pond to the irrigated area, can both be by gravity over the full range of pond level. Low-lift pumping would be resorted to in Western systems, but is not yet generally acceptable in South Asia One situation in which gravity inflow/outflow pondage can be achieved is by supply from the primary canal system into existing village reservoirs, tanks, where these are available.