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.
Within a few years of project completion actual cropping patterns usually differ considerably from the originally conceived pattern. Nevertheless the design of an irrigation system requires assumptions at least as to the class of crops to be grown in each season, and economic and financial analyses (farm budgets) require more specific assumptions, in short a "project" cropping pattern. In the case of farm budgets several alternative patterns may be explored, as an individual cultivator is unlikely to replicate the whole project-wide pattern.
Eventual departure from the "project" pattern (sometimes referred to appropriately as a "notional" pattern) can be due to a number of factors. These include a change in price structure, the advent of a new type of crop in the area, or simply a difference between the view of the cultivator and that of the designer, who is preoccupied with projecting an acceptable internal rate of return. The original pattern may also be the product of a balancing act by the designer, between seasonal water supply and demand.
The departure from the design pattern may be as radical as a change from the conceived use of irrigation primarily for pre-monsoon and supplemental monsoon-season irrigation of paddy, to irrigation of hot weather ground-nuts, (an actual case). The advent of a surplus situation in rice in another area has emphasized the need for diversification and radically changed irrigation scheduling requirements. In a third case, in a classically wet tropic area with high population density and deficit in rice, the emancipation of family members from working in the paddy fields and the high cost of hired labor has made paddy financially unattractive. In spite of government edicts forbidding it, conversion of paddy areas to other crops is widespread, producing such odd rotations as bananas (twelve month variety) rotated with paddy, and irrigated coconuts "interplanted" in traditional paddy lands. Finally, in the initial years of operation of a major project when only a portion of the service area is served by canals, a condition which may extend over a decade or more, the available supply of water per unit of area in service may be considerably greater than under design conditions, permitting (in the view of the cultivator) a very different interim cropping pattern.
In some situations future changes in cropping pattern, particularly the introduction of small scale specialty crops, can be catered to by invoking on-farm groundwater development. However, this is not a universally available solution. More generally, it is expedient to examine any proposed canal system to determine how it could adjust to possible major changes in the demand pattern, and whether provisions could be built in which would facilitate meeting such future changes without complicating initial operation.