Fundamentals of smallholder irrigation : the structured system concept

The intensity of system adjustments required to meet individual farmer demands, and the administrative complexity of measuring and accounting for water deliveries have generally proven excessive when attempting to meet "on demand" schedules, resulting in chaos (often characterized by illegal tampering with infrastructure, and vast differences of water use intensity at different locations in the system). The alternative -- provision of a simple service, based on proportional sharing of available supplies on the basis of landholdings has been resilient for many years over vast areas. The approach is based on a clear delineation between the part of the irrigation system that is actively managed (at various flow rates and water levels) and the part of the system that either operates at full supply level (with proportional division of water down to the level at which farmers rotate among their individual farms) or is completely shut. This operational design is known as a "structured" system, and has well-defined hydraulic characteristics, thereby simplifying operation and management and in turn allowing a clearer definition of water entitlements and the responsibilities of agency staff and farmers. The approach is particularly suited to areas where water is scarce and discipline is needed to ration water among users. An additional benefit, which has been demonstrated in modeling studies using a well-proven model relating to water and yield, is that the productivity of water (which is more important than the more traditional productivity of land when water is scarce) is substantially increased when deficit irrigation is practiced -- a widely observed and predictable response to rationed water supplies. Structured systems are most suited where water is scarce, clear definition of water entitlements is needed, management capacity is limited, and investment resources are limited. The approach to determining critical aspects of a structured system design is described in this report. (Author abstract)