Effective rainfall

In a monsoonal environment rainfall can provide a major part of crop water requirements in the wet season, and a much lesser part, or none at all, in the dry season. However, not all rainfall can be utilized by the crop. During periods of heavy precipitation much is lost from the field by run-off and during very light showers most rain is intercepted by leaves and reevaporated without ever reaching the ground. Bunding of fields provides temporary pondage of heavy rain, although where crops other than paddy are being grown impondment has to be limited. On the other hand, where paddy is being grown, the bunded plot is likely to have standing water prior to the rainstorm, which limits its capacity for further storage. The soil moisture situation prior to a rainstorm also influences the extent of retention of rainfall, for instance pre-monsoon or early monsoon rain on dry soil may be fully retained, while later in the season it would not be.

Procedures for estimation of the "effectiveness" of rainfall are set out in the paper previously referred to (Doorenbos 1977). However, operational factors make it desirable to view each project separately. Also to be considered are the operational implications of unusual deficiencies in rainfall at particular times, for instance late arrival of the monsoon or rainless periods in mid monsoon. Hold-over storage may be included in the design of the project operation as insurance against delayed rains. Aside from the amount of storage to be reserved for this purpose, irrigation distribution system capacity may be determined by its function during such times of rainfall deficiency. Simulation ("paper operation") of the system, under various historic or postulated rainfall conditions, is the only satisfactory means of testing the system under these circumstances.