ABSTRACT
Two performance variables related to on-farm water use and costs are used in this chapter to characterize smallholder irrigation. The first is a standard variable used to describe irrigation water use by relating crop water demand (Id) to the actual amount of water supplied to the field (Js) (Burman et al., 1983). The resultant demand-supply ratio is given as
D/S = y (1) ■*-s
where Id is the estimated crop water demand at the field level (m3) and Is is the observed irrigation supply (m3). A minimum D /S value of 0.6 (or an “irrigation efficiency” of 60%) is included in the design of most surface irrigation systems to accommodate crop water needs and antici pated losses. Values below this limit would normally be considered unacceptable, indicating that significant overirrigation is occurring and excess water is being lost below the crop root zone. D /S values in excess of 1.0 may indicate that maximum potential crop yields are not being attained. However, this may often be a deliberate choice of smallholder farmers when they perceive the cost of additional water to be in excess of the benefits from the anticipated maximum potential yields.
