ABSTRACT
A time-based concept which provides a quantitative means for estimating the plant water stress imposed on a crop during its growing season to provide a criterion for characterising the need for irrigation is presented, based on the stress -Day Index (SDI) concept. Wheat (Triticum aestivum L.) was used as the experimental crop, at the Ahmadu Bello University Farm during the 1991/92 irrigation season. The Time-based concept was evaluated by comparing; (1) estimates of plant-based plant water deficits corresponding to specified time intervals at different crop growth stages; (2) the values of water used efficiencies from the time-based concept for irrigation scheduling with results obtained from a conventional irrigation scheduling approach; and (3) Estimating the Correlation Coefficient between seasonal crop water stress and grain yield from the time-based concept. The results indicated that the time-based form of characterising plant water deficit is less effective with regards to the definition of crop susceptibilities to water deficits as a function of the timing of occurance of the deficit. However, using the time-based concept for irrigation scheduling, an increase of 20.5 per cent in water use efficincy was obtained over that of the conventional approach to irrigation scheduling.