LIGHT INTERCEPTION AND EVAPOTRANSPIRATION IN HEDGEROW AGROFORESTRY SYSTEMS

Quantifying water lost through evaporation and transpiration in a crop ping system is an important tool in adapting a system for semi-arid co nditions. During two cropping seasons in eastern Kenya, light intercep tion and soil water content were measured in several different croppin g systems: monocultures of cowpea (Vigna unguiculata (L.) Walp,), maiz e (Zea mays cv. Katumani), Senna spectabilis cv. Embu managed as a hed ge, and hedge intercrops of cowpea and maize. These systems differed w ith respect to plant population density, maximum light interception (4 4-75%) and canopy height (0.5-2.0 m). Parameters in the EPIC model for leaf area development were derived from periodically-measured light i nterception in the different systems. Daily light interception predict ed from the leaf area was used to partition potential evapotranspirati on into potential evaporation and potential transpiration. A simple wa ter balance model was used to predict actual transpiration and actual soil evaporation. Predicted values of water loss during the two season s correlated closely with measured values (r(2) = 0.85 and 0.91; slope = 1.00 and 1.01). In both seasons, the model predicted that soil evap oration comprised approximately half (42-58%) of the estimated evapotr anspiration. This study suggests that evapotranspiration can be predic ted for a variety of cropping systems when light interception measurem ents are used in conjunction with a simple model of plant water uptake . It also demonstrates the difficulty of maximizing plant water use in agroforestry systems in semi-arid environments when the canopies of b oth annual crops and hedges develop simultaneously.