CHANGES IN EVAPOTRANSPIRATION FROM AN OIL PALM STAND (ELAEIS-GUINEENSIS JACQ) EXPOSED TO SEASONAL SOIL-WATER DEFICITS(1)

Evapotranspiration (E) and interception losses (I(n)) from a stand of oil palm trees (Elaeis guineensis Jacq.) were measured weekly from 198 8 to 1990 using a water balance method. Stomatal conductance, leaf are a index and climatological parameters were measured and used to calcul ate the rate of transpiration (T) from canopy of oil palm stand using the Penman-Monteith equation. At high values of soil water storage in the root zone the evapotranspiration rate of the stand was 81 % of the potential evaporation (E(p), Penman, 1948; modified by Van Bavel, 196 6), the ratio of the transpiration rate to the potential evaporation ( T/E(p)) varied from 0.69 to 0.72 and the stomatal conductance (g) vari ed from 6.0 to 6.6 mm s-1. During the 1988 dry season, which was the m ore pronounced one, E/E(p) and T/E(p) were reduced to respectively 0.5 6 and 0.35. Net interception during rainy and dry seasons were respect ively 11 % and 5 % of the potential evaporation. During the three dry seasons observed, the fraction of extractable water in the rooted zone of the soil (5.2 m deep) remained above 0.4 as a consequence of early stomatal closure which occured when fractional extractable water of t he top 80 cm of soil (f80) decreased below 0.67. When f80 decreased be low 0.35, E/E(p) decreased sharply to a value of 0.1 corresponding to a stomatal conductance of about 1.5 mm s-1.