COMPUTED EVAPOTRANSPIRATION OF ANNUAL AND PERENNIAL CROPS AT DIFFERENT TEMPORAL AND SPATIAL SCALES USING PUBLISHED PARAMETER VALUES

Land use changes from perennial to annual crops and vice versa may aff ect water balances, in particular through changing evapotranspiration rates with great implications for modelling. Direct comparison of evap otranspiration rates published in the literature is hardly possible, s ince differences in soils and climate make data incomparable. This pap er presents literature data on crop parameters and environmental condi tions that determine transpiration for four crops (oil palm, cocoa, ma ize and rice) and evaporation, Transpiration at leaf scale and soil ev aporation as well as evaporation of intercepted rainfall have been com puted using these data and are scaled-up to canopy scale. The objectiv e is to quantify evapotranspiration of the crops under identical envir onmental conditions at two temporal scales: daily and annual evapotran spiration. Variation among crops in transpiration at leaf scale owing to crop characteristics is levelled out in scaling up to canopy scale. Differences in annual evapotranspiration between perennial and annual crops mainly are due to the fact that perennial crops transpire durin g the dry season, although at low rates, but still considerably higher than evaporation rates of bare and dry soils. Apparently, the degree of soil cover with vegetation in space and in time is of major importa nce to evaluate differences in annual evapotranspiration caused by cha nges in land use.