MODELING PLANT AND SOIL-WATER DYNAMICS IN SEMIARID ECOSYSTEMS WITH LIMITED SITE DATA

The processes and mechanisms involved in the soil moisture balance of semi-arid ecosystems are discussed. A model of water dynamics (WATDYN) is developed for estimating plant available soil moisture (PAM) on pa rticular sites. Potentially the model could be applied anywhere in the savanna regions of the world, using a minimum of readily available si te data and standard weather records. It calculates water distribution in the soil profile and models water uptake using a modified Penman-M onteith equation involving separate calculations of soil evaporation a nd transpirational loss from each soil layer. Default data are provide d where site data are unavailable. Vegetation is modelled only to acco unt for radiation interception and for resistances to transpiration an d evaporation. Growth is calculated on the basis of transpiration effi ciency (adjusted for temperature, vapour pressure deficit, soil fertil ity and the time of the year) and death as a function of temperature, vapour pressure deficit, water stress and the amount of green biomass. Dead biomass is reduced as a function of soil moisture, temperature a nd the amount of litter. Maximum leaf biomass of woody plants is consi dered to be constant from year to year.