Modelling plant and soil water dynamics in semi-arid 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 particular sites. Potentially the model could be applied anywhere in the savanna regions of the world, using a minimum of readily available site data and standard weather records. It calculates water distribution in the soil profile and models water uptake using a modified Penman-Monteith equation involving separate calculations of soil evaporation and transpirational loss from each soil layer. Default data are provided where site data are unavailable. Vegetation is modelled only to account for radiation interception and for resistances to transpiration and evaporation. Growth is calculated on the basis of transpiration efficiency (adjusted for temperature, vapour pressure deficit, soil fertility 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 and the amount of litter. Maximum leaf biomass of woody plants is considered to be constant from year to year.