On the validation of a coupled water and energy balance model at small catchment scales

Catchment runoff is the most widely used catchment scale measurement in mod elling studies, and we have a reasonable degree of confidence in its accura cy. The advent of satellites gives access to a new suite of measurements ta ken over a defined spatial range. These measurements, principally reflected or emitted radiation, provide hydrologists with new possibilities for quan tifying the state of a catchment. Surface temperatures can be readily measu red by a satellite on a scale comparable to the size of a small catchment. In this paper we show that satellite sensed temperatures can provide an imp ortant measure of catchment status, which can complement runoff measurement s in water balance studies. A one-dimensional model, which couples the land surface energy balance with the soil and surface water balance is tested b y comparison with runoff and with remotely sensed surface temperature measu rements. Simulations have been run over four years for two small catchments which have a fairly homogeneous vegetation, one being forest and its neigh bour pasture. Satellite "surface" temperatures have been interpreted in ter ms of the energy balance, and used as a test of modelling accuracy. An "eff ective" surface temperature is calculated as a weighted mean of temperature s of the separate soil and leaf surfaces. This modelled "effective" tempera ture correlates well with Landsat TM surface temperatures. When pasture replaces forest, the model predicts a reduction in evapotransp iration of around 30%, a three-fold increase in runoff, and an increase in mean soil moisture status. The change to pasture also results in a rise in mean effective surface temperature of about 4 degrees C, and an increase in summer diurnal temperature range from 10 to 22 degrees C. The winter diurn al temperature range is similar for both vegetation systems. Inclusion of soil moisture variability in thermal properties results in an increase in mean daily maximum temperature of about 2 degrees C in summer a nd winter, without much change in daily minima. The daily mean temperature is not significantly affected. (C) 1999 Elsevier Science B.V. All rights re served.