2 SOURCE MODEL OF SURFACE FLUXES FOR MILLET FIELDS IN NIGER

Fluxes from land surfaces with partial vegetative cover are being esti mated with two source (soil and leaves) models. A network of aerodynam ic resistances linking the two sources allows interaction of these flu xes after incident solar energy is partitioned between them using leaf area index. Many of these interactive two source models use a simple soil resistance approach to estimating fluxes from the soil which is r arely validated by field measurement. In this study a detailed model o f soil water and heat fluxes is combined with a two source surface flu x model to form the SWEAT model, which avoids use of the soil resistan ce approach. Output from the SWEAT model is compared with measurements of evaporation from soil, transpiration, soil temperature and soil wa ter content made within a sparse millet field in Niger during the rain y season of 1993. Simulated and measured values of daily evaporation f rom soil are not significantly different. Simulated maximum soil tempe ratures at 0.1 m underestimate measurements by approximately 2.5 K. Wi th a few exceptions, simulated soil water content is within one standa rd deviation of the mean measured value. The other field data provide further confirmation of the accuracy of simulation using SWEAT. Using the SWEAT model it was estimated that the greatest increase of transpi ration (MJ) above the incident net radiation partitioned to the canopy , would occur at a leaf area index between 1.0 and 1.5. This increase in transpiration can be attributed to sensible heat fluxes from the so il underlying the canopy being absorbed by the canopy and dissipated a s latent heat. To test the soil resistance approach, soil resistance w as defined as a linear function of time since rain. It was shown that this linear resistance model for estimating fluxes from the soil may b e adequate in a two source model when simulating fluxes from a land su rface with a leaf area index of 2.0 or higher, but it is unlikely to b e adequate for lower leaf area indices.