Heat and water exchanges of fallow land covered with a plant-residue mulch layer: a modelling study using the three year MUREX data set

The MUREX (Monitoring the Usable Soil Reservoir Experimentally) experiment was conducted on fallow land in the Southwest of France. A three year continuous data set, including climatic variables, energy fluxes, surface water content, soil moisture profiles, surface and soil temperature, and evolution of vegetation characteristics was collected. The field possessed a plant-residue mulch layer, formed naturally by the accumulation of decaying and dead biomass. The three-year data set was used to analyse and model the long-term water and heat exchanges of the field using the SiSPAT (Simple Soil Plant Atmosphere Transfer) model. The original version was modified to take into account heat and water transfer within the plant-residue mulch layer. The 1995 data set was used for calibration of unmeasured parameters. Years 1996 and 1997 were used for validation of the approach, using the same parameter set obtained in 1995. Model results and observations were in good agreement for the three years when the plant-residue (mulch) layer effect was considered. The model properly reproduced contrasting responses to different rainfall conditions. Model simulations were used to understand some physical processes modified by the mulch layer. A decrease of 5–10% of annual total evaporation was obtained, as compared to the residue free case, associated with a decrease in soil evaporation and increase of transpiration. The decrease in soil evaporation was responsible for higher surface soil moisture. Daily soil and air temperature profiles were shown to be considerably modified by the mulch layer, an inversion occurring within the mulch, leading to colder averages and a smaller amplitude for soil temperature.