RETRIEVING THE ROOT-ZONE SOIL-MOISTURE FROM SURFACE SOIL-MOISTURE OR TEMPERATURE ESTIMATES - A FEASIBILITY STUDY BASED ON FIELD-MEASUREMENTS

The bulk soil water content must be estimated accurately for short-and medium-term meteorological modeling. A method is proposed to retrieve the total soil moisture content as well as the field capacity from ob served surface parameters such as surface soil moisture or surface tem perature. A continuous series of micrometeorological and soil water co ntent measurements was obtained in southwestern France over a fallow s ite in 1995. In addition, the database includes measurements of the su rface temperature and soil moisture profiles within the top 5-cm soil layer. The surface soil moisture measurements are available twice a da y during two 30-day intensive observing periods in spring and autumn 1 995. Once calibrated, the ISBA (Interactions between Soil, Biosphere, and Atmosphere) surface scheme is able to properly simulate the measur ed surface variables and the bulk soil moisture. Then an assimilation technique is applied to analyze the field capacity and the total soil water content from the surface data. In particular, it is shown that k nowing the atmospheric forcing and the precipitation, four or five est imations of the surface soil moisture spread our over a 15-day period are enough to retrieve the total soil water content by inverting ISBA. The use of the surface temperature seems more problematic because its sensitivity to the value of the total water content is meaningful in relatively dry conditions only.