Étude des composantes du bilan hydrique d'un sol par télédétection

Study of soil water balance components by teledetection

Ingénieurs EPF-L

Abstract

Various agrometeorological data are required for determining the water balance of soil in a given area, especially precipitation, evaporation and infiltration data. Teledetection can provide an answer for point measurement spatialization problems if the required parameters can be extracted from the teledetection data. Soil water balance is given by formula (1). Precipitation cannot yet be determined by teledetection. Actual consumptive use can be found from the energy balance (formulae (2) and (3)) ; net radiation can be determined by teledetection (formula (5) and Fig. 1) ; the convective energy term R_c has to be found by other methods. Soil water storage variation can be determined by microwave teledetection, though only down to a limited depth (no deeper than 20 - 50 cm). French rural engineers of the "Institut de Génie Rural" have completed a programme of field measurements of soil moisture by hyperfrequency radiometry (e.g. Figs. 3 and 4). The Burke and Paris radiative emission model has been tried and found to systematically give radiance temperatures below the measured values, probably owing to the use of dielectric constants determined in the laboratory for various typical soils and underestimation of reflected celestial radiance. A hybrid model simulating the behaviour of the thermal and water profile of the ground is proposed (Fig. 5) as a means of "filling in the gaps" in teledetection and conventional measurement data. Its special features are as follows : In-situ measurement data are used for reference purposes and to calibrate the model. Profiles at time t are calculated from weather data. Occasional teledetection data are used for periodic correction of the calculated profiles. This simulation is applied locally to a certain number of points in the considered area. The required parameters (e.g. soil moisture) are extended in space on the basis of the teledetection data, the relationship between radiometer measurements and soil data being established on the "simulation points".