Calcul des indices d'humidité des sols dans le bassin versant du Gardon d'Anduze à partir des données météorologiques

Calculation of soil moisture indexes in the Gardon d'Anduze catchment area from meteorological data

Ingénieur en Chef des Ponts-et-Chaussées Ex-Directeur Adjoint de l'Agence de Bassin Rhône Méditerranée Corse

Abstract

The object of this study was to attempt to calculate soil moisture indexes from observed rainfall and evaporation data. The considered data had been collected by M. Hofer over a period of four years (1971-1974) for a flood-forecasting model of the Gardon d'Anduze catchment area in the Gard Department of France. During this period, the soil moisture index was measured once weekly at various points of the catchment area by taking core samples from the soil surface. Rainfall and evaporation observations provided daily data which were used to calculate daily moisture indexes, each one of which was taken as basic data for calculating the next index from observed rainfall and evaporation over a period of 24 hours. The following daily factors are taken into account in calculating the daily index-correction factor : - An increase in moisture expressed as the percentage of rainfall actually stored in the top 20 cm soil layer, varying from 0 for satured soil to 0.9 for dry soil, based on a peak daily rainfall figure of 30 mm. - A decrease in moisture expressed as a percentage of measured evaporation, varying from 0 for dry soil to 0.7 and 0.95 for saturated soil in summer and winter respectively. Comparison of the calculated and measured soil moisture indexes confirms the validity of the calculation for a general approach. Variations of mean deviation over a year are as follows : - 0.2 to - 2.5 index points for one field with extreme indexes of 8.5 and 39. - 1.3 to + 3.5 index points for a second field with extreme indexes of 15 and 40. - 0.7 to + 0.3 index points for a third field with extreme indexes of 6 and 20. Mean deviations on a quarterly scale for the above fields do not exceed - 5.3, + 4.7 and - 1.4 index points respectively.