Soil-vegetation-atmosphere relationship: A model for estimating evapotranspiration of different land covers in the Ivory Coast (West Africa)

A key factor in defining agrometeorological parameters is to be able to simulate efficiently soil evaporation and plant transpiration. These two terms represent the main part of the water balance; this information is of great importance in developing countries where agriculture plays a significant role in the economy.

In this study, an agrometeorological model is used to compute daily evapotranspiration for a variety of land covers within a savannah area in Ivory Coast (West Africa). Over a one-year period (1998) agrometeorological data were collected on Khan basin (6 000 km2) in the centre of the country. The model, CoupModel (Jansson et al., 2000), describes the water balance for each land cover. It uses the Lohammar implementation (Lindroth, 1985) of Penman-Monteith equation to simulate soil evaporation and plant transpiration. The results of the model provide time series of data on a complete one-year period.

The results show that the runoff by drainage is very limited (< 5% of precipitation) in such a savannah area. Annual evapotranspiration corresponds to about 97% of the precipitation. The importance of the evapotranspiration depends on the state of surface wetness following frequent rainfall events. In rainy season (april to july) surface resistances are small (50-100 sm-1) and daily evapotranspiration occurs nearly to the potential rate. But in the Harmattan, the so called dry season, the level of water loss by plants and soil contribute slightly to provide atmosphere with water.

The advantages of using such a model as compared to deriving evapotranspiration from short time measurement or satellite data are that the model obtains results for each day of the year as opposed to occasional snapshots of remote sensing or short period of measurements.