AVAILABLE WATER OF A FERRALLITIC SOIL ALONG THE KOSSOU LAKESIDE SLOPE (IVORY-COAST) AS AFFECTED BY SOIL TEXTURE

The neutron probe method for in situ measurement of field capacity was applied to ferrallitic soils on the Kossou Lakeside slope (Ivory coast). For better utilisation of the natural resources, a study was conducted to determine the influence of the lake on the availability of soil water to plants along the lakeside slope. Two methods, one gravimetric and the other neutronic, were used to evaluate soil water content and its variation with time. Five access tubes were placed along the slope and were monitored during the study. The infiltration rate determined at three of the measuring sites was higher in the top soil layers than in the deeper soil layers. Drainage and redistribution data show a juxtaposition of two infiltration kinetics, the first being exponential and the second linear. The, extrapolation of the second kinetic (slow) to the start of drainage gave, for the first 600 mm of soil, field capacities of 154, 184 and 163 mm for sites T1, T3 and T5 respectively. These differences were specifically related to the porosity of the sites under study. An analysis of the soil water profiles showed that the soil moisture near the lake was always higher than the permanent wilting point. The influence of the lake was not apparent during the long rainy season because of the steady decline of the water table. The water availability then depended upon the rainfall distribution and the specific characteristics of each of the sites, especially soil texture and structure. A linear relationship was found between soil moisture estimated by the neutronic and the gravimetric methods with correlation coefficients ranging from 0. 80 to 0. 96. The data also showed that the neutron probe method did not systematically underestimate the soil moisture of the top layers. This response seemed to be associated with the fraction of coarse sand. As soil moisture increased, data from the neutron probe method converged toward the gravimetric data regardless of the site.