Spatial variability of soil water properties in tropical soils of nigeria1

Soil water management requires a reliable estimate of soil water properties in order to predict and control the dynamics of water in a field soil. This estimate should take cognizance of the spatial variability of the soil water properties in the field. The internal drainage technique, which employs a neutron moisture meter and tensiometers, was used to evaluate the spatial variability of soil water properties of a 0.34 and 91.6-ha field. Five infiltration plots were established on each field. The precision with which the soil hydraulic conductivity K, as a function of moisture content α, could be obtained for a single soil type in the field was also determined. High variability in soil physical properties, such as percent gravel, sand, silt, and clay, and bulk density, resulted in a high variability in soil water retention characteristics and hydraulic conductivity. The coefficients of variability of these soil properties were only slightly higher for the 91.6-ha farm area than for the 0.34-ha area, consisting of one soil type on a nearly level topography. Although the spatial variability of soil water content was relatively low at any time and any depth after cessation of infiltration, ranging from 2.48 to 12.06 percent, the hydraulic conductivity values were far more divergent, having coefficients ranging from 47.6 to 146.9 percent. Steady-state moisture content and hydraulic conductivity were normally and log-normally distributed, respectively. Agreement between the plots of K versus α for the two drainage periods in 1975 and 1976 were generally satisfactory. The magnitude of soil water flux at any depth was predicted with some success, using a simple prediction equation that requires a knowledge of saturated hydraulic conductivity and the slope of the K(α) curve near saturation. The result was not expected, in view of the large coefficients of variability of soil water flux among the five plots on a soil.