RELATIONSHIP BETWEEN SOIL WATER DIFFUSIVITY AND EVAPORATION: A STUDY UNDER DIFFERENT AGRICULTURAL LAND USE SYSTEMS

Evaporation from soil surface is a significant process of water loss in the field. Soil evaporation studies were conducted in the laboratory on finite soil columns from three different fields, namely, arable, oil palm and cocoa at the Plantations section of the Department of Crops and Soil Sciences, Kwame Nkrumah University of Science and Technology, Kumasi. Soils were randomly taken from three different spots at depths of 0-5 and 5-10 cm. The soil cores were saturated for 24 hours and subjected to evaporation under an incandescent bulb and an electric fan. Evaporation measurements were made at 30 minutes interval for 3 hours. Cumulative evaporation (E) was plotted against time and the curve obtained indicated that the evaporation occurred at constant atmospheric evaporativity. The plot of E against the square root of time (√t) at the falling rate stage showed a linear relationship which agreed with the existing theory. Soil Water Diffusivity (D) was calculated from the slope of E against√t, termed as desorptivity (Sd). The highest for D were observed in the cocoa field with parameter values of 0.037-0.040 cm2s-1 for the 0-5 and 5-10 cm depths, respectively. Parameter values of 0.030-0.035 and 0.032-0.035 were obtained for the respective soil depths for the arable and oil palm fields, respectively. The D values were found to be positively correlated with sand, silt and organic carbon and negatively correlated with clay. The results showed that D increased with increase in sand and organic matter contents and decreased with increase in clay content. Additionally, diffusivity was higher within the 5-10 cm depth than the 0-5 cm depth in both fields.