SPATIAL VARIABILITY OF INFILTRATION AND RELATED PROPERTIES OF TROPICAL SOILS

The levels of variance associated with measuring the infiltration proc ess and modelling it by means of a regression model are compared to se e which approach yields the best results in terms of effort and accura cy. A nested sampling scheme has been used in the three major physiogr aphic units of central Guyana, South America: 'White Sands' (Haplic an d Ferralic Arenosols), 'Brown Sands' (Haplic Ferrasols) and 'Laterite' (Xanthic and Dystric Leptosols). Cluster analysis yields three sample groups that reflect the sharp landscape boundaries between the units. Multiple regression analysis shows that each unit has a different com bination of soil properties that explains the variance in final infilt ration rate and sorptivity satisfactorily. Nested analysis of variance indicates that clear spatial patterns with distances of variation of several hundred metres exist for final infiltration rate in White Sand s and Laterite. Infiltration rate in Brown Sands and sorptivity in all units have large short-distance variabilities and high 'noise' levels . The correlated independent variables behave accordingly. For the maj ority of the soil properties, sampling at distances of 100 to 200 m re sults in variance levels of more than 80 per cent of the total varianc e, which indicates that only a detailed investigation can assess spati al variation in soil hydrological behaviour. The use of simple soil pr operties to predict infiltration is only possible in a very general se nse and with the acceptance of high variance levels.