QUASI-STEADY INFILTRATION RATES OF HIGHLY PERMEABLE TROPICAL MOIST SAVANNA SOILS IN RELATION TO LANDUSE AND PORE-SIZE DISTRIBUTION

A knowledge of the physical properties influencing the quasi-steady in filtration rates (Ic) of soils is useful in the hydrological modelling of the infiltration process. In this study, the Ic of 18 highly perme able soils in the derived savannah zone of south-eastern Nigeria were characterized and related to land use, bulk density and pore size dist ribution. Unmulched soils with or without conventional tillage and soi ls in continuous pasture/grass cover had relatively slow to moderate I c, whereas mulched soils, soils amended with plant residues or under s econdary forests/legume cover had rapid to very rapid Ic. The saturate d porosity and void ratio gave low correlation with Ic with correlatio n coefficients (r) of 0.388 and 0.217, respectively. Mesoporosity (i.e . pores with equivalent radius of 1.5-15 mu m) and microporosity (i.e. pores with equivalent radius of 0.1-1.5 mu m) had negative influence on Ic, with respective 'r' values of -0.566 and -0.404. Preferential p orosity (Pe) (i.e. pores with equivalent radius > 15 mu m) and dry bul k density (rho(b)) were the most important soil physical properties in fluencing Ic. Their correlation coefficients (r) with Ic were respecti vely, 0.852 and -0.806 (p = 0.001). This shows that soil management sy stems which increase the bulk density due to compaction with concomita nt reduction in the proportion of the preferential pores will reduce I c substantially on these soils. The Philip (1957) model, Ic = A + 1/2 St(-1/2) (where A and S are the fitting parameters), could not predict the measured quasi-steady infiltration rates very satisfactorily.