Quasi-steady infiltration rates of highly permeable tropical moist savannah soils in relation to landuse and pore size distribution

Abstract

A knowledge of the physical properties influencing the quasi-steady infiltration rates (Ic) of soils is useful in the hydrological modelling of the infiltration process. In this study, the Ic of 18 highly permeable soils in the derived savannah zone of south-eastern Nigeria were characterized and related to land use, bulk density and pore size distribution. Unmulched soils with or without conventional tillage and soils in continuous pasture/grass cover had relatively slow to moderate Ic, whereas mulched soils, soils amended with plant residues or under secondary forests/legume cover had rapid to very rapid Ic. The saturated porosity and void ratio gave low correlation with Ic with correlation coefficients (r) of 0.388 and 0.217, respectively. Mesoporosity (i.e. pores with equivalent radius of 1.5–15 μm) and microporosity (i.e. pores with equivalent radius of 0.1–1.5 μm) had negative influence on Ic, with respective ’r’ values of −0.566 and −0.404. Preferential porosity (Pe) (i.e. pores with equivalent radius > 15 μm) and dry bulk density (ϱ_b) were the most important soil physical properties influencing Ic. Their correlation coefficients (r) with Ic were respectively, 0.852 and −0.806 (p = 0.001). This shows that soil management systems which increase the bulk density due to compaction with concomitant reduction in the proportion of the preferential pores will reduce Ic substantially on these soils. The Philip (1957) model, $\text{Ic}\text{=}\text{A + 1}\text{2St}{}^{\mathrm{<mtext>-1</mtext><mtext>2</mtext>}}$ (where A and S are the fitting parameters), could not predict the measured quasi-steady infiltration rates very satisfactorily.