UNSATURATED HYDRAULIC CONDUCTIVITY MEASURED BY TIME-DOMAIN REFLECTOMETRY UNDER A RAINFALL SIMULATOR

We used time domain reflectometry (TDR) probes installed vertically at the soil surface beneath a constant-rate rainfall simulator to measur e cumulative water storage and the soil's unsaturated hydraulic conduc tivity. The slope from linear regression of water storage on time befo re any applied water infiltrates to the bottom of the TDR probe gives an estimate of the local infiltration rate. Local filtration rates mea sured by TDR in the field were plotted against the corresponding local steady state water contents to give an estimate of the soil's unsatur ated hydraulic conductivity over a range in water content of 20% using only two applied rainfall rates. The spatial variability in local inf iltration rates may be the result of infrequent high-intensity pulses of rainfall leading to temporary pending and redistribution of water a t the soil surface. Nonlinear optimization was used to estimate the sa turated hydraulic conductivity and inverse capillary length scale from TDR data.