THE EFFECTS OF FIRE ON THE SOIL IN A DEGRADED SEMIARID WOODLAND .2. SUSCEPTIBILITY OF THE SOIL TO EROSION BY SHALLOW RAIN-IMPACTED FLOW

Small (500 by 250 by 100 mm) soil monoliths collected from experimental plots subjected to various fire frequencies in a semi-arid woodland near Coolabah, N.S.W., were tested for theirsusceptibility to erosion by rain-impacted flows under laboratory conditions. In the tests, the erosive stress applied to the soil surfaces was controlled by using drops of uniform size(2-7 mm) falling 11 - 2 m onto flows whose depths were controlled between 1 and 2-7 dropdiameters. Experimental data and thin-section observations showed that cryptogams providea high degree of protection against erosion. Sediment concentration (measured in g L_1) fromcryptogam-covered plots was considerably less than that from both bare-crusted surfacesand surfaces dominated by depositional material. Surfaces with cryptogamic crusts weregenerally free of debris, but showed only minor signs of erosion, whereas bare-crusted anddepositionary surfaces suffered pronounced lowering and tended to disintegrate. Becauseincreasing fire frequency reduces the spatial distribution of the cryptogamic cover on thesoil surface in the semi-arid woodlands, increasing fire frequency must increase the erosionhazard. © 1990 CSIRO. All rights reserved.