ROLE OF PLANT COVER AND STOCK TRAMPLING ON RUNOFF AND SOIL-EROSION FROM SEMIARID WOODED RANGELANDS

Relationships between plant cover, runoff and erosion of a massive red earth were investigated for a runoff zone of an intergrove area in a semi-arid wooded rangeland in eastern Australia. The measurements were carried out in small experimental paddocks with different stocking ra tes of sheep and kangaroos. A trailer-mounted rainfall simulator was u sed to apply rainfall at a time averaged rate of 30 mm h-1 to obtain r unoff rates and sediment concentrations. There was a significant negat ive relationship (r2 = 0.58; P < 0.01) between final runoff rate and p lant cover. It is probable that the plants increase infiltration and d ecrease runoff by (i) funnelling water down their stems and (ii) provi ding macropores at the base of the plant through which water can rapid ly enter the soil. However, there was no significant effect of plant c over on sediment concentration. Probable reasons for this are: (i) eve n though plant cover will absorb raindrop energy and decrease the eros ive stress on the soil, the nature of the plants investigated is such that they may not be 100% effective in protecting the soil beneath the m, and (ii) the distribution of contact cover provided by the base of the plants is highly patchy and thus relatively inefficient at reducin g sediment concentration. At zero cover final runoff rates from paddoc ks with a high and low stocking rate were similar, i.e. 23.4 and 22.3 mm h-1 respectively. However, at zero cover, the sediment concentratio n from the high stocking rate paddock was significantly (P < 0.01) gre ater than that from the low stocking rate paddock. Greater hoof activi ty and lower organic matter (and hence lower structural stability) of the 0.20 mm layer in the high stocking rate paddock caused the soil su rface to be more susceptible to erosion. These results show that grazi ng by removing perennial grasses and pulverizing the surface soil can have a major impact on local water balances and erosion rates respecti vely within the intergrove areas. The implications of these results fo r the long-term stability of semi-arid mulga woodlands is briefly disc ussed.