Runoff and soil and nutrient losses from an improved pasture at Ginninderra, Southern Tablelands, New South Wales

Abstract

Plot and catchment measurements of runoff and of soil and nutrient losses were carried out on a moderately to heavily grazed (12-30 sheep/ha) phalaris-subterranean clover pasture at Ginninderra. The effects of an intense summer storm were examined by applying artificial rains of 20 mm/ 15 min on 5.6 m2 runoff plots in 1964. The pasture topsoils had high infiltration capacities (50-75 mm/h), and when dry could absorb more than 40 mm of water. Surface runoff and soil loss were inversely related to cover (as pasture, detached litter, and sheep dung). Cover values less than about 70% were associated with some large increases in runoff and soil loss, whereas at higher cover values there was relatively little reduction in runoff and soil loss. Most soil losses were small (< 5 g m-2) when runoffs were less than about 15 %, but increased rapidly with increasing runoff. Acceptable conditions of ground cover were mostly maintained on the improved pasture but their potential for soil and nutrient losses was greater than on native pasture. Pasture renovation substantially reduced surface runoff. The effects of natural rains on runoff and soil and nutrient losses were measured from 1966 to 1971 on a 88 ha experimental catchment. The amount of runoff varied with the amount and season of rainfall, from less than 1 mm (0.2% of rainfall) in a dry year to 51 mm (7%) in a wet year, with an average of 29 mm (4%) per year. Most runoffs in autumn and summer were relatively small, reflecting the high infiltration capacity of the surface soil when topsoil moisture storage was available. In spring and winter, when the topsoil was mostly wet, runoffs were greater, reflecting the much lower infiltration capacities (c. 5 mm/h) of the subsoil. Soil losses were related to the runoffs. They ranged from 4 kg to 376 kg/ha/year, with an average of 179 kg. Most of the soil was in fine suspension, little as bed load or coarse floating debris. Losses of nitrogen, phophorus, potassium and sulfur were at average rates of 0.62, 0.12, 1.93 and 0.06 kg/ha/year. The present rate of soil loss is less than the estimated past rate of topsoil development, and the rates of loss of nitrogen, phosphorus and sulfur are less than the inputs from fertilizers and legumes. With management to retain adequate ground cover, the Ginninderra pasture effectively controls runoff and soil and nutrient losses, and could be used in similar environments as a standard for soil and water conservation, as well as livestock production.