Cover-runoff equations to improve simulation of runoff in pasture growth models

Abstract

An Australian native pasture growth–water balance model (GRASP) was modified to include the USDA curve number runoff method from PERFECT, with the aim of providing a more general model than the regionally derived Scanlan runoff approach used previously and to improve runoff and water balance prediction. The modified model was calibrated against measured runoff and soil water data for a range of pasture treatments in Central Queensland. Optimised curve numbers were related to cover; curve number reduced from 97 for bare conditions to a minimum of 57 for 53% cover, with no difference between soils (mudstone or sandstone derived, or eroded phases) or types of cover (grass biomass and litter, or tree litter). In the modified model this equation provides feedback of effects of grazing management and tree density on cover to the water balance.

Runoff prediction using the Scanlan runoff sub-model (derived for soils in the Burdekin catchment) was satisfactory (r²_1:1 = 0.70), indicating that soils at the Central Queensland site had similar runoff responses to those in the Burdekin. Addition of the curve number sub-model improved runoff prediction (r²_1:1 = 0.87). Adjusting curve number each day according to cover improved runoff predictions for low cover situations where cover varied through time, which is important in degraded pastures. Total soil water prediction was very good using the Scanlan runoff sub-model (r²_1:1 = 0.88) and slightly improved using the modified model (r²_1:1 = 0.91). Soil water in the surface layer was predicted well, giving confidence in prediction of soil evaporation.

This study provides important new runoff parameter values for modelling water balance and degradation of hard setting soils under pasture. The parameters derived from this study can be used with historical climate data to provide a long-term assessment of effects of grazing management on runoff on such soils.