Effect of vegetation states on the simulated water balance in Poplar Box (Eucalyptus populnea) lands.

Abstract

Climate, soil and vegetation data were used in a simulation of the soil water balance of different vegetation states common to regions of south-western Queensland characterized by the presence of poplar box (Eucalvptrrs populrira). Published information on water use by the four vegetation states, natwe grassland, buffel (Cetzchms czlraris) grassland, mulga (Acacla aneura) shrubland and niulga-poplar box woodland was used to derive input relationships fo~ the water balance model, AGWATMD. Good agreement was obtained between the observations from which the parameters for the model were derived and the predicted soil water availabilities over a two year period for shrubland and woodland communities. Inadequacies in the model for simulating natural communities, particularly those composed of spaced plants, are discussed in relation to the hydrologic processes affecting their water balance. A comparison of predicted long term patterns in the water balance of the four vegetation states using 82 years of climate records indicates that the highest probabilitjt of experiencing wet soils occurs in winter. Wettest conditions in winter are likely to occur in grasslands, although the expected variation in soil water content is also higher for these communities. Therefore, in years uith above-average rainfall, wet soils under grassland could favour enhanced shrub encroachment, particularly where grasslands are g-azcd. The driest soils were predicted under shrubland, indicating that shrubs encroaching on lands recently cleared of woodland will compete with, and suppress the growth of herbage species..