VALIDATING MODELLED DEEP DRAINAGE ESTIMATES FOR THE QUEENSLAND MURRAY-DARLING BASIN

Accelerated rates of drainage below the root zone can lead to rising water tables, which may in turn lead to land and stream salinity. Estimating rates of deep drainage is an important part of predicting salinity risk. Water balance modelling is widely used to estimate deep drainage. However, model outputs of deep drainage have rarely been validated in the past because of a lack of data. Estimates of deep drainage are now available for the Queensland Murray Darling Basin from several sources. This paper presents preliminary validation of modelled deep drainage against these data. Two daily water balance models, GRASP and PERFECT, were used for different soils and land uses, using ‘default ’ parameter values. Relationships of modelled water excess (runoff plus drainage) and average annual rainfall were similar to published relationships (Zhang et al. 2001), although modelled estimates give much greater differentiation between soils. Modelled drainage rates were of a similar order of magnitude to estimates determined from a transient chloride mass balance approach for native vegetation and cropping sites, and for a detailed catchment study. This improves confidence in drainage modelling, which is used to assess salinity risk in areas where measurement is impractical