Watershed groundwater balance estimation using streamflow recession analysis and baseflow separation

By the analysis of the observed time series of streamflow from catchments, the main components of the underlying groundwater balance, namely, discharge, evapotranspiration loss, storage and recharge, can be identified and quantified. This holistic (as opposed to reductionist) estimation method is demonstrated for the Harris River catchment in southwest Western Australia. The relationship between the groundwater discharge and the reservoir storage of shallow unconfined aquifers was found to be nonlinear based on the analysis of numerous streamflow recession curves. However, depletion of groundwater by evapotranspiration losses, through the water uptake of tree roots, was found to bias the recession curves and the estimated reservoir parameters. As a result of the seasonality of both rainfall and potential evaporation, analysis of the recession curves, stratified according to time of the year, allowed the quantification of evapotranspiration loss as a function of calendar month and stored groundwater storage. Time series of recharge to the groundwater aquifer were computed from the observed total streamflows, and the estimated discharge and evapotranspiration losses, by inverse nonlinear reservoir routing. Using traditional unit hydrograph methods unit recharge responses to rainfall were computed by least squares fitting. The shapes of the estimated unit response functions showed no significant seasonal variation.