Abstract
Abstract. Deep drainage estimates are required for effective management of water resources. However, field measurements are time consuming and costly so simple empirical relationships are often used. Relationships developed between clay content of the surface soil and deep drainage have been used extensively in Australia to provide regional estimates of deep drainage but these relationships have been poorly justified and did not include rainfall in the relationships. Here we present a rigorous appraisal of clay content of soils and rainfall as predictors of deep drainage using an extensive database of field observations from across Australia. This study found that annual average rainfall and the average clay content of the top 2 m of the soil are statistically significant predictors of point scale deep drainage. Relationships have been defined for annual, perennial and tree type vegetation as a line of best fit along with 95% confidence intervals. This allows the uncertainty in these deep drainage estimates to be assessed for the first time.
Highlights
The capacity of effective groundwater management practices are expanded as our understanding of the driving processes, timescales and spatial distribution of groundwater recharge develop
Recharge is the flux of water that contributes to the saturated groundwater reservoir whereas deep drainage is the flux of water that moves through the unsaturated zone past the root zone
When we investigated the statistical fit more thoroughly we found that even though there was an overall significance to the multiple linear regression (MLR) for average clay content (0–1 m), (Significant F = 2.17E-12), there was no correlation between log deep drainage and clay (Pvalue = 0.837)
Summary
The capacity of effective groundwater management practices are expanded as our understanding of the driving processes, timescales and spatial distribution of groundwater recharge develop. Accurate measurement of recharge may be difficult to attain due to the small deep drainage fluxes that occur below the root zone. Groundwater recharge is associated interchangeably with deep drainage or potential recharge albeit a distinct difference defines the two terms. Recharge is the flux of water that contributes to the saturated groundwater reservoir whereas deep drainage is the flux of water that moves through the unsaturated zone past the root zone. Assuming one-dimensional flow and that deep drainage will eventually recharge the unconfined aquifer, unsaturated soil zone profiles can provide long-term mean annual recharge estimates for both steady state and transient environments (Walker, 1998). Point scale methods are used to estimate low deep drainage fluxes in a transient environment scaling up is often not practical because of the costs involved to obtain the required data. It is necessary to use a surrogate measurement that is more readily available as a proxy for deep drainage
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