Groundwater recharge to a hydrogeologist is equivalent to deep drainage to a soil scientist. They should be equivalent, but in many circumstances the predictions differ greatly, leading to uncertainty in system understanding and potentially unreliable conceptual and numerical models. This paper explores some of the reasons these discrepancies exist, and suggests approaches for practitioners and water resource managers to achieve alignment despite the different approaches to analyse a system.
Groundwater recharge is a key parameter that affects groundwater behaviour. Hydrogeologists commonly estimate recharge by analysing groundwater level data with rainfall data using soil-water balance methods. Soil scientists typically focus on characterising soil-water behaviour in the near surface zone, often with multi-layer soil-water balance models.
Discrepancies generated when relying on deep drainage estimates derived from lumped parameter models are often apparent when conducting impact assessment of irrigation schemes. Often these projects do not have the resources to undertake detailed soil and groundwater modelling and calibration, and therefore judgment-based decisions and interpretation are required.
There are some simple checks of model outputs that can be performed (but rarely are) to verify model predictions are reasonable. For example, deep drainage estimates could be compared to recharge estimates made using calibrated groundwater models which simulate groundwater level fluctuations (e.g. from government groundwater models).
There is no single method that will produce good estimates of recharge in all cases. However, estimating recharge using an integrated approach by linking groundwater level fluctuations to recharge would reduce uncertainty in the assessment of the sustainability of irrigation schemes.
It is important that water resource managers are aware of the different approaches used in system conceptualisation and modelling when assessing and interpreting model predictions so that the soil-water regime of the system is well represented and any outcome based decision making can be done with greater confidence.