Impact of uncertainties in modelling soil water on recharge estimates

Quantifying the evaporative losses and the changes in soil water content to an acceptable accuracy is a critical component of estimating groundwater recharge using a water balance approach. In particular, the soil water store plays an important role in moderating the evaporative losses during periods of low rainfall coinciding with high evaporative demand, with a consequent impact on the timing and quantity of recharge. Therefore, we have focussed on investigating the uncertainties arising from using different soil water models. There are three sources of uncertainty in the soil water model, i.e. the FAO56 soil water model, that will be used by the Environment Agency: • The model structure; • The model parameters; • The driving variables. Uncertainties in the model structure have been investigated by comparing the output of the FAO56 soil water model with those from other models. Comparing the simulated model output with the historical observations has allowed us to assess the uncertainties due to the parameter estimation. The uncertainties in the driving variables have not been assessed in this study.