Using Monte Carlo simulations to estimate the surface run-off for a distributed recharge model in a catchment area in Scotland.

Recharge modelling has been undertaken using the distributed recharge model ZOODRM to estimate recharge to an alluvial aquifer in the Borders region of Scotland. The alluvium is flat lying and lies on bedrock which outcrops to form the adjacent hills. Recharge is calculated by the ZOODRM model for a water balance for use in a groundwater flow model of the alluvium. ZOODRM estimates the daily run-off as a percentage of the daily rainfall using run-off coefficients and applies a simple approach to route the run-off to rivers based on an aspect map derived from a topographical map. The comparison of simulated river flows to those observed allows the calibration of the recharge model without using the groundwater flow model. Monte Carlo simulations have been undertaken to estimate the run-off coefficient values of the two topographically different zones and to place confidence limits around these values. The run-off coefficients are either specified (Method 1) or related exponentially to the surface gradients and the antecedent soil moisture conditions (Method 2). The advantages and disadvantages of using these methods are discussed. Both methods produced simulated river flows that are in good agreement with those observed but Method 2 provided a slightly better match because run-off coefficient values were dependent on the antecedent soil moisture.