Modelling the water balance of a free‐draining lysimeter using the downward approach

A 2600 m 3 free‐draining lysimeter constructed in a Pinus sylvestris plantation forest in Colbitz, Germany, has been monitored daily since 1974, with the intent of improving the understanding the effect of afforestation upon groundwater recharge (drainage). The objective of this research was to employ a downward approach in the development of a conceptual water balance model applicable to the site. Observed annual and inter‐annual trends were successfully replicated by a simple capacitance model using a simplified representation of vegetation growth. An important limitation of the capacitance approach in simulating the timing of drainage at sub‐annual time scales was identified, which could not be overcome by adding complexity to the model basis. Given the a priori use of capacitance approaches for simulating drainage in many hydrological models, the findings of this study suggest that (a) these approaches are sufficient where the prediction of annual and inter‐annual drainage behaviour is the primary objective, and that (b) an alternative basis accounting for the time delay between precipitation inputs and drainage generation is required for modelling if the prediction of drainage at sub‐annual time scales is a priority.