Hydrological modelling of changes in the water balance due to the impact of woody biomass production in the North German Plain

Several studies have implied that cultivation of willow and poplar short-rotation coppices influence the area’s water balance. Due to the high density of sites suitable for SRCs in the Northern German Plain, this study focusses on four different model areas representative of the climatic, soil, and morphological heterogeneity of this landscape. The river basins selected for the study are the Ems, Treene, Aland, and Uecker–Randow–Welse basins. The water balance modelling was performed with the Soil Water Assessment Tool (SWAT) and automatically calibrated with SWAT-CUP. The implemented scenarios were set to 10 % SRC cover on suitable sites, based on the area’s predicted need to meet domestic demands for woody biomass by 2020. Additionally, an extreme scenario of 100 % cultivation on all suitable sites was implemented to determine the maximum effect of SRC on the regional water balance as well as to allow for a direct comparison with annual crops, pasture, and deciduous forest. For parametrization, field measurements were used to characterize the key physiological parameters for willow and poplar in SWAT. The results for the 10 % SRC scenarios did not show a substantial impact on the investigated water balance components at the water basin level. But at the local level, the effects of conversion to SRC are more pronounced. In general, actual evapotranspiration of SRC is 16 % higher compared to annual crops and average groundwater recharge decreases by 48 %. Also, available water capacity in the soil increases by 26 % for SRCs in comparison with annual crops.