The effect of forest conversion from coniferous to broadleaved forests on deep seepage rates in North-west Germany

Forests affect the water balance and thus deep seepage rates differently depending on species composition. Here, the effect of forest conversion on deep seepage rates is investigated in pure and mixed forests of pine (Pinus sylvestris), beech (Fagus sylvatica) and Douglas fir (Pseudotsuga menziesii) at three groundwater-distant and sandy forest sites in Northwest Germany. Based on continuous meteorological, stand precipitation and soil moisture measurements between January 2019 and October 2020, deep seepage rates are modelled with the hydrological model LWF-Brook90. Compared to pure pine forests, modelled deep seepage rates do not increase in mixed forests, but strongly increase in pure beech forests. The results further indicate that tree thinning increases deep seepage rates significantly. These modelled deep seepage rates serve as a basis for an economic evaluation of groundwater recharge as an ecosystem service to promote groundwater-emphasized forest conversion.