Analyzing the hydrological impact of afforestation and tree species in two catchments with contrasting soil properties using the spatially distributed model MIKE SHE SWET

Groundwater depletion occurs at a global scale but requires regional strategies for sustainable management of freshwater resources. In Denmark the groundwater quantity and quality is under pressure, and forested areas are considered to protect groundwater reservoirs. However, little is known on how afforestation or forest conversion impacts the water resource at the catchment scale. We hypothesize that the groundwater formation and streamflow is increased when water consuming conifers are replaced with the less consumptive broadleaf tree species. To test this a distributed hydrological model with an energy-based description of evaporation and transpiration processes (MIKE SHE SWET) was used. Large scale hydrological models were established for two geologically (sandy/clayey) contrasting catchments in Denmark; Skjern and Lejre catchments. Land use scenarios were defined with forest vegetation (conifer/broadleaf) and agricultural crops (grass, maize, wheat and barley) in different areal combinations. Initially, the SWET component was calibrated against plot scale field data from two forest sites to obtain vegetation parameter estimates for conifers and broadleaves. Subsequently, the catchment models were run for 10 years with predefined land use scenarios. MIKE SHE SWET simulated canopy interception and throughfall for conifers and broadleaf forests satisfactorily. The catchment simulations showed that replacing current conifer forests with broadleaves, resulted in a significant increase in groundwater recharge and groundwater level, especially in the Skjern catchment with predominantly sandy soils. Also, doubling the forest area, as intended by national legislations, using only broadleaves did not negatively affect the groundwater generation or minimum stream discharge compared to current conditions at Skjern. However, because the shallow geology in the Lejre catchment are dominated by clayey glacial moraine deposits with low hydraulic conductivity, increased net precipitation in areas covered by broadleaf forests would primarily discharge as overland flow or drainage flow rather than contributing to groundwater formation.