Assessing modelled hydrological responses to afforestation using hectometre-scale cosmic-ray neutron soil moisture

Since the 1990´s, and in particular during the last decade, afforestation has become a common water management practice. Afforestation improves the quality of the groundwater resource by reducing the leaching of nutrients and pesticides in the soil. Furthermore, planting of trees is also used to capture carbon from the atmosphere as an integral element of carbon emission mitigation, for biodiversity restoration and for biofuel. With the more extensive implementation of afforestation, it is important to understand the hydrological responses and to predict and quantify these adequately using hydrological modelling.

The hydrology of the forest system is characterized by high spatial variability. The forest vegetation intercepts and redistribute a considerable fraction of the precipitation resulting in an uneven input of water at the forest floor. The transpiration and soil evaporation vary in space according to the tree root distribution and soil texture. All these factors influence the soil moisture in the unsaturated zone, the percolation, and the groundwater recharge. Hydrological models are often used to estimate the groundwater recharge rate and to obtain information of the timing of the recharge to ensure sustainable groundwater exploitation and sufficient streamflow. The high spatial variability makes it difficult to predict forest hydrology and it is important that the observations are representative of the forest plot to assess the performance of the hydrological model.

In this study, we predict the water balance for bare ground conditions and for a coniferous forest to examine the hydrological responses to afforestation. We use a physically based and spatially distributed hydrological model with an energy-based description of evapotranspiration processes (MIKE SHE SVAT). The forest model was calibrated against timeseries of throughfall and point-scale soil moisture. Simulated soil moisture is evaluated against forest plot cosmic-ray neutron and point-scale estimates. Further assessment of the model is obtained through comparison to time-series of forest plot eddy-covariance evapotranspiration estimates and observation-based and predicted interception loss. We find that the forest plot and point-scale soil moisture estimates differ which in turn affects the assessment of the reliability of the model performance. The hydrological responses of afforestation are significant, influencing the total evapotranspiration, the soil moisture and the groundwater recharge.