Using Sap Flow Data to Parameterize the Feddes Water Stress Model for Norway Spruce

Tree water use is a key variable in forest eco-hydrological studies and is often monitored by sap flow measurements. Upscaling these point measurements to the stand or catchment level, however, is still challenging. Due to the spatio-temporal heterogeneity of stand structure and soil water supply, extensive measuring campaigns are needed to determine stand water use from sap flow measurements alone. Therefore, many researchers apply water balance models to estimate stand transpiration. To account for the effects of limited soil water supply on stand transpiration, models commonly refer to plant water stress functions, which have rarely been parameterized for forest trees. The aim of this study was to parameterize the Feddes water stress model for Norway spruce (Picea abies [L.] Karst.). After successful calibration and validation of the soil hydrological model HYDRUS-1D, we combined root-zone water potential simulations with a new plant water stress factor derived from sap flow measurements at two plots of contrasting soil moisture regimes. By calibrating HYDRUS-1D against our sap flow data, we determined the critical limits of soil water supply. Drought stress reduced the transpiration activity of mature Norway spruce at root-zone pressure heads