## 2 Materials and Methods  
Our study used data from seven monitoring sites in northeast Germany (Figure 1). The sites are situated in the German states Berlin, Brandenburg, and Mecklenburg-Pomerania. Northeast Germany is located in a transition zone between oceanic and continental temperate climates. The coastal parts are directly influenced by the Baltic sea. Within the region,there is a climatic gradient from northwest to southeast toward an increasing continentality. The annual average temperature varies regionally between 8-9 \({}^{\circ}\)C. The average annual climatic water balance ranges from minimum values of about -200 mm in central Brandenburg to maximum values of about +200 mm in the north of Mecklenburg-Vorpommern. The parent materials are quaternary sediments. According to the results of the National Forest Soil Inventory for the topsoil, mainly glacial cover sands (60% of soils), glacial fluvial sand (17%), aeolian dune sand (10%), and glacial loam (5% in 10 cm depth; 10% in 100 cm depth) occur in forest soils [28]. Glacial marl appears in the subsoil of less than 5% of the forest soil. The study sites are mainly stocked with Scots pine ( five sites, Table 1) with a comparable age structure [29]. One site is a mixed Scots pine, pedunculate oak site, and one site is a European beech site. Sites 1203 and 1303 exhibit dense ground vegetation. Most of the soils are characterized as Brunic Arenosols by nutrient-poor sandy substrates. On sites 1203 and 1303, podzolization has advanced. Site 1202 contrasts to the other sites as Lamellic Arenosol with a higher water holding capacity due to the occurrence of bands of loamy sands in the subsoil. Moreover, sites 1203 and 1302 exhibit relatively higher water holding capacities. The substrate in 1203 is fine sand, and site 1302 is characterized by a low silt and clay content. Figure 1: Location of the sites. Since 1996, the seven monitoring sites are part of the ICP Forests Level II monitoring program, with the consequence that measurements on the sites are conducted according to a common protocol. The sites are equipped with a meteorological station and soil moisture content is measured in several soil layers by time domain reflectometry (TDR) sensors [30]. Forest mensuration data are collected [31], ground vegetation is surveyed by vegetation releves [32], and soils are examined [33]. Soil analyses include measurements of carbon concentration, gravel content, bulk density, and particle size distribution. Within the last 10 years, also on-site measurements of leaf area index (LAI) started on all sites [34]. For water budget modeling, we used the LWF-BROOK90 hydrological model [35], which is a modification of the BROOK90 model [36]. BROOK90 is a process-based hydrologic model that simulates at point-scale vertical soil water movement and evapotranspiration at a daily time-step throughout the year. The model includes macropore-assisted infiltration, streamflow, and groundwater flow, in addition to modules to handle snow. Evaporation is calculated by the Shuttleworth-Wallace approach, which is a modification of the Penman-Monteith method to differentiate between soil evaporation and canopy transpiration [37]. It was designed to be applicable to canopies of any leaf area index (LAI) and developed further by [PERSON] et al. [38] to be applicable for any land surface. The model estimates interception and transpiration from a single-layer plant canopy. Interception rates are assumed as a constant fraction of rainfall or snowfall per unit LAI and stem area index (SAI) for both forest canopy and ground vegetation. Partial canopy wetting and drying [39] are not considered. Transpiration is related to radiation, air temperature, and vapor pressure deficit due to stomatal control, according to [40]. The aerodynamic resistance for water vapor fluxes is obtained using daily mean wind speed and an approximated vertical wind profile. Soil water limitation of transpiration is calculated recognizing root density profiles, plant resistance, and a critical leaf water potential.