Soil Water Budget and Drought Stress

Abstract

The availability of water to roots is a central property for the growth and productivity of forests. Because nearly all plant nutrients are taken up from the soil solution, there is a close connection between forest nutrition and the water budget. Furthermore, seepage water transports substances between the atmosphere, the soil, and the hydrosphere and determines the soils' filter function. Information on the water budget is therefore an important basis for the assessment of forest sites. Through processing and combination of data on soil, root and forest stand parameters in the NFSI data set, it was possible to reliably estimate the factors that control the soil water budget. Soil hydraulic properties such as water retention and water conductivity were derived from measured and estimated soil properties using pedotransfer functions. Additionally, the depth profiles of fine roots for each of the NFSI profiles were estimated in a correlation analysis between fine root density and soil properties (Hartmann and Wilpert 2014). Based on the processed NFSI data, the soil water budget model LWF-Brook90 was parameterized, and the dynamic water budget for all NFSI profiles was modelled. The aim was to identify in what way water flow and availability are affected by the variability of soil properties. All simulations were carried out in daily resolution, and model output, such as seepage water rates, soil water storage and evapotranspiration, was derived in daily resolution. This chapter gives an overview on (1) the soil water budget model LWF-Brook90, (2) the methodologies used to derive the input parameters for the soil water budget modelling (specifically the soil hydraulic and rooting properties) and (3) the results of the soil water modelling. The discussion of results focusses on the assessment of drought indices. It is shown that the plant-available soil water storage and other drought indices display similar trends. According to the model results, the intensity of water shortage has increased since 1990 and years with above-average soil water supply.