D3: Water budgets

„Water budgets“ is a demonstration action that focuses on the development and implementation of water budget modelling on intensive monitoring plots. The focus of interest in this action is the reaction of trees on drought stress, which may occur as a consequence of climate change and the better understanding of nutrient uptake and growth. From the demonstration action conclusions regarding the selection of intensive monitoring plots and the monitoring on intensive monitoring core plots will be drawn.

The water supply in the soil is one of the key factors affecting tree vitality and forest condition. Moreover, the determination of the water budget has been shown to be of major importance in understanding a number of physiological processes like nutrient uptake, growth and response to biotic stress factors. The existing intensive monitoring programme (Level II) covers the most important impact factors, as well as the reaction parameters of the forests at the same sites. The impact of environmental stressors like drought, physiological water shortage and elevated ozone concentrations may result in reduced tree growth, increased occurrence of pests and diseases as well as deterioration of crown condition. In these cause-effect relationships, water availability to forest trees plays a vital role. So far, meteorological and hydrological measurements for a better description of water availability at forest sites have been conducted at a limited number of Level II plots. Such intensive soil moisture measurements are needed to adopt water budget models, which will explain water fluxes in the soil and water availability at given site conditions. Therefore, it is suggested to consolidate and amend the existing monitoring activities within the Level II network and to aim at the implementation of complete sets of surveys including meteorology, soil moisture, phenological observations, forest growth assessment, biotic damage, soil solution and crown condition on the same plots. In this respect, the demonstration action “Water budget” will test and demonstrate the feasibility of more intensive soil moisture measurements. Based on the measurements, water budget models will be further developed and validated at the plots included in the action and will be applied to the other Level II and all Level I plots. The water budget models will yield water fluxes through the soil required for the calculation of nutrient fluxes.

Implementation

Action D3 will be conducted on plots on which the full set of surveys from action IM1 is carried out (114 plots from 21 associated beneficiaries).

• Additional measurements of soil moisture will be implemented at different depths on selected plots. This should preferably be performed using technology that measures the volumetric water content (TDR).
• On some plots also the matric potential will be measured using tensiometers. The implementation of both methods offers the possibility to determine water retention functions (field pF curves). pF curves will also be measured in the laboratory on samples from all those plots, covered by the Action, for which water retention function curves are not available.
• Water retention functions are needed for the parameterisation of water budget models and for the improvement of existing pedo-transfer-functions.
• Within the corresponding action C1-Met-29(BY) water budget modelling will be carried out on the basis of soil moisture measurements. These models need the input data from many other surveys, like meteorology, soil analysis (physical parameters and mainly C content), stand characteristics, increment and leaf area index (LAI).
• Water budget models will be calibrated and the results will be validated with soil volumetric water content, soil matric potential, soil temperature and stand precipitation.