Materials and methods
2. Materials and methods
2.1 Concepts and indicators
The impact of forest management upon the water budget of forest stands/watersheds is evaluated based on the terms of the water balance equation (Equation 1).
Equ. 1 Po = E s + I +T + SW + GW + Q
Po…Precipitation, E s …Soil evaporation, I…Intercepted rain evaporation, T…Transpiration, SW…delta soil water storage, GW…delta ground water storage; Q…runoff; all units are [mm.a -1 ].
With respect to the service ‘water yield’, water use by the ecosystem, namely EIT (E s +I+T = EIT) is a suitable indicator. A second indicator is runoff, as this value is frequently given as an output of paired catchment studies. This term is meaningful under the assumption that over longer periods, SW + GW will equal zero. With respect to the protective function of forested systems, the impact upon surface runoff and peak discharge is also of importance. The analysis of hydrographs from paired catchment studies (control versus treated watershed) provides information on such response patterns. Peak discharge is dependent upon a number of factors – timing and intensity of precipitation, snow accumulation and snowmelt patterns, site factors such as watershed size and topography, surface roughness and soil properties (porosity, pore size distribution and continuity), water content of soils prior to a precipitation event and vegetation cover. Results gained from watershed investigations are therefore usually of limited general validity.
2.2 Review
The current study includes a literature review of publications regarding the effects of forests and forest management upon hydrological processes. The review includes results pertaining to management effects on runoff from single and paired catchment experiments, stand level data on sub-components of the water cycle and information on physiological response of trees (leaf area and stomatal conductance, which affect ET) to fertilisation. With respect to the service ‘reduction of peak runoff’ few references to published experiments and runoff coefficients are included. The results are structured into species effects, effects of site preparation and fertilization, effects of stand density (including thinning and harvesting), effects of road construction and effects of land use changes (grassland or arable land to forest). A synthesis is drawn for different FMA’s defined by Duncker et al. (2010a).
2.3 Meta analysis of literature values
Paired basin experiments provide a continuous, and continuously changing record of vegetation structure, composition, climate, and their effects on streamflow (Jones and Post, 2004). Single or paired catchment approaches have been applied for more than a century and a considerable amount of literature has been generated. A literature search on paired catchment studies provided a broad range of data, comprising of reviews (Hibbert, 1967; Bosch and Hewlett, 1982; Sahin and Hall, 1996; Stednick, 1996; Best et al., 2003; Pike and Scherer, 2003). which were completed by additional studies (Jones and Grant, 1996; Troendle et al., 2001; MacDonald and Stednick, 2003; Robinson and Dupeyrat, 2005). In the course of constructing a meta-database, empirical data of water yield, evapotranspiration and interception before and after forest management activities were collected. This approach was based on two different scales: either the whole catchment or just a managed stand was considered. Water yield data exclusively originated from watershed studies while evapotranspiration and interception data referred to the stand scale. The available data were hierarchically classified regarding the affiliation to the Köppen-climate classification, the size of the investigated drainage area, the vegetation type and the management Paired basin experiments provide a continuous, and continuously changing record of vegetation structure, composition, climate, and their effects on streamflow (Jones and Post, 2004). Single or paired catchment approaches have been applied for more than a century and a considerable amount of literature has been generated. A literature search on paired catchment studies provided a broad range of data, comprising of reviews (Hibbert, 1967; Bosch and Hewlett, 1982; Sahin and Hall, 1996; Stednick, 1996; Best et al., 2003; Pike and Scherer, 2003). which were completed by additional studies (Jones and Grant, 1996; Troendle et al., 2001; MacDonald and Stednick, 2003; Robinson and Dupeyrat, 2005). In the course of constructing a meta-database, empirical data of water yield, evapotranspiration and interception before and after forest management activities were collected. This approach was based on two different scales: either the whole catchment or just a managed stand was considered. Water yield data exclusively originated from watershed studies while evapotranspiration and interception data referred to the stand scale. The available data were hierarchically classified regarding the affiliation to the Köppen-climate classification, the size of the investigated drainage area, the vegetation type and the management
2.4 Modelling approach
To show the applicability of physically based hydrological models as support tools for forest management decisions, modeling results for generic forest systems of different species (beech vs. spruce) and thinning intensity under steady site conditions are provided. The hydrological model BROOK90 (Federer, 1995) was used to calculate water balances for the generic forest systems. Details on parameterization are given in Katzensteiner and Eckmüllner (2010) and Duncker (2010b).