This project was funded by the Senate of Berlin and the European Union (European Regional Development Fund).
Peat soils by comparision provide an approx. 10 % to 15 % higher evaporation service than mineral soils (Eggelsmann et al. 1990). Heat energy is taken from the environment for this evaporation process and the surface-near air circulation is cooling (Baumgartner & Liebscher 1996). This translates into a cooling effect which is particularly useful at times of higher air temperatures, and has a balancing effect on the local climate and the regional environment (Luthardt 2014).
The assessment of a peatland's cooling service, therefore, is based on the assessment of its evaporation function, or evapotranspiration (the sum of evaporation and plant transpiration from the Earth's land and ocean surface to the atmosphere). This again depends on the water budget and energy balance of a site (DVWK-M 238, 1996). Radiation intensity and temperature, wind speed, vegetation stocks as well as vegetation types and the groundwater table are all parameters, which heavily influence the energy and water budget balance, and whose characteristics are defined by a large number of local as well as regional correlations (Joosten et al. 2013).
The direct influence of the sun's radiation onto a peatland's surface (radiation intensity) is reduced by the coverage of moor woods. This reduces the peat soil evaporation. The surface roughness of the wooden plants also lowers the impact of the wind, which in turn lowers the potential peatland evaporation (Edom 2001).
The evaporation rates of moor plants vary from one species to another (Overbeck & Happach 1957), and their concentration impacts the evaporation function. According to Eggelsmann (1990), the evaporation rate is lower with low-growing communities or forms of exploitation (low sedge reed, meadow, grazing land, fallow, or tilling field) than it is with taller communities (tall sedge reed, forest). In the same vein, reed marshes (phragmites) also display a higher evaporation function due to their leaves' larger surface areas (Behrendt 1996).
It can be expected next to the influence of coverage density that the size of the peatland area itself also influences the cooling function directly. However, a simple causality between area size and influences on the environment could not be detected. When the fetch length increases, peatland evaporation by contrast decreases, which means that the so-called advection effects exert their impact in larger peatland areas and most of all on their margins (Edom 2001).
The potential cooling capacity decreases with an increasing depth to the groundwater or moor water table. Peatland soils with topical dryness contribute to the evaporation effect, as capillary water rises towards the surface (ATV-DVWK-M 504); however, with greater depth the effect of the capillary water rise on the surface evaporation decreases. According to Mundel (1982), only water tables with a depth of up to a maximum of 50 cm below the surface can be considered optimum conditions for the water supply. Edom (2001) defines a critical depth for the moor water level, or table, with 25 to 40 cm for sphagnum bogs, within which the capillary water rise is interrupted and evaporation suddenly decreases. The range from 80 cm below surface is ineffective for the capillary rise. Evaporation is based only on precipitation there and retained water (Edom 2001).
The net evaporation cooling service of a peatland site is a complex process and can only be quantified by equally complex analyses or modelling, for example Glugla et al. (1999, ABIMO), Frahm (2010), Münch (2004, AKWA-M®). A simplified approach is represented by the EEST Process (Evapotranspiration Energy Site Types; Joosten et al. 2013), which utilizes a model-based matrix (Edom 2001, Edom et al. 2010) together with site-specific climate data. For the time being, however, site types for peatlands are only verifiable with a view to climate stations or locations in Mecklenburg-Vorpommern.
