Assessing the influence of the vegetation on the evapotranspiration from a wetland – a case study from northwest Germany based on in-situ measurements at different scales

Evapotranspiration (ET) from wetlands is often considered to occur at a potential rate. However, depending on the structural and physiological traits of the dominating plant species, actual ET can deviate substantially from potential ET. Here we present a case study from a restored wetland in north-western Germany which is dominated by moor grass (Molinia caerulea). ET was measured over three years by means of the Bowen Ratio method, leaf transpiration and leaf resistance of moor grass were measured with a porometer, and both green and total leaf area index were measured optically and manually.

Whilst actual and potential ET were practically similar during the period from late summer to the end of winter, they differed significantly from the beginning of spring to early summer and on hot summer days. Two likely reasons for this marked seasonality could be identified. (1) Molinia leaves responded very sensitively to the vapour pressure deficit of the air, independent of the unlimited water supply to its roots. (2) A thick mat of dead leaves covered the surface in spring before and while the new leaves emerged and acted as an efficient protection cover against evaporation.

The SVAT model ‘AMBAV’ was developed by the German Meteorological Service and is operationally used in agrometeorological applications. Based on the Bowen Ratio and in-situ plant physiological data, it was newly parameterised for the investigated type of wetland. If run with weather data from a nearby station, AMBAV could verify the observed seasonal pattern of actual ET from the moor grass dominated wetland. The results demonstrate that the present vegetation reduces wetland ET and thus contributes to the maintenance of a high water table in the restored wetland.