Effects of climatic fluctuations and land-use changes on the hydrology of temperate fluviogenous mire

The upper part of the Narew River valley (NE Poland) belongs to the last extensive undrained, non-reclaimed valley wetlands in Central Europe. However, in recent decades, the fluviogenous mire has experienced a significant change in water resources which may be linked to climatic fluctuations and changes in vegetation. In NE Poland during the last three decades, a clear trend toward milder and drier climate has been observed that has resulted in lower river discharge. Reduction in surface water inflow into the mire, diminished duration and degree of flooding, decrease in summer rainfall and increase in evapotranspiration have led to the substantial fall of the groundwater table in the valley and a decrease in volume of water stored in organic sediments. A comparison of water table measurements made in the 1970s with contemporary observations showed that presently, summer minimum groundwater levels are as much as 60 cm lower than in the past. A significant drop of water table elevation might be additionally attributed to increased ET caused by cessation in agricultural use of the mire and expansion of Phragmites. Between the 1970s and 2002, the total amount of water lost by ET increased by 42%, whereas ETo rose by 7% only. We found that patches of common reed rushes were hot spots for ET across the Narew wetland landscape. The evaluation of the effectiveness of restoration measures aimed at restoration of the water conditions in the Narew mire led us to the conclusion that raising the river water level by use of in-stream structures may be not adequate to make up the summer moisture deficit and ensure a sufficiently high groundwater level in the adjacent wetland. The restoration goal may be partly achieved by large vegetation manipulation in the form of mowing or grazing. Vegetation management could reduce ET by 170 mm m−2 year−1, potentially minimizing the extent and the duration of groundwater drawdowns.