Climate impacts on water balance of a shallow steppe lake in Eastern Austria (Lake Neusiedl)

Abstract

Lake Neusiedl, the largest lake in Austria, is especially sensitive to climate variations due to its extreme shallowness and a small catchment area. Historical records indicate that large variations of the lake area have occurred naturally (0% to >150% of present) but contemporary touristic uses of the lake require a largely constant water level. This dependence increases the regional economic vulnerability. Water balance of the lake as influenced by weather conditions was studied in detail. 79% of water input was due to precipitation, whereas more than 90% of water output was caused by evapotranspiration. Long-term observation of annual and seasonal precipitation sums, starting in 1865, revealed a slow downward trend of 15 years moving averages by 6 ± 1 mm/decade, masked by large interannual variations of the original data (s.d.: ±120 mm). Multidecadal oscillation indices (AMO, NAO, MOI) were tested against patterns of precipitation, air temperature and hydrological parameters of Lake Neusiedl. The clearest relation was observed between air temperature and North Atlantic oscillation index (p < 0.0001). Water level and volume of Lake Neusiedl are very sensitive to precipitation changes with after effects of individual years lasting up to 2 years. Summer precipitation is more important for lake water amount than the other seasons. The major surface water input to Lake Neusiedl is coming from River Wulka. Its annual discharge (15 years moving averages) showed a variable, moderately decreasing trend for the period 1961–2010 by −1.2 ± 0.6 × 106 m3/decade. Waste water treatment plants contributed up to 68% of monthly flow of River Wulka into the lake. Precipitation of the current and the previous year, and in some months also temperature influenced Wulka’s flow significantly. Evaporative losses from the lake and its reed belt were shown to increase over the last 33 years (+48 ± 11 mm/decade); as main drivers decreasing relative air humidity and increasing water temperature were identified. Based on the outputs of a regional climate model scenario and seasonal regression models, water balance projections for the period 2035–2065 showed a significant risk of hydrological deficits, leading to lower water levels than at present.