Analysis of water quantity and water quality for climate and management scenarios: development and application of a model cascade for the Spree River catchment

Climate-induced changes of the natural water balance as well as socio-economic developments may impact water quantity and water quality management. In this study, a model cascade consisting of (i) the hydrological models SWIM and EGMO, (ii) the long-term water management model WBaIMo and (iii) the water quality model GGM was developed, implemented and applied to assess the impacts of regional changes on water quantity and quality. Moreover, using the model cascade adaptation measures in terms of water quantity and water quality management can be assessed. The study area is located in the Spree River catchment which is exposed to various challenges both in terms of water quantity and water quality (e.g. low pH-values in post-mining lakes, high sulphate concentrations in post-mining lakes and the river system) due to lignite mining activities and the rapid decrease of these activities in the 1990s. The fundamental question is to what extent problems in terms of water quantity and water quality are exacerbated under potentially drier climatic conditions and how these problems can be mitigated by adaptation measures. Stochastically generated time series (realizations) of the regional climate model STAR are used as climatic drivers of the model cascade. The realizations of the scenario STAR OK serve as stationary stochastic replicates of the recent climate conditions. To determine the vulnerability of the water resources to a potentially drier future, we have used the scenario STAR 2K, which assumes a further temperature increase by 2 K as well as a decrease in the mean annual precipitation of the study region.The socio-economic development of the region is considered in terms of a management scenario assuming a further decrease of lignite mining activities. Based on STAR OK, only minor changes in natural discharge in the scenario period 2018-2052 are simulated, while managed discharges slightly decrease due to declining mining discharges. In the STAR 2K scenario, natural and managed discharges decrease, resulting in negative consequences on reservoir volumes and on water availability to the users. Additionally, the risk of a re-acidification of post-mining lakes and increasing sulphate concentrations is much higher in STAR 2K scenario than in STAR OK. In order to compensate for negative impacts on water quantity and water quality, adaptation measures were analysed. Water transfers from the River Elbe into the study region showed positive impacts on both water quantity and water quality. In-lake-neutralisation of post-mining lakes can solve local water quality problems.