Abstract: Integrated Catchment Biophysical Modelling: Synthesis Report
This document synthesizes key results from the application of biophysical models to the REFRESH Demonstration Catchments. These catchments are very diverse in size and characteristics, and were intended to be a sample of important representative climate and land-use types over Europe. The models were applied to assess the effects of environmental change (change in climate, land use, nitrogen deposition and water use) on water quantity and quality. In particular, those aspects relevant to the Water Framework Directive were modelled: river flow, river and lake nitrogen and phosphorus concentrations, and indicators of ecological status.
The robustness and uncertainty of the models used are analysed in this document. Long-term trends and seasonal variations in all the major modelled variables (flow, nitrate, phosphorus and lake water chlorophyll-a concentrations) were simulated well and can be used with some confidence. Dynamic models are however not sufficiently developed to simulate extremes, and typically produce results with a lower variance than observed data. For simulating shorter-term variations, the overall model performance reflected the relative complexity of the underlying processes. Thus hydrology simulation was excellent, nitrate was good, but phosphorus and chlorophyll-a dynamics were not generally well simulated on short timescales. To do this is a severe test of a model, and weak performance here does not preclude valid results on longer timescales.
The effects of the outputs from three different Global Circulation Model-Regional Climate Model (GCM-RCM) combinations derived during the ENSEMBLES project were compared in this work. Flow and water chemistry were modelled during a baseline period (1981-2010) and a scenario period (2031-2060) and compared. The effects of up to four land use changes were also modelled, as were possible mitigation strategies. Land use changes modelled reflected a mixture of “economic” storylines, maximising agricultural productivity, and “environmental” storylines, with a focus on protecting water quality. The actual land use and mitigation strategies employed were determined on a catchment-specific basis using local knowledge.
All three GCM-RCM models predicted a rise in temperature over Europe, with the Hadley Model consistently and significantly higher than the others, with a mean rise of 2.2 °C, followed by KNMI (1.4 °C) and SMH (1.0 °C). The highest temperature rises were in Finland: apart from this site there was a north-south gradient with the greatest temperature rises in the south, though the differences between the models are generally greater than the differences between the sites. For precipitation, there is a distinct north-south divide, with small increases in the north and mid-latitude sites, and large decreases in the south. At Arbúcies in the Pyrenees, there is considerable variability between models, with the Had Model predicting a 17% decline in precipitation, and the SMH Model a 15% increase. All models concur, however, in predicting substantial decreases in precipitation at the Greek and Turkish sites.
The predicted effects on water flows differed between the northern and southern sites. In the north and mid-latitudes, the increased temperatures are balanced to some extent by the increased precipitation, leading to relatively small effects on water flows, though seasonal effects may still be important. In the south, increased temperatures and lower precipitation act in the same direction to reduce water flows considerably. In the case of Lake Beysehir in Turkey, this may even lead to the lake drying up in the foreseeable future, and this effect would far outweigh any nutrient-related problems. Seasonal effects also need to be taken into account, particularly as precipitation change in winter has a much larger effect on river flows than precipitation change in summer, and these seasonal differences may have significant ecological consequences.
In general, the effects of climate change alone on nutrient concentrations were rather small. The effects of credible land use changes were larger, and generally, the land use changes representing the “environmental” storylines reduced nutrient concentrations, and those from the “economic” storylines increased them. However, there were exceptions and considerable differences in response between sites. The responses seem more dependent on the mixture of nutrient sources (e.g. agriculture versus wastewater) than the degree of climate change. Modelled ecological changes were not generally proportional to the changes in nutrients. Ecological change was predicted to be both less and greater than the nutrient change, at different sites. Modelled mitigation options were able to reduce nutrients, and there was no evidence here that they were less effective under a future climate. With less certainty, mitigation options could affect the ecological status of waters at these sites in a positive manner, leading to an improvement in Water Framework Directive status at some sites. Uncertainty in the climate models, as represented by the differences between the three GCM-RCM combinations used in this study, did not affect this overall picture much, though there were variations in response to the different models at individual sites.
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