Long-term water temperature modelling in semi-arid alpine basins

Abstract

Temperature plays a critical role in the functioning of river ecosystems. Hence, understanding the processes that control water temperature in river networks across daily to multi-year scales is important when trying to manage river thermal regimes. This is particularly urgent in alpine semi-arid basins with substantial human impact, and, especially within the context of global change, where river ecosystem integrity is at risk. A process-based model has been developed to simulate water temperature in lakes and rivers at a regional (watershed) scale. The physically based and fully distributed hydrological model provides comprehensive hydrological and hydraulic simulations of river flow, including contributions from snowmelt, groundwater, and direct runoff at each node of the network. Additionally, the model accounts for the discharge of urban wastewater at its respective nodes. To overcome the computational cost and numerical problems associated with Eulerian methods in long-term simulations, the model uses a semi-Lagrangian approach to discretize the one-dimensional heat conservation equations in river reaches. Reservoir stratification and withdrawal temperatures are simulated with a 1D Lagrangian model (General Lake Model). This methodology ensures the accurate and detailed simulation of water temperature dynamics in rivers by integrating meteorological, hydrological, and hydraulic data, along with the impact of urban wastewater discharges and reservoir outflows. The model is applied to simulate water temperature in a small semi-alpine watershed upstream of the city of Granada that includes two water-supply reservoirs (Canales and Quéntar). Autonomous temperature sensors deployed at different sites are used for model validation. The model is forced with climate databases (reanalysis, regional climate simulation conducted with WRF, and measured data bases) and used in hindcast/forecast exercises to assess the impact of climate change on the thermal regime of inland waters.