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[1] This paper describes a one-dimensional stream temperature model that is computationally efficient and highly scalable in both time and space. The model is developed within the framework of state space structure. The time-dependent equations for the conservation of thermal energy in a flowing stream or river are solved using a mixed Eulerian-Lagrangian, or semi-Lagrangian, numerical scheme. Solutions are obtained by tracking individual water parcels along their flow characteristics and storing the simulated results at discrete points on a fixed grid. Computational efficiency and accuracy of the numerical scheme are demonstrated by comparison of model estimates with observations of stream temperatures from rivers in the Pacific Northwest as well as with results from a closed-form solution of the energy equation. A preliminary analysis of the impact of climate changes on stream temperature in the Columbia River system illustrates the strengths of the semi-Lagrangian method for addressing water quality issues of regional, national, and, ultimately, global scale. Further development of the semi-Lagrangian method has the potential to improve the ability of water quality planners to perform uncertainty analysis, risk analysis, and forecasting for large, complex river systems.

Abstract:Stream temperatures in urban watersheds are influenced to a high degree by changes in landscape and climate, which can occur at small temporal and spatial scales. Here, we describe a modelling system that integrates the distributed hydrologic soil vegetation model with the semi-Lagrangian stream temperature model RBM. It has the capability to simulate spatially distributed hydrology and water temperature over the entire network at high time and space resolutions, as well as to represent riparian shading effects on stream temperatures. We demonstrate the modelling system through application to the Mercer Creek watershed, a small urban catchment near Bellevue, Washington. The results suggest that the model was able to produce realistic streamflow and water temperature predictions that are consistent with observations. We use the modelling construct to characterize impacts of land use change and near-stream vegetation change on stream temperatures and explore the sensitivity of stream temperature to changes in land use and riparian vegetation. The results suggest that, notwithstanding general warming as a result of climate change over the last century, there have been concurrent increases in low flows as a result of urbanization and deforestation, which more or less offset the effects of a warmer climate on stream temperatures. On the other hand, loss of riparian vegetation plays a more important role in modulating water temperatures, in particular, on annual maximum temperature (around 4°C), which could be mostly reversed by restoring riparian vegetation in a fairly narrow corridor -a finding that has important implications for management of the riparian corridor.

Water scarcity threatens people in various regions, and has predominantly been studied from a water quantity perspective only. Here we show that global water scarcity is driven by both water quantity and water quality issues, and quantify expansions in clean water technologies (i.e. desalination and treated wastewater reuse) to ‘reduce the number of people suffering from water scarcity’ as urgently required by UN’s Sustainable Development Goal 6. Including water quality (i.e. water temperature, salinity, organic pollution and nutrients) contributes to an increase in percentage of world’s population currently suffering from severe water scarcity from an annual average of 30% (22%–35% monthly range; water quantity only) to 40% (31%–46%; both water quantity and quality). Water quality impacts are in particular high in severe water scarcity regions, such as in eastern China and India. In these regions, excessive sectoral water withdrawals do not only contribute to water scarcity from a water quantity perspective, but polluted return flows degrade water quality, exacerbating water scarcity. We show that expanding desalination (from 2.9 to 13.6 billion m3 month−1) and treated wastewater uses (from 1.6 to 4.0 billion m3 month−1) can strongly reduce water scarcity levels and the number of people affected, especially in Asia, although the side effects (e.g. brine, energy demand, economic costs) must be considered. The presented results have potential for follow-up integrated analyses accounting for technical and economic constraints of expanding desalination and treated wastewater reuse across the world.