With changes in weather patterns and intensifying anthropogenic water use, there is an increasing need for spatio-temporal information on water fluxes and stocks in river basins. The assortment of satellite-derived open-access information sources on rainfall (P) and land use/land cover (LULC) is currently being expanded with the application of actual evapotranspiration (ET_act) algorithms on the global scale. We demonstrate how global remotely sensed P and ET_act datasets can be merged to examine hydrological processes such as storage changes and streamflow prior to applying a numerical simulation model. The study area is the Red River Basin in China in Vietnam, a generally challenging basin for remotely sensed information due to frequent cloud cover. Over this region, several satellite-based P and ET_act products are compared, and performance is evaluated using rain gauge records and longer-term averaged streamflow. A method is presented for fusing multiple satellite-derived ET_act estimates to generate an ensemble product that may be less susceptible, on a global basis, to errors in individual modeling approaches. Subsequently, monthly satellite-derived rainfall and ET_act are combined to assess the water balance for individual subcatchments and types of land use, defined using a global land use classification improved based on auxiliary satellite data. It was found that a combination of TRMM rainfall and the ensemble ET_act product is consistent with streamflow records in both space and time. It is concluded that monthly storage changes, multi-annual streamflow and water yield per LULC type in the Red River Basin can be successfully assessed based on currently available global satellite-derived products.

Water managers around the world face the increasingly challenging task to evaluate the impacts of technological measures and policy mechanisms from the local to the river basin scale. A toolset providing quantitative, actionable information on dependencies and trade-offs between upstream and downstream water users is currently lacking. Yet, any intervention needs to be assessed in terms of consequences for downstream water users. This study evaluates the potential of a tracer-like approach, implemented in the water allocation software WEAP, to quantitatively track return flows and their downstream reuse in the river basin context. The WEAP-Virtual Tracer (WEAP-VT) approach was successfully applied to one of Europe’s driest river basins, the Segura River Basin in Spain. For each water demand site, the different original sources of water supply, dependency on upstream return flows, and downstream reuse of its return flow were assessed. Based on these results, agricultural, urban, and environmental water users were evaluated in terms of their suitability for water saving measures and their vulnerability to the reduction in upstream return flows. A scenario analysis simulating the improvement of local efficiency improvements shows that specific irrigation schemes and ecosystems become deprived of water. Hence, efficiency improvement in water-scarce basins should be considered with caution. The demonstrated ability to quantify key water reuse indicators for individual water users and at different aggregation levels makes WEAP-VT a valuable tool to support water resource management decisions.