Rapid screening of operational freshwater availability using global models

Abstract

Freshwater shortage already affects large parts of the world, and is expected to increase rapidly over the coming decades as a result of increased water demands and the impacts of climate change. Global-scale water risk or stress maps are available online, but these lack quantitative information on local freshwater availability, rendering them unsuitable for water risk assessment from an operational perspective, i.e. when comparing water availability to a specific quantified water demand (in m3/s rather than generic risk indicators). Therefore, our main goal was to develop a rapid screening method to estimate current and future operational freshwater availability using global-scale models. Operational Freshwater Availability (OFWA) was computed using the PCR-GLOBWB global hydrology and water resources model, coupled to a global MODFLOW groundwater model. PCR-GLOBWB was forced with rainfall and temperature fields from the IPSL-CM5A-LR climate model under the RCP6.0 climate scenario, with water demands based on the SSP2 socio-economic scenario. Output from PCR-GLOBWB provided recharge and river stage height boundary conditions to the MODFLOW model. To our knowledge, the combined PCR-GLOBWB-MODFLOW model is the first coupled global surface water-groundwater model that is locally downscaled to a 90 m resolution, and used to provide quantitative estimates on long term trends in operational freshwater availability. Our results showed a high, operationally relevant, accuracy for operational surface water availability, while the uncertainty about operational groundwater availability remained high due to limited availability of subsurface data. With this method, we developed a modelling capacity for rapidly generating scenario-based water availability projections with operational relevance in a rigorous, systematic way, such that it enables like-for-like comparisons. Further refinement is required for accurate estimates of operational groundwater availability.