Improving the robustness of Upper Rio Grande water supply forecasting in the face of climate trends and variability

Abstract:

In the Rio Grande Basin, recent significant drought and declining runoff efficiencies driven by a warming climate have been shown to undermine the skill of conventional streamflow prediction methods that support seasonal water supply forecasts. Such streamflow volume forecasts form a critical input to management decisions made by the water management agencies (e.g., the Bureau of Reclamation) and for stakeholder decision making. Strategies for enhancing the robustness of operational seasonal streamflow prediction methods in the face of strong climate trends and increased hydroclimatic variability is thus imperative for the region. To address this need, we apply a new water-applications tailored implementation of the SUMMA hydrologic modeling framework at the watershed scale (USGS HUC12) to a suite of current Reclamation forecast points in the Upper Rio Grande Basin. Using this process-oriented modeling approach, we ask: 1) When and where are hydroclimate trends and/or decadal variability leading to changing predictability and systematic biases in waters supply forecasts? And 2) What practical adjustments can be made to current forecasting approaches to make seasonal water supply forecasts robust in the face of such phenomena? We present several prototype strategies for improving the robustness of these forecasting methods to hydroclimate trends, building off of prior research in the basin. Subseasonal to seasonal climate forecasts from sources such as SubX and the National Multi-Model Ensemble (NMME) are applied, for example, to guide streamflow forecast ensemble-member weighting and interpretation. We also assess the potential for improved prediction of seasonal hydrograph shape to guide reservoir operation planning, as compared to existing analog-based practices. Finally, we comment on the potential for such strategies to aid in developing climate-resilient water supply forecasts across the western U.S.