Evaluating an Earth system model from a water user perspective

Abstract. The large spatial scale of global Earth system models (ESM) is often cited as an obstacle to using the  output by water resource managers in localized decisions. Recent advances in computing have improved the fidelity of hydrological responses in ESMs through increased connectivity between model components. However, the models are seldom evaluated for their ability to reproduce metrics that are important for practitioners, or present the results in a manner that resonates with the users. We draw on the combined experience of the author team and stakeholder workshop participants to identify salient water resource metrics and evaluate whether they are credibly reproduced over the conterminous U.S. by the Community Earth System Model v2 Large Ensemble (CESM2). We find that while the exact values may not match observations, aspects such as interannual variability can be reproduced by CESM2 for the mean wet day precipitation and length of dry spells. CESM2 also captures the proportion of annual total precipitation that derives from the heaviest rain days in watersheds that are not snow-dominated. Aggregating the 7-day mean daily runoff to the watershed scale also shows rain-dominated regions capture the timing and interannual variability in annual maximum and minimum flows. We conclude  there is potential for far greater use of large ensemble ESMs, such as CESM2, in long-range water resource decisions to supplement high resolution regional projections.