We assess the significance of groundwater storage for seasonal streamflowforecasts by evaluating its contribution to interannual streamflow anomaliesin the 29 tributary sub-basins of the Colorado River. Monthly and annualchanges in total basin storage are simulated by two implementations of theVariable Infiltration Capacity (VIC) macroscale hydrology model – thestandard release of the model, and an alternate version that has beenmodified to include the SIMple Groundwater Model (SIMGM), which representsan unconfined aquifer underlying the soil column. These estimates arecompared to those resulting from basin-scale water balances derivedexclusively from observational data and changes in terrestrial water storagefrom the Gravity Recovery and Climate Experiment (GRACE) satellites. Changesin simulated groundwater storage are then compared to those derived viabaseflow recession analysis for 72 reference-quality watersheds. Finally,estimates are statistically analyzed for relationships to interannualstreamflow anomalies, and predictive capacities are compared across storageterms. We find that both model simulations result in similar estimates oftotal basin storage change, that these estimates compare favorably withthose obtained from basin-scale water balances and GRACE data, and thatbaseflow recession analyses are consistent with simulated changes ingroundwater storage. Statistical analyses reveal essentially no relationshipbetween groundwater storage and interannual streamflow anomalies, suggestingthat operational seasonal streamflow forecasts, which do not account forgroundwater conditions implicitly or explicitly, are likely notdetrimentally affected by this omission in the Colorado River basin.
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