Abstract
Abstract. We assess the significance of groundwater storage for seasonal streamflow forecasts by evaluating its contribution to interannual streamflow anomalies in the 29 tributary sub-basins of the Colorado River. Monthly and annual changes in total basin storage are simulated by two implementations of the Variable Infiltration Capacity (VIC) macroscale hydrology model – the standard release of the model, and an alternate version that has been modified to include the SIMple Groundwater Model (SIMGM), which represents an unconfined aquifer underlying the soil column. These estimates are compared to those resulting from basin-scale water balances derived exclusively from observational data and changes in terrestrial water storage from the Gravity Recovery and Climate Experiment (GRACE) satellites. Changes in simulated groundwater storage are then compared to those derived via baseflow recession analysis for 72 reference-quality watersheds. Finally, estimates are statistically analyzed for relationships to interannual streamflow anomalies, and predictive capacities are compared across storage terms. We find that both model simulations result in similar estimates of total basin storage change, that these estimates compare favorably with those obtained from basin-scale water balances and GRACE data, and that baseflow recession analyses are consistent with simulated changes in groundwater storage. Statistical analyses reveal essentially no relationship between groundwater storage and interannual streamflow anomalies, suggesting that operational seasonal streamflow forecasts, which do not account for groundwater conditions implicitly or explicitly, are likely not detrimentally affected by this omission in the Colorado River basin.
Highlights
Among the most important contributors to the skill of a streamflow forecast are the estimation of initial hydrologic conditions (IHCs, i.e., basin water storage at the time of forecast) and prediction of future meteorological anomalies (Mahanama et al, 2012)
In the American West, this condition is the basis for the statistical seasonal streamflow forecasts issued by the Natural Resources Conservation Service (NRCS; Garen, 1992) and is an implicit attribute of the dynamically generated ensemble streamflow predictions issued by the National Weather Service (NWS; Day, 1985)
The objective of this paper is to evaluate the contribution of groundwater storage to interannual streamflow anomalies, and to seasonal and interannual streamflow predictability, in the Colorado River basin, which is iconic in the American West
Summary
Among the most important contributors to the skill of a streamflow forecast are the estimation of initial hydrologic conditions (IHCs, i.e., basin water storage at the time of forecast) and prediction of future meteorological anomalies (Mahanama et al, 2012). At seasonal lead times, accurate streamflow forecasts are possible mostly in situations where future runoff is more strongly related to catchment water storage than to meteorological anomalies during the forecast period (Wood and Lettenmaier, 2008; Shukla and Lettenmaier, 2011). In the American West, this condition is the basis for the statistical seasonal streamflow forecasts issued by the Natural Resources Conservation Service (NRCS; Garen, 1992) and is an implicit attribute of the dynamically generated ensemble streamflow predictions issued by the National Weather Service (NWS; Day, 1985). The opportunity to exploit the relationship between soil moisture and runoff in statistical streamflow forecasts was recognized in early studies (e.g., Boardman, 1936; Clyde, 1940), accumulated precipitation is more typically used as a proxy due to a paucity of in situ soil moisture observations
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