Southwest Hydrology: The Resource of Semi-Arid Hydrology Volume 4 Number 2

Home Browse Search

Naturalized Flows Used for Comparison Streamflow in the Colorado River is substantially affected by human - caused depletions and reservoir operations. Valid comparisons of droughts must be made on "naturalized" flows, derived by adjusting observed flows for human - caused depletions, including reservoir operations. Recent comparisons published by the USGS (Webb et al., 2004) and researchers at Scripps Institute of Oceanography at the University of California at San Diego and the University of Nevada, Las Vegas (Piechota et al., 2004) did not make such adjustments. Many of the hydrologic processes that affect streamflow in the Colorado River go unobserved and unmeasured. As such, reconstructing natural hydrology is time - consuming and fraught with uncertainty. Reconstructed naturalized flows since 2002 are still provisional, so a clear picture of the hydrology of the current drought has yet to emerge. But based on preliminary reconstructions (and shown in charts, left), the onset of the current drought is intense, even compared to the SSD study drought. The SSD study drought covered a period of unusually low flows lasting about two decades, followed by a period of high flows that lasted long enough for mean annual inflow to return to its long- term average. It is shown in Chart I as the trace of annual flows at Lees Ferry, Arizona. Current Lake Powell Decline Steeper Than SSD Chart 2 compares water volume recently measured at Lake Powell with the reservoir volume as modeled for the SSD study. The charts were synchronized on the last year before reservoir levels began their steady decline (1999 and SSD Year 5). Based on this decision, the first year of the SSD drought corresponds to 1995, the starting point for all comparisons in this article. What is apparent is that the current decline in the contents of Lake Powell is substantially steeper than was the case during the onset of the SSD study drought. A fundamental assumption in the SSD was that the study drought would be a very rare event, probably occurring many years in the future, and having been preceded by numerous less-severe droughts. Accordingly, it was assumed that all water allocations would be fully implemented and strictly adhered to. The SSD team also assumed (quaintly, in retrospect) that the high cost of pumping water through the Central Arizona Project would reduce the demand for and use of water through that project. The sudden onset of the current drought, however, caught water managers unprepared. While depletions in the Upper Basin are consistent with the assumptions in the SSD study, depletions in the Lower Basin are substantially higher. Lake Mead Depletions Far Exceed SSD Depletions are the water removed from a river or river system by processes such as evaporation or evapotranspiration. Man - caused depletions arise from things such as domestic or agricultural water use and evaporation from reservoir surfaces. Depletions can only be estimated, and with much effort; this work has not yet been completed for the most recent years. As a surrogate, releases from Lake Mead were used, with estimates of evaporation from that reservoir added in. Chart 3 shows that the total withdrawals and depletions from Lake Mead for the last ten years are greater than the values from the SSD study. The net effect of increased withdrawals is lower reservoir levels. Lake Mead is substantially lower today than at an equivalent point in the SSD drought, as shown in Chart 4. The rules by which the federal reservoirs on the Colorado River are operated force "equalization" releases from Lake Powell when its reservoir volume is greater than a prescribed amount and also greater than Lake Mead's volume. Although the SSD study showed some equalization releases at the initiation of the drought, flow measurements at Lees Ferry (downstream of Lake Powell) show (Figure 5) that releases made over the last See SSD, page 38 Innovative Solutions in Hydrology • Vadose Zone Characterization and Modeling • Groundwater Recharge Investigations • Water Resources Management • Mine Reclamation Studies • Copper and Gold Heap Leach Optimization Hydrologic Testing Laboratory - Saturated and Unsaturated Flow Properties - Calibration of Monitoring Instruments - Large Core Testing - Custom Testing and Research March /April 2005 • Southwest Hydrology • 25