OCR Text |
Show XII-30 UPDATING THE HOOVER DAM DOCUMENTS met in full because of limitations on basin water supply. Although California would still be receiving its basic Colorado River supply and even though it has available more expensive alternative supplies to offset the cutback, it could be said that a shortage exists; 2. At that time when the total annual water demands of the basin, including reservoir evaporation and other water losses, exceed the long-term average annual water supply, the basin will be in a water shortage status even though water demands could be met for a limited period by drawing on reservoir storage; 3. When Arizona must reduce its Colorado River consumptive uses below 2.8 million acre-feet per year a new shortage will occur; 4. When the upper basin reaches its assured annual consumptive use of 5.8 million acre-feet annually, a still different type of shortage will be reached. (The 5.8 million acre-feet annually is that assured amount remaining for use in the upper basin under adverse runoff conditions after it has met its obligation to deliver 75 million acre-feet to the lower basin each 10 years and if it is required to contribute 750,000 acre-feet annually toward meeting the Mexican Water Treaty obligation. The assured supply thus estimated is not to be construed as the limit of the upper basin apportionment, as it is recognized that there is not agreement among the States of the Colorado River Basin on how the Mexican Treaty obligation is to be shared prior to the time augmentation of the Colorado River is accomplished under the terms of the Colorado River Basin Act of 1968). Because of the long-term carry-over storage of Lake Mead and the upper basin storage project reservoirs, whenever shortages occur in the above categories such shortages likely will span several years; 5. Another category of shortage common to the river systems can occur in the upper basin tributaries above main storage units where the water supplies are not sufficiently regulated by long-term carry-over storage. In these circumstances, shortages can vary from as little as a month at a time to a year or more; and 6. Still another category of shortage is represented by the Gila River basin where the present consumptive uses far exceed the combined surface and ground-water recharge of the basin. A shortage has existed here for years. To help predict future situations relating to both water supply shortages and water quality under varying assumptions, a mathematical model of the Colorado River referred to as the Colorado River Simulation Model (CRSM) was developed. The model receives projected hydrologic sequences and projected water demands, processes them through a comprehensive systems operation, and evaluates the results in terms of water supply versus water demand. To explore various possible future Colorado River runoff conditions, hydrologic traces, or theoretical 30-year sequences of Colorado River flow, are generated for selected stations over the entire Colorado River Basin based on statistical parameters derived from historical data and modified as necessary to reflect the runoff period of 1914-65. This period of record was selected because the modified flow analysis for benchmark gaging stations was available. These data were prepared for the Upper Colorado Region Comprehensive Framework Study. An almost infinite number of hydrologic traces can be generated. For the purpose of illustrating a range of possible future runoff sequences three hydrologic traces were structured. Trace A reflects a reasonably low runoff cycle with a 30-year mean estimated virgin flow at Lee Ferry of 13.2 million acre-feet. Trace C reflects a reasonably high runoff cycle of 15.5 million acre-feet. Trace B (14.1 million acre-feet) reflects a reasonable intermediate situation. Alternative Future Water Demand Schedules.-The element of uncertainty in projecting the rate at which future water demands in the 40 |