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Show considered, as well as the alternative uses which can be made of electric power. C. Character and Rate of Development 1. Basic Data Deficiencies The Problem Basic data essential to planning, construction and operation of a comprehensive program in the basin. The Situation Probably no large drainage basin in the country is as poorly equipped with basic data as is that of the Colorado River. This lack of data has come to be widely recognized and great effort has been made to fill gaps. However, much information is still lacking. Hydrologic data.-Hydrologic data are insuf- ficient. In part this is due to an inadequate number of recording stations, and in part because of the nature of the climate. In a region with great local variation in precipitation and with many localized storms of variable intensity, a dense network of precipitation stations would be required to obtain real knowledge of rainfall behavior. In the ab- sence of such a dense network, a long period of record from key stations is the next best expedient. Both the network and the long period are lacking except for relatively few stations, most of which are along the edges rather than in the center of the area. Data, on quantity and intensity of precipi- tation are needed. The precipitation that affords the main source of water supply comes as snow at the higher eleva- tions. This fact makes possible the use of snow surveys to obtain much requisite information on stream behavior and on available yields. How- ever, only about half of the needed snow surveys are now made annually. The relatively short records from surveys do not adequately sample the great variation in snow water from one year to another in the different tributary areas. More precipitation stations are needed at the upper elevations to obtain information on summer storms. Although these storms are usually local, they do replenish soil moisture and occasionally are sufficiently large in volume or intensity as to cause stream flow or local floods. Because of the paucity of precipitation data gen- erally in the basin, all the data should be released currently for general use by all agencies. Stream flow.-Knowledge of stream volume and behavior is more plentiful than that of precipita- tion, and these records permit a better understand- ing of the precipitation than the precipitation rec- ord itself. Although records of flow have been taken on the main stem and some of the major tributaries for a considerable period, the data are not sufficient for full development of the basin. The construction of the San Carlos Reservoir was based on records at the site of only a few years, and on records of other stations over a period of 30 years. Whereas the computed runoff exceeded the mean during 15 of the 30 years prior to construction, it has exceeded the mean during only three of the years since construction. This is an outstanding example of the need for long records at the site of a proposed structure in an arid area, and of the lack of confidence that can be placed on interpolation from records of conditions at supposedly similar sites. Although nearly 350 stream gaging stations have been installed in the basin, few have records of adequate length. Study reveals the need for at least 100 additional stations. Many of these should be installed on small watersheds to obtain better information on water behavior. Water losses.-Because water is so important in this and dependent basins, its disposition is a mat- ter of great concern. Although numerous records have been taken on the lower river, these are inade- quate to meet the basin requirements. Various es- timates have been made of the amount of evapora- tion that takes place from free water surfaces. Es- timates for Lake Mead are as much as 7 feet a year. If such losses are to be expected from the large water surface area in various reservoirs, they could be most serious, especially in years of water shortage. Complete information concerning evaporation rates, and factors affecting evaporation such as wind direction and velocities, temperature and air pres- sure should be available prior to the selection of any reservoir in the Colorado River Basin. And steps to secure this information on a basin-wide basis should be initiated immediately. (See Problem C-S.) Another significant water loss is that from plants along water courses. The tamarisk (salt cedar), cottonwood, willow, and other similar trees and shrubby growth along streams, on out-wash fans, and on sediment deposits in reservoirs are said to 414 |
