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Show VI. Basic Physical Data on Water Resources A. Hydrology and Sedimentation This section of the report was prepared from material supplied by the subcommittees on Hydrology and Sedi- mentation of the Federal Inter-Agency River Basin Com- mittee. It was recognized that these subcommittees, the membership of which is made up of representatives of basic data agencies as well as resource development agencies, were well qualified to advise on the status and adequacy of hydrologic and sedimentation data. The subcommittees had been engaged for about 5. years in the review of exist- ing sources of hydrologic and sedimentation data and had considered recommendations concerning anticipated future needs for sucl data and the most effective means for its collection. In the following sections, statements as to the adequacy of all types of hydrologic and sedimentation data are pre- sented. Included also are statements on research in these fields. In most cases maps are used to show graphically the status of such data and data-collecting facilities. The boundaries of the major drainage basins and sub-basins are delineated on the maps and each sub-basin is identi- fied by a number. Under each of the classes of data certain recommendations are made for expansion of exist- ing facilities. Provision of these facilities is an essential step toward efficient development of the water resources of this country. For reasons of efficiency and economy the additional facilities should be installed and in operation within 10 years. Precipitation, Evaporation, and Other Meteorological Data The Weath_er Bureau is the principal agency collecting precipitation, evaporation and other meteorological data. Although many of these data are employed by the Weather Bureau in its own river forecasting opera- tions or in serving the general public, they have many important applications in the planning, design, and op- eration of woiks for water control and utilization. Figure 24 sliows the adequacy of facilities for gathering precipitation data. The current percent of adequacy is equal to the number of existing stations divided by the sum of the existing and recommended new stations. Ap- proximately 2 5 percent of the sub-basins have precipita- tion-gathering; facilities between 0 and 50 percent ade- quate, while 75 percent have facilities between 50 and 100 percent adequate. In addition to consideration of the adequacy of facilities based solely on the number of stations, it is appropriate to comment briefly on the adequacy witti respect to instrumental equipment. A nonrecor-ding rain gage does not supply adequate information regarding rainfall intensity for durations of less than 24 h.ours. The present percentage of recording gages in the network (about 30 percent) is too low. A distribution of about 50 percent recording gages should be adopted as a reasonable target. Recording gages now in general use have a capacity of 12 inches of precipita- tion, which is insufficient to accommodate the most intense storms in many areas. Wherever amounts in excess of 12 inches in 24 hours can be expected, larger capacity gages should be installed. In many remote areas where daily or weekly attendance is not feasible, large capacity nonrecording storage gages are in use. A sufficient number of recording gages capable of operating from 30 days to 6 months without attendance should be installed to make possible reliable determination of daily rainfall increments in remote areas. The definition of an adequate network is based upon the considered judgment of field representatives of the various Federal agencies. The recommendations are limited to the needs which can be foreseen within the next 10 years. A change in national policy, or new technical discoveries, may, in the future, require a redefi- nition of an adequate network. The adequacy of existing data, and the extent to which these data have been processed and summarized, must be considered on the basis of rather arbitrary standards. It is generally considered that 50 years of observations from a network such as recommended would provide data of minimum adequacy. Also, these data should be proc- essed by the use of punch cards, adjusted where necessary for such changes as station location, exposure, and instrumentation. Finally, certain standard summaries and analyses such as frequency curves and monthly and annual isohyetal maps should be prepared and published. The average length of precipitation record for the United States from gages now installed is approximately 23 years. These same records when applied to the recom- mended network would give an average length of record of approximately 16 years, which is generally conceded to be too short for completely reliable planning of the development of our water resources. Data now available on punch cards represent only a small percentage of the total, while few data have been adjusted for changes in location, exposure, and instrumentation. Analyses and publication of rainfall intensity-duration-frequency rela- tionships are badly needed for the entire United States. Figure 25 shows the adequacy of facilities for gathering evaporation data. The need for evaporation data gather- ing facilities are the greatest in arid areas having low average annual runoff. In areas represented by sub- basins, as shown in figure 2, having an average annual runoff of 10 inches or less, the minimum number of re- quired evaporation stations is 6. In those sub-basins having an average annual runoff in excess of 10 inches, the minimum requirement is 3. Deficiencies in other meteorological elements are less easily evaluated. Temperature is reasonably uniform 336 |