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,. 5 " those five years, shortages would be materially reduced. In addition the reservoirs will provide complete protec- tion to the two areas from floods. There is indicated on Drawing SLv-46 the water supply of the Rio Gr?~de near Del Norte for each year from 1890 to 1938 and the net irrigation demand on that supply. The mean annual water supply as represented by the upper hor- izontal line is about 702,000 acre-feet. About 50,000 acre~feet of usable return flow enters the river between Del Norte and Alamosa; thus, the net irrigation demand on the water supply passing Del Norte is shown as 670,000 less 50,000 acre-feet, or 620,000 acre-feet. It may be noted that during a large number of years the water sup- ply was above and during a large number the supply was materially below the demand. '" In common with the ethor streams, Rside from years of low run-off being interspersed at interv~ls between years of high run-off, during certain periods .there s"ems to be a grouping of yo?rs of lO~T run-off and during other periods a grouping of years of high run-off. For example, during the l3-year period from 1892 to 1904, there were only three years when the run-off was above the leng-time mean, and only four years when the total water supply was in ex- cess of the 620,000 aore-foot re~uiroment. Again, during the 9-year period, 1930-1938 inclusive, there were only two years when the water supply exceeded the long-time mean and only three years when it exceeded the irrigation requirement. On the other hand, during the 25-year period from 1905 to 1929, there were 17 years when the water sup- ply exceeded the long-time moan n11d 22 years when it ox- ceeded the re~uirement. On the graph, the white area between the demand line and the top of the short bars indicates the extent of the an- nual water shortages. Actual shortages are greater,sinoe all tho water ropresented by the bars is not usable under present condi tions. This grouping of yee.rs of low run-off and years of high run-off is the reason large storage capac- ity is requirod to equalize the flow to a certain draft over a long period of years. , The water supply not only fluctuates materially from year to year but the seasonal supply does not occur parallel with the irrigation requirements. In the early part of the season much more water is available than is re~uired D-~d in the late season the water supply is much less than 1778 "