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. r /- ~ (...- Ii. , Ii 8541 \ . 0........-_..--'.. L SL V fd~ APPENDIX E Ict 3 9 Rs Tipt"- PERIOD OF STUDY Two periods of record were used for the operation studies made in this investigation. For irrigation purposes, the studies were started at the end of the flood season of 1924. This date was used for two reasons, the first being that Elephant Butte Reservoir spilled in 1924 in all pre- liminary studies allowing the irrigation study to be started without debits or credits on January 1, 1925, and second, the stream discharge records prior to 1925 are not adequate for determination of tributary inflows and for routing of water from otowi to Elephant Butte Reservoir. Flood control studies were started at the end of the flood season in 1919. Both the irrigation and the flood control studies were started at the end of the flood seasons in order to obtain contents of all reservoirs on the river at the beginning. of 1920 and 1925. Both studies were continued through 1951, the date of the latest published water supply records available at the time the studies were made. Fqr the period 1920-1924 water was routed only as far as Alamosa on a monthly basis and no Compact compliance was computed. With the inclu- sion of the period, 1920-1924, the flood control benefits of a reservoir at Wagon 1r~beel Gap are easily recognizable. Also the inclusion of this period providlls a period of ten years of high flows to test the flood con- trol criteria upon which \'jagon ',1heel Gap Reservoir must be operated in order to obtain maximum flood control benefits. CLIHATE The climate of the San Luis Valley is typical of an arid, high altitude region. The summers are dry, with warm days and cool nights; the winters are cold l~ith bright sunny days; and the springs are generally windy and unpleasant. Rainfall is light, except in the higher mountain sections, and averages from six to nine inches a year over the Valley floor. Total snowfall averages over 100 inches at Wagon wbeel Gap and amounts to about half of the total precipitation .at that location. Heavy rainfalls are rather rare. The maximum 24 hour rainfall on record is 2.60 inches at Wagon \'1heel Gap, and 2.00 inches at Del Norte. Most of the precipitation in the Valley occurs as light rainfall during the summer. Humidity generally is low. Evaporation ranges from twenty inches per year at Hagen loJheel Gap to fifty-seven inches per year at San Luis Lakes. Wind is generally from the southwest and averages 50 miles per day at \vagon \\Jheel Gap. The growing season in the valley is rather short, averaging about 110 days. Figures B-1 and B-2 show typical temperature and precip- itation data. The small amount of precipitation which occurs over the valley illustrates the need for irrigation for crop production. The short growing season greatly restricts the variety of crops that can be grown profitably. r,"''';-- . l'\ '....., , ~- ,,",'... i:.J B-2 , (... r.+!' , v fJ'