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;.1 .i1 ":j :j STREAMFLOW FORECAST FROM SNOW AND RAINFAll DATA by HOMER J. STOCKWELL Snow Survey Leader, Soil Conservation Service U. S. Department of Agriculture, Fort CoIlins, Colorado ~;l ." Runoff in the Rio Grande for the past few years has been so low that water shortages seem almost normal. In fact, the average flow of the Rio Grande for the ten-year period, 1945 to 1954, has been only about two-thirds that of the previous ten years. Now, if we say that streamflow is going to be average J it also can be assumed that it will be short. Several years of deficient precipitation, along with rapid increases in water requirements, are responsible for the current shortage. According to a report of the National Resources Committee a water shortage such as the present one occurred about 1900.' Long range forecasting of streamflow can be done only when it is possible to measure the major factors affecting streamflow well in advance of their occur- rence. Practically, this means streamflow that originates from snow..melt. The snow accumulates during the winter months and melts during a rel- atively sbort period in April, May, and June. The amount of the snow accumulation during the winter bears some relation to the amount of streamflow during the following spring and early summer. The effect of snow accumulation on streamflow varies throughout the West. Forecasts based on seasonal snow accumulation are most accurate in the Pacific Northwest and along the Sierra Nevadas in California, On the east slope of the Continental Divide, large errors can be expected. On the West Coast, most of the annual precipitation comes as rain or snow in the late fall and winter. Summers are dry, East of the Continental Divide, the heavy precipitation months are April, May, and June. Water in the snow- pack on the West Coast exceeds 30 inches in most places at the high elevations. In Colorado or New Mexico, a seasonal snow water accumulation of over 20 inches is rare. The main stem of the Rio Grande is somewhat of an exceptional stream in this area. Seasonal snow.. pack is rather high along the Continental Divide in Colorado and northern New Mexico. Annual precip.. itation, even in San Luis Valley, is almost neg.. ligible in providing sustained runoff. These condi.. tions make forecasting more accurate than on other east slope watersheds. On the other hand, fore- casting of snow-melt season runoff for streams originating in the Sangre de Cristo Ra.nge is very "". ~; ~-,' " ~.; ~;1 '< :~ .~ :'~ 1Reiional Planning.. Part VI - The Rio Gl'ande Joint InvestiAstion in the Uppel' Rio Grande Basin, 1936-37. National Resources CommIttee. Washington. D. C., February 1938. Page 17. difficult. The winter snowfall is light and spring and summer rainfall varies within wide limits. In general, there are three classifications of hy.. drologic information affecting the runoff of the stream which must be considered in making a streamflow forecast. These are: 1. Known information such as snow course meas- urements, precipitation over a definite period, past streamflow, ground-water levels, temperatures and others. 2. Information that could be made available at the time of the forecast but for which there is no adequate record, such as soil moisture under the snow and ground-water levels in the mountains. 3. Information on climatic behavior following the forecast date, Such data are not available to the streamflow forecaster. Therefore, in making a fore.. cast it is assumed that these occurrences will be near average. The forecaster even then may make wide errors in his estimates if the index of the factors he is meas.. uring is inadequate, or if there is a wide divergence from normal in the factors not measured, or in those yet to occur. Soil Conservation Service procedures relate runoff to the snow-pack minus expected losses. Most of the snow-melt goes into the soil before runoff occurs. Little surface runoff occurs unless the soil under- neath is saturated and then only in the presence of high temperatures or rainfall. Snow-melt must replace any deficiency in soil moisture under the snow and perhaps some of the ground water beneath the plant root zone. Then there are losses to evaporation. In my opinion, and this is backed by research of others, evaporation losses are not as great as the general public might suppose. Also, evaporation losses do not vary much from year to year. In practice, current forecasts are based on an index of precipitation and snow-cover. Some fore- casters use temperatures and streamflow in the fall as a small part of the index of a forecast formula. Whatever index may be us~d, it is compared to the subsequent summer runoff, thus building up a rela- tionship between an index of events that occur during the winter months and the resulting streamflow. There are two general systems for public fore- casts, One is based on a proportional part of monthly rainfall at stations' near the mountains. The index usually starts with August rainfall of the previous -\ .. I t;_~ " ,"'i '! l;,,~ 4... .11.. 3