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I 01 " ~" , b-. , g 0_ o~ , I I 0"'~ ! I, iRIO GRANDE AT OTOWr BRIDGE I APRIL I SNOW COVER ONLY 0.. , !05.3 0"'7 I I 1 1 , I ! I 0;-1 , ! "" I year with a relatively low weighting for this month. Rainfall during subsequent months is also weighted and the sum total of the weighted rainfall data at a number of stations is used as an index to runoff for each forecast date. Monthly forecasts are made from January through May, Errors in forecasts become less as the season progresses. The other forecast method is based on seasonal snow accumulation in the high mountains, plus a correction for soil moisture. Fall precipitation (August, September, and October) is generally used as an index of soil moisture conditions under the snoW. On the Rio Grande, for example, this fall precip- itation factor improves accuracy of forecasts very materially. On Figure 1 (below), a relationship is shown between an April 1 snow-cover index and the subsequent April-September runoff for the Rio Grande at Otowi Bridge gaging station. This is for the years 1940-53, Figure 2 shows the relationship for the same station using snow-cover plus a correction based on fall precipitation. The improvement in forecast accuracy is obvious. The effect of the condition of mountain soils under the snow is a major factor in how much water comes out of a given snoW. pack. We are now experimenting with electrical soil moisture units in an attempt to get a direct index of soil moisture in the mountains. These are called Colman Units and were developed by E. A. Colman, - . . < . w o ~ $ '00 Forest and Range Experiment Station, U. S. Forest Service, Berkeley, California. We now have six of these stations on the Rio Grande, but none with more than two years' of record, We hope they will provide us with a tool for improving forecasts. Using fall precipitation as an index of mountain soil moisture under the snow does not work so well on most water. sheds as it does on the Rio Grande. Numerical forecasts of streamflow for annual and seasonal periods published in "Water Supply Fore- casts for the Western United States," by the U. S. Weather Bureau, and in the basin HSnow Reports," published by the Soil Conservation Service, now are coordinated between the two agencies. If forecasts from other reliable sources are known their opinions are refl~cted in these published forecasts. Finally, 1 want to emphasize that the suow survey and water supply forecasts are a cooperative effort between numerOUS Federal, state, and private or. ganizations concerned with water. Among these are the Bureau of Reclamation, Corps of Engineers, U. S. Forest Service, National Park Service, State Engineers, Agricultural Experiment Stations, and several power and irrigation companies. The role of the Soil Conservation Service is to coordinate the snow survey program, make many of the actual field measurements, and to provide data for snow-cover to anyone interested. Forecasts are prepared prin- cipally for irrigation planning for use by Soil Con- servation and Irrigation Districts. . " ~ ~< ~" r , ~.~ ~~. j,:;. F~ ~'.: -'. :.~ " C." [, <;. ~~: !~. ~ ~ -': ~ -;. ~ ~:~ ~~" ," $00 7()O !loo /lot) .oco 1$00 1700 DISCHAAGE 1000 AC.I"T APRIL-SEPT. "DO JOO _0 Figure 1. Relationship Between April 1 Snow-Cover Index and Sub- sequent April-September Discharge of Rio Grande at Otowi Bridge Station. 4 ,"' ' ,.. ~ ; t'*' 0~)~J-'.lJ