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e e e EM 1110-2-1406 5 Jan 60 vapor pressure difference between the snow surface Hnd the air, and the wind speed. 1\11en the vapor pressure of the air is less tban tbat of tbe snow surface, evaporation occurs. Observations have shown tbat, at middle latitudes during tbe winter 11Ild early spring, tbere is usually evaporation from the snow surface, averaging less than 0.5 inch of water per month, During the melt period in the late spring, the vapor pressure of the air is genrrally in ('xcC'ss of the snow surface vnpor pressure, and condensation results. 7-04. GROUND-WATER STORAGE. Delay to runoff due to ground and ehannd storage is it basic hydrologic phenomenon. Direct enlluat.ioIl of ground-water storage throug-h tile use of \n~ll records is impractical in mountainous areas bCCllllSC of the wide variability of conditions on a drain- age basin. Streamflow-recession analysis provides an indirect means of evaluating basin storage. Determination of volumes of ,vater "generated" in a. given period can be accomplished by use of standard recession analysis techniques. 7-05. TIME DELAY TO RUNOFF. Hydrograph synthesis requires a method for evaluating tbe time-delay to runoff for all component.s of basin storage, including the effect of transit.ory storage in the sllO\vpuck, soil, ground-water aquifers, and surface stream channels. Two commonly used techniques for evaluating basin storage arc unit hydrographs and storage routing. Either met.hod gives acceptable results, as sho\vn by the reconstitution of streamflow from snowlllrlt ilnd rainfall on the Boise River near Twin Springs, Idaho (D.A.=S:30 sq wi), presented on figure 9. In this case the runoff ,vas separated into two major components, i.e., direct runoff (surface and subsllrf:lce) and base flow or gound-water discharge. The runoff components were routed separately, to reflect the differences in storage times. The base flow component represented 30 percent of the total. The storage routing method is convenient for routing snowmelt through basin storage', par- ticula.r with the use of an electronic digital computer. By routing snmvnlclt input amounts through two or more "phases" of reservoir-type storage (as was done for the example shown in fig. 9(a)), a realistic distribution of outflows can be developed. This procedure 45 has the advantage of flexible fit of data by trial-and-error reconstitution of historical data, through use of time of storage whicb is allowed to vary with discharge. Another advantage is the case of adjusting computed strearn- flows to observE'd conditions on a day-ia-day basis, which is of value in rate-of-flow forecasting procedures. It is not within the scope of this chapter to discuss geIlerally the derivation of routing methods. Figure 10 sbows the results of routing a hypotbetieal continuous snowmelt of fixed diurnal variation, from zero flow at the beginning time, by use of the "lllultiphasc" rrsrrvoir-typc storage routing technique, for a drainage basin of 8,760 square miles. The three curves shown on figure 10 show the effect of varying tbe routing coefficients in evaluating the time-delay of rUIloff. 7-06. FROZEN SOIL. Cuses are known where runoff has been accelerated and losses reduced significantly because of frozen ground. The ground is gerwl'nlly unfrozen beneath deep mountain snowpacks hccnuse of the flO\\~ of heat from the ground, togrthcr with the inslllilting efTl'ct of thr snowpaek. Frozen ground will occur during winter or early spring, in areus where snowpaeks are shallow and where prolonged periods of subfreezing air temprrnturcs prevail. Such conditiollS arc characteristic of the northern Great Plains regions of thC' Cnited States. In general, the efl'cd of frozen ground is to inhibit. infiltration. In caseS \\"l1('re the soil pores arc small, liquid water entering the ground will refreeze within tbe surface layer and will retard further infiltration. Accordingly, the concept of satisfying soil moisture deficits for unfrozen soil would not apply and, in addition, the basin time-delay for ,yater in transit would be considerably reduced. Quantitati\'c evaluation of these factors cannot presently be made; t.lwir effect would vary \vith soil conditions, depth of freezing, topography, vegetation, and other ph:ysicul character- istics. In extreme cases, it would be possible that essentially no loss would oerur to the underlying soil or ground-water storug(',\ so that runoff rates \yould be considerably greater than for the normal unfrozeIl ground conditions. 25