. Compendium of meteorology. Meteorology. T 1 1 —T RAPPAHANNOCK RIVER NEAR FREORICKSBURG, VA. DRAINAGE AREA= 1599 SQ, Ml. -1 I 1 L. 3 4 5 6 7 8 TIME IN DAYS Fig. 4.—Typical unit hydrographs. The problem of predicting the basin-outflow hydro- graph is quite similar in its theoretical aspects to that of routing a flood wave downriver. The ordinary rout- ing procedure makes it necessary to adopt routing periods so short that the computations for a headwater basin would be too time-consuming for forecasting pur- poses. However, the electronic routing machine [6] makes this approach a practical rea
. Compendium of meteorology. Meteorology. T 1 1 —T RAPPAHANNOCK RIVER NEAR FREORICKSBURG, VA. DRAINAGE AREA= 1599 SQ, Ml. -1 I 1 L. 3 4 5 6 7 8 TIME IN DAYS Fig. 4.—Typical unit hydrographs. The problem of predicting the basin-outflow hydro- graph is quite similar in its theoretical aspects to that of routing a flood wave downriver. The ordinary rout- ing procedure makes it necessary to adopt routing periods so short that the computations for a headwater basin would be too time-consuming for forecasting pur- poses. However, the electronic routing machine [6] makes this approach a practical reality. In fact, the use of headwater forecasting techniques may be ex- tended to areas much larger than can now be success- fully treated with the unit hydrograph. Snow-Melt Forecasting. Forecasting the runoff from melting snow poses many problems which have not yet been solved satisfactorily. In level terrain, the rate of melting of snow has been forecast with reasonable accuracy by use of factors expressing melt per degree day above 32F. If melting is concurrent with rainfall, the water equivalent of the snow may be added to the rainfall and a relation such as Fig. 3 used to compute runoff. In mountainous terrain the situation is much more difficult. Here the snow pack accumulates in the form of a wedge with its greatest depths at high elevations. tapering to zero depth at intermediate and low levels. Owing to the variation of temperature with elevation, melting normally occurs over a fairly narrow range of elevation immediately above the snow line. The term "snow line" is used to describe the lower limit of the snow pack and does not imply a contour of constant elevation. As this melting zone moves upslope with the annual rise of temperature, the distance to the outlet of the basin increases and the effect of a unit of melt on the outflow changes. Although it is believed that our understanding of the problem is sufficiently complete to permit a solution by the u
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