3 The Hydrologic Cycle Water input and output is in balance globally P = R + ET
4 The Hydrologic CycleWater input and output is not always in balance locallySomething is missingΔS is the change in groundwater storageP ≠ R + ETP = R + ET + ΔS
5 The Hydrologic CycleLocal variations can have serious consequences (floods or drought)
6 Hydrologic BalanceMeasuring the amount of water coming in and going out to assess availability
7 Precipitation (Input) How do we measure it?Rain gauge collects 1 inch of rain. What does that mean?In a one-inch rain, what is the volume of water that falls on an area of one square mile? (Don’t have to do the numerical calculation; just outline the steps).
8 Measuring Precipitation (P) 1 inch = 1/12 feet1 sq. mi = 5,280 ft x 5,280 ft = 27,878,400sq.ft.27,878,400 sq.ft. x 1/12 feet = 2,323,200 cu.ft.2,323,200 cu.ft. x 7.48 gal./cu.ft =17,377,536 gals. (17 x 106 gals.)If rain fell over 1 day, the rate is2,323,200 cu. ft. ÷ 86,400 sec/day =26.9 cu. ft/sec.
9 Measuring Runoff (R)Measure how fast the water is moving in the stream (velocity, ft./sec)Measure the width and depth of thestream = area (sq. ft.)velocitydepthwidthvelocity (ft/sec) x area (sq. ft) = cu.ft/sec
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