3The Hydrologic Cycle Water input and output is in balance globally P = R + ET
4The Hydrologic CycleWater input and output is not always in balance locallySomething is missingΔS is the change in groundwater storageP ≠ R + ETP = R + ET + ΔS
5The Hydrologic CycleLocal variations can have serious consequences (floods or drought)
6Hydrologic BalanceMeasuring the amount of water coming in and going out to assess availability
7Precipitation (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).
8Measuring 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.
9Measuring 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