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What is the water (hydrologic) cycle? Why is it important?. The hydrologic cycle. http://observe.arc.nasa.gov/nasa/earth/hydrocycle/hydro2.html. The Hydrologic Cycle. Water input and output is in balance globally. P = R + ET. The Hydrologic Cycle.

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## What is the water (hydrologic) cycle? Why is it important?

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**The hydrologic cycle**http://observe.arc.nasa.gov/nasa/earth/hydrocycle/hydro2.html**The Hydrologic Cycle**• Water input and output is in balance globally P = R + ET**The Hydrologic Cycle**• Water input and output is not always in balance locally • Something is missing • ΔS is the change in groundwater storage P ≠ R + ET P = R + ET + ΔS**The Hydrologic Cycle**• Local variations can have serious consequences (floods or drought) • http://observe.arc.nasa.gov/nasa/exhibits/flood/flood_1.html • http://ma.water.usgs.gov/floods/flood_2006_05.html**Hydrologic Balance**• Measuring the amount of water coming in and going out to assess availability**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).**Measuring Precipitation (P)**• 1 inch = 1/12 feet • 1 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 is 2,323,200 cu. ft. ÷ 86,400 sec/day = 26.9 cu. ft/sec.**Measure how fast the water is moving in the stream**(velocity, ft./sec) Measuring Runoff (R) • Measure the width and depth of the stream = area (sq. ft.) velocity depth width • velocity (ft/sec) x area (sq. ft) = cu.ft/sec

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