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

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

The hydrologic cycle

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)

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.

Measuring Runoff (R) Measure how fast the water is moving in the stream (velocity, ft./sec) 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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