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Water Budget

Water Budget. A budget is a statement of income vs. output. In water budgets the income is precipitation and the output is runoff and Evapotranspiration. You can not “spend” more then you have in a water budget. In computing water budgets we use millimeters of water (mm).

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Water Budget

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  1. Water Budget

  2. A budget is a statement of income vs. output. • In water budgets the income is precipitation and the output is runoff and Evapotranspiration. • You can not “spend” more then you have in a water budget.

  3. In computing water budgets we use millimeters of water (mm). • We are measuring the depth of water falling, or evapotranspiring, over an area. • Do not confuse mm for ml – units measuring volume.

  4. Terms • To calculate a water budget you must be familiar (as in know cold) a number of terms and their symbols.

  5. Terms • Potential Evapotranspiration • Ep • This is a fixed value determined by an areas climate and vegetation. • Tropical forests will have very high Ep • Cold regions with little plant life will have Eps close to 0

  6. Terms • Actual Evapotranspiration • Ea • This is the actual amount of water that evapotranspires. • It is determined by the Ep and the amount of water available

  7. The Ea can equal the Ep or be less. • Ea can never exceed the Ep.

  8. Terms • Precipitation • P • The number of mm of water falling during the month.

  9. Terms • Precipitation minus Potential Evapotranspiration • P – Ep • This is the difference between what precipitates and what can potentially evapotranspire. • It can be positive, negative or 0

  10. Terms • Storage • St • The water held in the soil. • We often assume the maximum storage to be 100mm • Storage can be used to supplement P when Ep > P

  11. Terms • Difference in storage (change) • DSt • The amount of water either added to (+) storage or taken out of (-) storage.

  12. If St = 100mm and P> Ep then DSt = 0 • If St > 0 and P< Ep then DSt = P-Ep or the amount in St (which ever is smaller) • If St = 0 and P<Ep then DSt = 0

  13. Terms • Recharge • This is when St is filling. • Anytime St < 100mm and P> Ep. • You can not exceed 100mm. • Ex. If P-Ep = 25mm and St = 90mm then DSt = 10mm and 15mm would be surplus

  14. Terms • Surplus • S • Occurs when P>Ep and St = 100mm • Or • When (P-Ep) + St > 100mm

  15. Terms Usage When water is removed from St when P<Ep You can not use more than what is in St

  16. Terms • Deficit • D • Occurs when St + (P-Ep) < Ep • When there is not enough water in storage to make up for the shortfall in evapotranspiration.

  17. Water Budget Calculator http://www.eserc.stonybrook.edu/ProjectJava/SoilMoisture/

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