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Topics for 9/6

Topics for 9/6. Annual water balance and the Budyko curve Project progress. Water Balance. Ratios can be used to “characterize” watersheds Q/P, ET/P… Energy limited vs Water limited systems Budyko (1974) Climate and Life. Physical Controls on Water Balance at Catchment Scales.

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Topics for 9/6

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  1. Topics for 9/6 • Annual water balance and the Budyko curve • Project progress

  2. Water Balance • Ratios can be used to “characterize” watersheds • Q/P, ET/P… • Energy limited vs Water limited systems • Budyko (1974) Climate and Life

  3. Physical Controls on Water Balance at Catchment Scales • Treat as a lumped system • Receives rainfall at the rate p(t), loses water through evapotranspiration e(t), and runoff q(t) • dS/dt = p(t) – q(t) – e(t) • Where S is the storage of water at any time (t) • At the annual scale we assume there is no carry over from year to year and • P = Q + E where P, Q, and E are annual totals • We will investigate the “partitioning” of the water balance at the annual scale • Key Point: Annual water balance is controlled by climate

  4. Bucket Model Assumptions • p and e are uniform through the year • e occurs at potential rate as long as water is available • No runoff if bucket is less than full

  5. Two Scenarios • Case 1: Dry Climate (arid) • Ep>P, and ep(t) > p(t) • … • Actual E is controlled by the amount of rainfall • Water limited system • S/Sb= 0 • No water storage, no vegetation…adverse to life

  6. Two Scenarios • Case 2: Wet Climate (humid) • P>Ep, p(t) > ep(t) • What happens to the bucket? • E = Ep, Q = P – Ep • Upper limit to e(t) is the evaporation demand • Energy limited • S/Sb = 1 • Too wet, adverse to life

  7. Budyko Diagram • Consider the limits • Plot E/P vsEp/P • If E = Ep (wet climate)… • If E/P = 1 (dry climate) … • E/P is a measure of actual water balance • Ep/P is a measure of climate, called dryness index

  8. Limits of Budyko Diagram • Whole world can be divided in to “energy limited” and “water limited” areas • Why doesn’t empirical curve match theoretical?

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