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Published byArmando Kenderdine Modified about 1 year ago

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

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

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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

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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

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

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

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

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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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