Report

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 • 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 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 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 Two Scenarios • Case 2: Wet Climate (humid) – P>Ep, p(t) > ep(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/Sb = 1 • Too wet, adverse to life Budyko Diagram • Consider the limits – Plot E/P vs Ep/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 Limits of Budyko Diagram • Whole world can be divided in to “energy limited” and “water limited” areas • Why doesn’t empirical curve match theoretical?