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Cooperative Institute for Research in Environmental Sciences <br />UNIVERSITY OF COLORADO BOULDER and NOAA <br />ET is supply of surface <br />moisture to atmosphere <br />E0 is atmospheric <br />demand for ET <br />ET = actual evapotranspiration <br />E0 = evaporative demand <br />ET is supply-limited: <br />ET drives E0 in a <br />COMPLEMENTARY <br />direction. <br />ET energy-limited: <br />E0 drives ET in a <br />PARALLEL direction. <br />Background <br />E0 / ET constraints and interactions <br />E0 is easy to estimate: <br />•Reference ET, ET0 •Potential ET, PET <br />•Pan evaporation <br />Temperature <br />Wind speed <br />Net radiation <br />Humidity <br /> <br />ET not easy to estimate: <br />•Moisture availability <br />unknown on useful <br />scales. <br />•Generally modeled or <br />remotely sensed. <br />Temperature <br />Wind speed <br />Net radiation <br />Humidity <br />Moisture <br />availability E0 <br />ET land <br />surface <br />Θ <br />water availability <br />at surface <br />supply of water from <br />surface to atmosphere <br />demand for ET <br />in atmosphere <br />evaporation under radiative, <br />meteorologic limits only; <br />unlimited moisture <br />evaporation and <br />transpiration under <br />prevailing conditions