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AIR SCIENCES INC. <br />TECHNICAL MEMORANDUM DENVER - PORTLAND <br />CALCULATE PAN EVAPORATION BACKGROUND <br />PREPARED FOR: Jeff Campbell <br />PREPARED BY. Tom Brauch <br />PROJECT NO.: 134-2 <br />COPIES: <br />DATE: June 22, 2006 <br /> <br />This document provides a quick summary?6f the empirical pan evaporation calculation. It <br />describes the important terms, discusses the limitations, demonstrates the comparability of <br />the calculation to measured values, and provides references. <br />Equation Summary. <br />The equation used for calculating pan evaporation (Ea) can be found below (Penman <br />combined equation, Reference 1): <br />Ea = (es - e.)0-88(0.42 + 0.0029vp) <br />es = The water vapor pressure at the water surface at ambient conditions. <br />ea = The water vapor pressure of the air at ambient conditions. <br />vp = The wind run above the pan (in kilometers/ day). <br />The term, es - ea, represents the vapor pressure difference between the surface and the air. <br />This term is the driving force for evaporation/ condensation. When es > ea evaporation <br />occurs. When ea > es condensation occurs. <br />The wind run term ( vp ) takes into account the transport of air over the water surface in <br />kilometers per day. The faster the air is moving across the surface of the pan, the greater the <br />evaporation/ condensation rate. The vp term is adjusted for height. The wind speed at this <br />station is measured at 10 meters. The wind run should be measured at 1 meter above the <br />pan. To correct the wind run for height, we use the following equation: <br />vp = (Average Wind Speed in km/day) x [LN(1m/0.15m)/LN(10m/.15m)] <br />where the average wind speed is measured on site at 10 meters and the 0.15m is the <br />surface roughness at the site. <br /> <br />&ScM"pC*.TECHMEMO06212006.d0C