Laserfiche WebLink
<br /> <br />-44- <br /> <br />Energy bud~et <br />The energy budget per unit area and time is given by the equationl <br /> <br />Qe = Qs - Qr + Qa - Qar - Qbs + ~ - Qh - ,~ - Q~ (~) <br /> <br />in which <br /> <br />Qe <br />Qs <br />Qr <br />Qa <br />Qar <br />Qbs <br />~ <br /> <br />= energy utilized by evaporation <br />= incoming short-wave (solar) radiation <br />= reflected short-wave radiation <br /> <br />~ <br />Qk <br /> <br />= incoming long-wave (atmospheric) radiat~on <br /> <br />= reflected long-wave radiation <br /> <br />= long-wave radiation emitted by the body of water <br /> <br />= net energy advected into the body of water, <br />including that of precipitation but excluding <br />that of evaporated water <br /> <br />Qh = energy conducted from the body of water to the <br />atmosphere as sensible heat <br /> <br />= energy advected by evaporated water <br /> <br />= increase in energy storage of the body of water <br /> <br />The computations are usually made in units of calories per square centi- <br />meter per day. <br /> <br />The energy-budget theory long has been accepted as sound, but only <br />subsequent to about 1950 have instruments of the required accuracy become <br />available. The main objections to the technique are the needs for <br />expensive instrumentation and careful maintenance to assure reliable <br />observations. The computations are laborious, but this limitation has <br />been partially overcome by the advent of high-speed electronic computers. <br /> <br />In principle, the most convenient method for evaluating the five <br />radiation terms of equation (2) would be by net radiometer installed on <br />a barge or float in the lake. However, net radiometry by a single instru- <br />ment currently is not sufficiently precise; also, a barge installation <br />involves serious difficUlties in operating and adequately maintaining <br />instruments of all kinds. Therefor~, the usual and more practical <br />installation is on land and includes (l) usually an Eppley pyrheliometer, <br />facing upward, to measure the incoming short-wave radiation (Q ); and <br />(2) a total or "hemispherical" radiometer, which measures the Bum of the <br />incoming short-wave and incoming long-wave radiations (Q + Q). Thus, <br />incoming long-wave or "atmospheric" radiation (Q ) is deiermifled by sub- <br />tracting the Eppley observation from the hemisph~rical observation. The <br />Eppley pyrheliometer shoUld be corrected for temperature effect although <br />this has not been common practice in the past. Currently, it is possible <br />to purchase temperature-compensated instruments. <br />