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<br />OOlZD4 <br /> <br />-45- <br /> <br />In principle, the reflected short-wave radiation (Q ) can be measured <br />by a separate Eppley pyrheliometer facing downward, overrthe water. Ho~- <br />ever, this leads to the difficulties of a barge installation, previously <br />mentioned. In the previously cited Lake Hefner investigations, Anderson <br />found that the reflected short-wave radiation could oe estimated ade- <br />quately by the equation: <br /> <br />b <br />Rt = as A <br /> <br />(3) <br /> <br />in which Rt = <br /> SA = <br /> a, b = <br /> <br />reflectivity ratio (reflected to incoming) <br /> <br />sun altitude, in degrees <br /> <br />"constants" which vary according to percentage <br />of cloud cover and its height: a, from 2.20 <br />to 0.20; b, from - 0.30 to - 0.98 <br /> <br />It is not always necessary to measure reflected long-wave radiation <br />(Q ) since a reflectivity of 3 percent may be assumed in most instances. <br />ar <br /> <br />To compute the long-wave radiation emitted by the lake (Q ), the <br />water temperature at lake surface must be measured and inserteBsinto the <br />equation: <br /> <br />Qbs = 0.97 d (To + 273)4 <br /> <br />(4) <br /> <br />in which d = Stefan-Boltzmann constant <br />T = water temperature at lake surface, in oc <br />o <br /> <br />As the Q term is one of the largest in the energy-budget equation it <br />must be B~termined accurately. To this end, the true mean water-surface <br />temperaturg must be determined as closely as is feasible, ordinarily <br />within 0.5 C. <br /> <br />Aside from radiation, two potentially decisive factors in the energy <br />budget are advected energy and change in energy storage. A part of these <br />two energy components is expended in modifying water-surface temperature, <br />which in turn modifies the amounts of back radiation and of sensible-heat <br />transfer. Methods for determining the part that is expended by the evapo- <br />ration process are described by'Kohler and others (1955), by Harbeck and <br />others (l959), and by Harbeck (1964). <br /> <br />Computing advected energy requires the measurement of the volumes of <br />water inflow and water outflow, also their mean temperatures. Precipi- <br />tationon the reservoir or lake surface must be included as an inflow <br />term; ordinarily, its temperature is assumed equal to the wet-bulb <br />temperature. <br /> <br />Computing the change in energy storage requires that the temperature <br />and volume of water in storage be measured at the beginning and end of <br />each budget interval. Usually, temperature profiles must be measured at <br />25 to 30 selected points to define adequately the thermal stratification <br />of the entire water body at the given times. From the temperature profiles, <br /> <br />~ <br /> <br />