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<br /> <br />-46- <br /> <br />mean temperatures are derived to correspond,with suocessive hori- <br />zontal layers of the water body. Then, the product of mean temperature <br />and corresponding volume of water in a particular layer indicates the <br />energy content of that layer above a selected temperature base. The sum <br />of the energy contents of the several layers is the total energy content <br />of the water body. The algebraic difference between the energy contents <br />at the beginning and end of the budget interval is the total increase in <br />, energy storage. For insertion in equation (2), this difference must be <br />divided by the product of the mean water-surface area during the budget <br />interval times the number of days in the interval, to express the in- <br />crease in the usual, energy-budget units (Qk' in "calories per square <br />centimeter per day). <br /> <br />Energy-storage computations, therefore, require:the availability of <br />area and capacity curves of suitable aocuracy. The change of energy <br />storage may be relatively large if the body of water is deep, especially <br />over periods during which the body overturns or changes its thermal <br />stratification notably. <br /> <br />The measured or estimated terms discussed thus far yield the sum <br />Q + Qh + ~, rather than the desired single term Q. However, ~ and, Qh <br />c~n be expressed in terms of Q according to the reiation: <br />e <br /> <br />'cTQe <br />o = <br />"W L <br /> <br />(5) <br /> <br />in which c = specifio heat of water <br /> <br />T = temperature at which evaporation takes place, <br />taken usually as that of the water surface <br /> <br />L = latent heat of evaporation <br /> <br />Also, according to the Bowen ratio (Bowen, 1926). Although there is <br />controversy over validity of the Bowen_ratio theory, it seems to be <br />generally applicable for computating reservoir evaporation. Bowen's <br />ratio is expressed as: <br /> <br />Qh <br />~=- <br />Qe <br /> <br />cP(T - T ) <br />_ ,0 a, <br />- l,OOO(e - e ) <br />o a <br /> <br />(6), <br /> <br />in which <br /> <br />c = a coefficient, 0.6l under ordinary atmospheric <br />conditions, but ranging between 0.58 and 0.66 <br /> <br />P = atmospheric pressupe in mb (millibars) <br /> <br />T = water-surface temperature in 00 <br />o <br />Ta = air temperature in oc <br /> <br />eo = vapor pressure of saturated air in mb, at To <br /> <br />e = vapor pressure in the air in mb, at the height <br />a of the T observation <br />a <br />