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<br />was that a suppression model based on the <br />equations developed herein clearly gives aconserva- <br />tive estimate of suppression for periods when the <br />temperature of the air averages not less than the <br />water surface temperature. This was the case for <br />the evaporation pans even during October and this <br />inequality should be even stronger for reservoirs <br />during the summer when the majority of evapora. <br />tion occurs. <br /> <br />The basic relationship for the detailed sup- <br />pression model described later is the equation <br />derived above: <br /> <br />Suppression = I . fIT c)!f(T n) <br /> <br />Exten.lon of model from pRD to <br />lake e.aporatloD <br /> <br />There appears to be no error introduced by <br />lreating wind as a constant and thereby eliminating <br />it from the ratios derived in the previous section <br />because thermal mixing has no measurable effect <br />on wind. The elimination of relative humidity, as <br />being independent of water temperature, however. <br />requires some qualification. It is no doubt true that <br />in the case of an evaporation pan the relative <br />humidity (or ea) is independent of a change in <br />water temperature. The air mass passes over the <br />pan too quickly to be effected by the lower <br />boundary temperature. This should also be true of <br />small reservoirs but probably will not be true in the <br />case of larger reservoirs. <br /> <br />The effect on vapor pressure of the air over <br />large bodies of water depends on the relative <br />temperatures of the air and water. The typical <br />period of principal concern (the period of high <br />evaporation) is a summer day when water is <br />typically cooler than air. In this circumstance the <br />air just above the water surface is cooled and it also <br />picks up additional water vapor (by evaporation) as <br />it moves across the water surface. Both of these <br />changes (temperature and quantity of vapor) tend <br />to increase relative humidity, to decrease the term <br />(l.R.H.) and therefore to decrease the evaporation <br />rate as the air mass continues across the water <br />boundary . <br /> <br />In order to understand the effect of artificially <br />cooling the water surface (by thenTIal mixing) and <br />the error introduced by the simplified model, it is <br />necessary to consider evaporation in the following <br />four situations, (initially using the Dalton equation <br />and then using the ratios developed for the model): <br /> <br />I. Natural condition at an upwind point <br />where an air mass is just beginning to pass <br />over the water surface. <br /> <br />2. Thermally mixed (cooled surface) condi. <br />tion at the same upwind point. <br /> <br />3. Natural condition at a downwind point <br />where the air mass has traveled across a <br />long expanse of water. <br /> <br />4. ThenTIally mixed condition at the same <br />downwind point. <br /> <br />Figure 5 shows the change in vapor pressure <br />for assumed temperature and humidity conditions <br />representing the four situations described above. <br />The following assumptions and parameter values <br />were used in developing Figure 5: Because of the <br />huge difference in specific heat of water compared <br />to air, the air temperature is effected (cooled) by <br />the water between points I and 2, but the water <br />temperature is not changed by this temperature <br />gradient. The air and water temperatures used <br />were measured at the reference evaporation pan (to <br />be discussed in the next section) on September 7. <br />This day was selected because it had a dewpoint of <br />.7AoC which was the average dewpoint for a <br />2.month August and September period. These <br />particular values were selected because they should <br />represent a typical relationship of air, water, and <br />humidity parameters at a canyon mouth site in <br />northern Utah. Other assumptions were that <br />thenTIally mixed water will be 50C colder than <br />natural surface water and the air temperature at <br />the downwind point is cooled by 50 percent of the <br />air/water difference. The increases in humidity at <br />the downwind point were selected arbitrarily but <br />with reasonable relative values. The notation is <br />defined on the figure. <br /> <br />Evaporation for the four cases can be <br />computed by the fOnTI of the Dalton equation, <br />which has been discussed previously and which <br />does not include the error introduced by the <br />approximations used in the model: <br /> <br />E = (esw . ea) K <br /> <br />(I) At point I under natural conditions: <br /> <br />Enl = (eswn . ea,)K <br /> <br />The vapor pressure deficit is shown in <br />Figure 5 as <br /> <br />l>enl' <br /> <br />(2) At point I under mixed conditions <br />(cooled surface): <br /> <br />EcI = (eswc - eal)K <br /> <br />10 <br />