WSP04999 - Laserfiche WebLink

Home Browse Search

becomes a primary source of added heat and possibly subsequent above normal evaporation. The quantity Qe is the latent heat of vaporization. It varies slightly with temperature at which the process occurs but is close to 585 calories per gram of water evaporaled at typical reservoir tempera- tures. This parameter is included in the model. Qh is the energy conducted out of the reservoir as sensible heat. This represents a very small portion of the heat budget (Harbeck, et al.. 1958) which will become a negative influence on suppression in summer and a positive influence in winter. Analysis of the size of this parameter compared to other factors in the energy budgets of the USGS studies indicates that only negligible error is introduced by ignoring the change in this parameter caused by thermal mixing. Qw is the energy advected by evaporated water. This is the smallest of all items in the USGS energy budgets and may be ignored (or could easily be added to the latent heat computation QeJ. In summary, two of the nine energy budget parameters appear to dominate the calculation of evaporation suppression secondary effects; these are Qe (evaporation latent heat) and Qv (the outflow component). It would appear that without any outflow, there would be no net annual suppression; that is, the savings during the summer season would be essentially completely dissipated by increased evaporation later during the decay of residual heat added by suppression. However, man-made impoundments do typically have large outflows and the increased r-utflow temperature should represent a significant net savings. The importance of the beneficial effect of outflow temperature increases due to destratifica- tion is interesting in relation to previous research on monolayer suppression. The fact that deep outlet temperatures are not increased during monolayer treatment suggests that the auded heat from reduced evaporation would tend to limit suppression by that melhod to a net seasonal amount which would tend to approach zero when analyzed on an annual basis. In addition to this advantage of thermal mixing over the monolayer concept in the long lerm. a similar advantage occurs with daily suppression rates. During the monolayer operation, above normal evaporation begins immediately upon wind stripping of the chemical film because of added heat which accumulated near the surface during film coverage. Although a similar amount of heat is added to the water during suppression by thermal mixing, it is continuously mixed and distributed equally throughout the reservoir rather than concentrated above the thermocline. At any point in time, therefore. the surface temperature increase due to suppression itself is much less than for the monolayer method. Summary of model conceptualIzation The suppression by thermal mixing model which has been described and justified in general terms here and which is developed in detail in a later section can be conceptualized in abbreviated form as follows: Basic Concept: Idealized suppression IS calculated as a function of change in water surface temperature (ergo change. in vapor pressure) caused by perfect mixing (isothermal temperature profile). Secondary Effects: The secondary effects which are sufficiently important to be included in the model are: (I) added heat due to the decrease in heat loss due to evaporation caused by suppression during a previous period. and (2) heat loss from the reservoir due to wam1er than normal outflow from the mixed reservoir. Time Resolution: Model Parameters will be determined on a monthly average basis. Heat flux will be accumulated between months up to six months for reservoirs on which a seasonal (May to October) analysis is appropriate and for annual or multi-year periods where carryover storage is important. Errors In the model A source of error which has not been discussed previously is that the model assumes perfect thermal mixing: that is, conversion of the normal thermocline into an isothermal temperature profile. Results on many reservoirs which have been mixed for water quality objectives indicate that this is feasible except for a very minor diurnal variation on the order of ICe. Many of the empirical results did not achieve this degree of mixing because pumping was limited to that necessary for desired dissolved oxygen levels. But those projects on which pumping with sufficiently large energy sources were operated continuously seemed to produce almost perfect mixing. No correction for this non.conservativE': error was incorporated into the model since it is considered to be relatively small. 14