WSP10994 - Laserfiche WebLink

Home Browse Search

001520 taken advantage of where possible. Selecting sites and designing the stock pond with the objective of securing minimum wind movement across the water surface can also have an effect on evaporation losses since one of the important factors influencing the rate in evaporation is air turbulence. Aligning the long dimensions of the pond north-south will reduce solar radiation on the water surface and should reduce evap- oration losses. A striking example of apparent savings resulting from wind insul- ation is represented by the so-called "charco" pits constructed in recent years by the Bureau of Indian Affairs and other agencies. These are off-stream ponds constructed in the form of an inverted cone with a depth ranging from 15 to 25 feet. Excavated material is piled around the periphery making a dike some 4 to 7 feet high, which keeps the wind currents well above the water surface. Water enters the pond through a pipe placed under the dike. Observations on thes e charcos show that invariably they hold water much longer than ponds of similar depth con- structed in conventional manner, in fact, once the charcos are filled, they seldom go dry. A part of the savings may be attributable to reduced seepage loss although in areas underlain by dense shale 'where seepage losses are known to be minimal, the recession in thecharcos is decidedly less than in other types of stock ponds, indicating that evaporation losses are lower. Another example of reduced evaporation attributable to favorable construction methods was obs erved in the Squaw Butte experimental area in eastern Oregon. This area contains a number of trench type charcos located in small playas containing dense volcanic soils. The trenches are aligned in random pattern, but it was noted that in charcos having a north-south alignment, which is about normal to the prevailing wind direc- tion and which also should give the least solar radiation, and having the spoil bank on the windward side, the water level on the average was from 1 to 2 feet higher than in other ponds. It was repo rted that all ponds had been full about 3 months earlier during the spring snowmelt. Also, there appeared to be little difference in the seepage in any of the ponds as the rate of recession in all ponds not aligned normal to the prevailing wind 'wa,s' about equivalent to the measured pan evaporation in the vicinity. Thus, the reduced evaporati,on appeared to be directly attributable to reduced solar radiation and to partial insulation from the wind currents resulting from the position of the spoil bank. Suppres s ion of evaporation by the application of monomolecular films has been the object of considerable research in recent years. Pro- cedures for applying the film and some of the results' obtained in reducing evaporation have been desc ribed in the 'literature and will be mentioned only briefly. 30 ~t'~l :1~~";,/ .,..... "~, ~f~~~ ~;~.{:>~;.~ ?~~r:~~~'~ ;.