Man’s attempt to control the amount of available water has taken two approaches, i.e., weather modification and water storage through the use of man-made reservoirs. In either case, it is necessary to ascertain the water vapor content of the atmosphere. In the case of weather modification, if the water vapor content is insufficient, it is futile to attempt to start precipitation. In the case of reservoirs, it is important to know the evaporation rate since it has been demonstrated that some reservoirs provide a net negative contribution to the total water supply by increasing the normal evaporation losses. Any attempt to control evaporation losses requires a measuring system to determine the effectiveness of the method. Presently available methods of measuring atmospheric water vapor content are severely limited in that they provide data only at the point of measurement whereas the value desired is an integrated figure over a long path (1/2 to 2 miles or more). Regarding the ineffectiveness of present methods, Malone et al. (1951) comment on the uncertainties of relating evaporation pan measurements to actual surface evaporation losses and say “A technique for determining evaporation, either by direct measurement of outgoing moisture or by computation from the energy balance, turbulent transfer, or other theory is urgently needed.” Although the above reference was published in 1951, the need for measurement mentioned is still unfilled. Donnan et al. (1964) say: “Point observations in space and time of variables affecting evaporation and consumptive use have limited value because of the difficulty making these observations representative of the area and time duration being studies. Increased effort should be directed toward observations which are integrated measurements. ”The system described herein is intended to fill the need outlined above. It will provide a measure of the integrated water vapor content over any chosen path provided only that each end of the path is accessible for equipment installation. The area required for installation at either end is not excessive since the base dimensions of the receiver are 13 x 17 inches and those of the light source are 1 x 3 feet.
Woffinden, Duard S., "Water Vapor Measurements Using Infrared Absorption" (1965). Reports. Paper 4.