1. Field of the Invention
The invention relates to instruments for measuring environmental evaporativity. Specifically, the invention relates to a device which models the diffusion resistance and albedo of field crops.
2. Related Art
A variety of atmometers or evaporimeters are known for measuring evaporativity of liquids. An atmometer is an instrument for measuring the evaporative capacity of air. The influence of the environmental complex on the vaporitization of water is known as evaporativity.
The simplest type of atmometer is an open pan. Water is placed in the pan, and a variety of methods are employed for determining the amount of water evaporation. Volume readings may be computed from depth measurements, or the pans may be weighed periodically in order to determine how much water has evaporated. Open pans have a number of disadvantages when used to estimate the crop water use or "evapotranspiration" of field crops. Pans are a free water surface which respond to heat absorption (advective energy) i.e., heat carried by the wind different than a crop surface. A free water surface does not exert any degree of resistance to the passage of water vapor into the air as a crop surface does. Also the large volume of water in a typical pan stores heat during the day resulting in evaporation at night which doesn't occur with a crop. Furthermore, wind tends to cause splashing out of the water, so that water is lost as a result of winds and not evaporation. In addition, the creation of ripples and waves increases the evaporating surface in an uncertain and variable manner.
A second type of atmometer is a wet paper or cloth atmometer. This form of atmometer employs a paper or cloth evaporating surface which is kept wet automatically by means of a distilled water supply from a reservoir. The evaporating surface is often in the shape of a disk or circle and can be made from thick filter paper or blotting paper. Evaporation occurs around the cylindrical edge of the circle as weel as from the two planar surfaces. One difficulty encountered by wet paper and cloth atmometers is that they are necessarily exposed unequally to wind from different directions. These atmometers are also free water surfaces offering no diffusion resistance to the water vapor evaporated.
A third type of atmometer is a porous porcelain atmometer. The porous porcelain atmometer comprises a porous porcelain piece having a hollow porous porcelain sphere or cylinder and a downwardly extending cylindrical neck. A rubber stopper seals the lower open end of the cylindrical neck. A tube extends from the cylindrical neck through the rubber stopper into a liquid reservoir. The porous porcelain piece is filled with distilled water and is connected to the water reservoir through the tube. Atmospheric pressure on the water in the reservoir keeps the tube and porcelain piece filled while evaporation takes place from the external porous porcelain surface. Small water-air menisci form over the external pore openings of the porcelain wall. This prevents air from entering into the porcelain structure. Evaporation takes place from the outer surface of the cylindrical neck and liquid water moves up from the reservoir and out through the porous walls at a rate adequate to keep the instrument filled and to maintain the microscopic menisci at the external pore openings of the porcelain structure.
One important application of atmometers is the estimation of water transpired by a green crop in a field. In measuring the crop consumptive water use or "evapotranspiration," it is important to consider a number of factors which will influence this parameter. Two key factors are the diffusion resistance of the crop and the albedo (amount of incident radiation which is reflected) of the crop. The known atmometers, being free water surface evaporimeters, do not account for these factors, and therefore, the estimated crop consumptive evapotranspiration is inaccurate.
The three above-mentioned atmometers are described in Livingston, "Atmometers of Porous Porcelain and Paper, Their Use in Physiological Ecology," Ecology, Vol. XVI, No. 3, July 1935.
U.S. Pat. No. 3,540,277 discloses an evaporimeter which raises the level of liquid to compensate for decreases in liquid level due to evaporation. While the actual amount of evaporated liquid can be fairly accurately measured by the disclosed apparatus, such measurements will not render extremely accurate estimates of crop evapotranspiration, as the diffusion resistance and albedo of crops have not been taken into account in the disclosed apparatus.
German Pat. No. 58953 discloses an evaporimeter having a valve at an upper end. Heat transmitted to the water in the evaporimeter expands and opens the upper valve to permit the evaporation of water. Here, the evaporation takes place from a free water surface, i.e., the surface is open to the atmosphere and does not account for diffusion resistance or albedo.
U.S. Pat. No. 3,898,872 and U.S. Pat. No. 4,412,477 also disclose apparatus which permit water movement or water evaporation from a "free" surface, which, as mentioned above, neglect diffusion resistance and albedo.
U.S. Pat. Nos. 3,886,057 and 4,050,995 disclose a method for determining water vapor transmission rate by mass spectrometer calculations of oxygen and hydrogen generated during electrolysis. Alternatively, the water vapor transmission rate can be determined by the electrical current needed to electrolyze the water vapor. All of the above-described devices do not account for diffusion resistance and albedo of crops. The need for a device which accounts for these parameters is expressed in Rosenberg, Microclimate: The Biological Environment, John Wiley and Sons, New York, Copyright 1974, page 198 of 315.