Comparisons between a class-A and a new East African Standard Pan: a physical approach

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University of Dar es Salaam
In this thesis, routine long term and special short term comparisons are reported between the worldwide used U. S. Weather Bureau Class A pan and the new East African Standard pan. We use a physical approach via the energy balance method in order to explain the evaporation indications of the two pans, and in particular to understand the differences in evaporation between them. The new East African Standard pan is a class A pan modified by painting the inside matt black and placing a wire mesh cover (bird guard) flush with the rim. Otherwise they are similar. First, a general revie is made of the methods measurements, including a detailed look at the problems affecting pan evaporation. Subsequently the complete energy balance equation is presented, using a rational symbolism and each term is discussed in detail to assess its importance in the total energy balance and more specifically, in differences between the pans. Three major parameters are singled out as contributing most heavily to differences in evaporation between the two pans. They are: the water surface temperature ( and its relation to bulk water temperature), the effective shortwave reflections coefficient (albedo) of the pans, and the air movement over their wate surfaces. These were studied more closely by means of special experiments. In the first stage of the work one year of intercomparions were made between the daily evaporation rates of a class A pan, a class A pan covered with a one inch hexagonal chicken wire mesh (solidity 17%) and an identically covered new East African Standard pan. Also, measurements with a Kenya pan (old East African Standard) were alo included covering was measured to reduce average evaporation of a class A pan by 12% (accuracy of all comparisons to nearest 0.5%). Blackening only, was derived to increase the evaporation rate by an average of 7%, which under the cover reduced to 5.5%. The net effect of the two (i.e. the difference in evaporation between the two main types of pans compared) is therefore a 6.5% reduction of evaporation on the average for the new East African pan compared with class A pan. In order to study the pan water surface temperatures thoroughly, a simple infra-red radiation thermometer after stoutjesdijk was studied in the laboratory using a simple dynamic calibration technique and tried out in the field. Surface temperatures measured using this “Stoutjesdijk”, and the water body temperature profiles obtained using fine wire thermocouples confirm recent conclusions from remote sensing research. Contrary to earlier belief, under several conditions very steep gradients exist in the first tens of um of a water surface. Surface temperatures higher than the water body ones up to nearly 3oC were found to exist during the early morning hours, while in the evenings the surface could be cooler than the body by up to nearly 2oC. By exploring the variation of the temperature profiles over the day a qulitative model of the possible circulation pattern in the pans is presented from literature study on the subject, so as to explain the obseved profiles. Albedo measurements above the pans using small net shortwave radiometers indicate that there are many problems of representativeness arising from the geometrical and radiation distribution complexity of the measured surface. However, the effective aledo measurements have given reliable results with respect to differences between pans. Blackening with cover increased the absorption coefficient by 6%. This is very close to the total of the observed evaporation difference between the two pans due to blackening under a cover, suggesting that most of additional absorption is used up in evaporation. We have also been able to explain largely from our albedo data the small seasonal trends we found in the differences in evaporation rates bwtween the pan. Measurements of air movement profiles above the pan water surfaces using small thermal anemometer probes show that an average reduction of about 25% can be expected due to the presence of the wire mesh cover. However. Our recorder traces suggest an increased turbulence intensity which must reduce the effect of lower air movement. Such a conclusion is effect of lower air movement. Such a conclusion is in line with suggestions from our albedo and temperature measurements that aveporation reduction through covering of a class A pan by a bird guard wire mesh is more a consequence of intergrated effects on the radiation regime than on the air movement regime. Our detailed work on the evaporation pans, using a physical approach, shows how peculiar are the surface temperature patterns, air movement patterns and short wave absorption and reflection patterns of pans only by studying such peculiarities in detail will we be able to understand in particular cases the sense or nonsense of applying pan factors and crop factors to pan evaporation for determining lake evaporation and the water requirements of crops.
Evaporation, Meteorology, Instruments and apparatus
Jiwaji, N. T. (1981) Comparisons between a class-A and a new East African Standard Pan: a physical approach, Masters dissertation, University of Dar es Salaam. Available at (