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Abstract
The climate of Central Iraq is continental subtropical. The long summer is characterized by a low atmospheric pressure, very high temperatures, low atmospheric humidity and frequent periods of strong wind. The maximum temperatures can be ranked among the highest on the earth. It is however ad- visable not to accept statements about record temperatures without critizism (Chapter I).
It was necessary to take special precautions such as screening against radia- tion to measure air temperatures accurately to some 0.1°C. The height at which the measurements are taken is highly important as is the nature of the surface. For instance temperatures measured at a height of 0.10 m were often 5-10°C higher than at a height of 2 m at noon (Chapter 11).
The usual formulas are not reliable for the calculation of the total global radiation of the sun. Owing to the absence of cloud in Iraq, the value of the radiant intensity arrived at with the help of the usual formulas is often excessive. When clouds are present the radiation is generally calculated as being too low. Better results were obtained with a new relationship between cloudiness degree and duration of sunshine, discovered by the author. It was important to relate the horizontal visibility to the total global radiation, but a strong shift from direct to diffuse radiation had been observed. Special attention was paid to the absorption coefficient of a free watersurface. Using a constant value for this magnitude may introduce considerable errors in the calculations of the radia tion absorbed by water surface (Chapter 111).
The usefulness of the evaporation pan has been discussed. It was explained that it is not possible to consider the evaporation from a given area only, since the nature and shape of the evaporating surface will have a great influence. The ratio of the evaporation from two evaporating surfaces depends on weather conditions and consequently on the time of the year. Thus the requirement that an evaporimeter has to indicate the evaporation from a crop, cannot in general be satisfied. At best it will be possible to satisfy it in a special case. Neither is it possible to establish a constant ratio between the evaporation from a surface and its potential" or latent" evaporation. In arid regions especially the assumption of a constant ratio may lead to considerable errors. An evapori- meter as an indicator for irrigation requirements can, however on the other hand be very useful. The U.S. Weather Bureau Class A evaporation pan was found to be satisfactory as an evaporimeter. This was not the case with the Piche evaporimeter.
In order to obtain a good understanding of the behaviour of the evaporation pan, the energy-balance of the pan was calculated. It was pointed out that the theory given by the U.S. Weather Bureau (ie. Kohler) leads to inaccurate results, whereas it has also appeared that the position and handling of the pan influences the evaporation considerably. It is necessary to study the heat exchange air/pan and to construct the pan of a material which in the open field will retain a con- stant absorption coefficient for long wave as well as for short wave radiation (Chapter IV).
The calculations are given in the appendix.
Evaporation from a lake can be calculated in several ways, provided that the temperature of the surface is known. In the present case a lake had to be con- structed for waterstorage for irrigation and surface temperatures were, there- fore, not available.
The author has attempted to calculate the temperature of the surface using elementary meteorological quantities. The method was checked on the Lake of Tiberias, and a reasonable agreement obtained. Finally the evaporation from a fictitious lake with a depth of 10 m was calculated and that from a fictitious lake with a depth of 50 m. The evaporation from these two lakes appeared to be unequal. Several border effects on the evaporation such as the influence of wind, atmospheric humidity and temperature could only be discussed quali tatively. A quantitative treatment of these problems is not yet possible owing to insufficient knowledge of the turbulent heattransfer in air. The yearly eva- poration from these two lakes appeared to be approximately 2500 num.
