After spring begins, evaporation intensifies remarkably with the rise in temperature, scarce precipitation, dry air and strong winds typical of North China. Potential evaporation from March to June accounts for about 52 per cent of the year's total, with a peak value in June. It drops during the subsequent rainy season from July to September, when it constitutes only 30 per cent of the annual total. It is also called field evapotranspiration. It is one of the most difficult water balance elements to determine because of the threefold influences of biology, soils and weather.
Numerous methods are available for determining and estimating field evaporation.
The Institute of Geography carried out a comparative study of all these methods except vorticity correlation. It was felt that the measuring error for the gradient method is the highest, with a difference of 20 to 100 per cent from evaporimeter measurements. This method needs to be improved and its suitability for China is pending further study. The energy balance method has a good theoretical basis in the concept of energy conservation, but it is difficult to obtain accurate observations when the sky is overcast or cloudy or when there is a temperature inversion. In recent years, the accuracy of observation abroad has been greatly improved by the application of computer techniques and the installation of automatic measuring and recording equipment.