Establishment of water surface evaporation model and determination of key parameters for plain reservoir in Northern Xinjiang

Abstract: The temperate continental arid semi-arid climate in plain area of Northern Xinjiang has some characteristics: Air is dry, temperature difference is large, air pressure change is obvious, and wind is large and speedly. In these areas, there are more than 400 plain reservoirs which have long dam, wide area and shallow water depth. Water surface evaporation is molecular exchange phenomenon that occurs between the water body and the atmosphere, which is one of the main reasons for the loss of water resources in the above arid regions, so the water resouce annual loss is about 40% due to evaporation. At present, there is an irreplaceable role in farmland irrigation in Xinjiang. The calculation error is relatively larger about water surface evaporation model, and it is impossible to estimate reservoir evaporation accurately because the model can not fully consider characteristics of plain reserviors in aird and semi-arid area. Based on the Dalton's model, fully considering the 4 influencing factors, i.e. air pressure difference, wind speed, humidity and temperature, a new calculation model is built on water surface evaporation. Each influencing factor of model was obtained by theoretical analysis, and key parameters in formula were obtained by 20 m2 evaporation pool. Based on complete observation data of hydrology and water resources in Daquangou Reservoir of Shihezi in Xinjiang, the evaporation was calculated by water balance method, and calculated results by 4-factor model were basically in accord with observations. At the same time, considering scale differences, the model is further modified by introducing correction coefficient. We find the research conclusion "20 m2 evaporation pool results can take the place of large water bodies on evaporation" is bigger error, and needs to be modified. Correction factor is finally determined by deep analysis of observation data, and correction coefficients were 0.886 and 0.939 in freezing and non freezing periods respectively. So the 4-factor calculation model after optimization can not only accurately simulate water surface evaporation of Daquangou Reservoir, but also fit the evaporation of 7 reservoirs in Northern Xinjiang, and accuracy and reliability of model are verified. The data required by model are basic meteorological data, easy to obtain, simple to calculate. To summarize, the evaporation model is correct, and it can accurately estimate evaporation of plain reservoir in inland arid area, which can provide theoretical support for rational utilization and scientific management of water resources in arid inland area. In the analysis of the evaporation model, the humidity, temperature, wind speed and air pressure are considered as the determinants, and the influences of other factors are ignored. The observation precision of evaporation on the surface of reservoirs depends not only on the above factors, but also on other factors, such as the fine degree of observation method, the size of the reservoir area, the surrounding occlusion rate, the climate change, the vegetation height, water depth, precipitation, and water quality. If the weight of the influential factors of the selected reservoir is different from the factors considered in this paper, a more accurate mathematical model will be needed again.