Evaporation from irrigation canals in the middle reaches of the heihe river in the northwest of China - A preliminary study

In the northwest of China, the leakage and evaporation from irrigation canals is an important issue from the stand point of efficient use of water resources, because long canals have been constructed for farmland irrigation. In order to estimate the evaporation from irrigation canals a preliminary study was conducted in the middle reaches of the Heihe River during the summer months based on the heat balance method. The rate of evaporation from irrigation canals depends on the temperature difference between canal water and air, the air humidity, and the wind speed. The present observations showed that the seasonal variations of the temperature difference and the wind speed were not large in the observation period, and the seasonal variation of air humidity had a large influence on the evaporation rate. Since the Heihe River rises from the Qilian Mountains and the river water is melt-water, the water temperature is rather low compared to the air temperature. As a result, daily evaporation depth was often almost zero or negative. The results of this study were obtained by applying the relationship between the heat transfer coefficients and the surface wind speed empirically derived by Mihara et al. (1959b), so the results are only approximate. Hence, these results should be verified against observational evidence before a final conclusion can be drawn.