Evapotranspiration of an oasis-desert transition zone in the middle stream of Heihe River, Northwest China

As a main component in water balance, evapotranspiration is of great importance for water saving and irrigation-measure making, especially in arid or semiarid regions. Although studies of evapotranspiration have been conducted for a long time, studies concentrated on oasis-desert transition zone are very limited. On the basis of the meteorological data and other parameters (e.g. leaf area index (LAI)) of an oasis-desert transition zone in the middle stream of Heihe River from 2005 to 2011, this paper calculated both reference (ETo) and actual evapotranspiration (ETc) using FAO56 Penman-Monteith and Penman-Monteith models, respectively. In combination with pan evaporation (E-p) measured by E601 pan evaporator, four aspects were analyzed: (1) ETo was firstly verified by E-p; (2) Characteristics of ETo and ETc were compared, while the influencing factors were also analyzed; (3) Since meteorological data are often unavailable for estimating ETo through FAO56 Penman-Monteith model in this region, pan evaporation coefficient (K-p) is very important when using observed E-p to predict ETo. Under this circumstance, an empirical formula of K-p was put forward for this region; (4) Crop coefficient (K-c), an important index to reflect evapotranspiration, was also analyzed. Results show that mean annual values of ETo and ETc were 840 and 221 mm, respectively. On the daily bases, ETo and ETc were 2.3 and 0.6 mm/d, respectively. The annual tendency of ETo and ETc was very similar, but their amplitude was obviously different. The differences among ETo and ETc were mainly attributed to the different meteorological variables and leaf area index. The calculated K-c was about 0.25 and showed little variation during the growing season, indicating that available water (e.g. precipitation and irrigation) of about 221 mm/a was required to keep the water balance in this region. The results provide an comprehensive analysis of evapotranspiration for an oasis-desert transition zone in the middle stream of Heihe River, which was seldom reported before.