Estimation of Evapotranspiration in the Desert–Oasis Transition Zone Using the Water Balance Method and Groundwater Level Fluctuation Method—Taking the Haloxylon ammodendron Forest at the Edge of the Gurbantunggut Desert as an Example

Abstract

Shallow groundwater is an important water source for Haloxylon ammodendron (H. ammodendron). The accurate estimation of evapotranspiration (ETg) from groundwater is of great significance for the water cycle and the maintenance of ecological stability. Using a combination of the water balance method and the groundwater level fluctuation method (WTF), the water balance components (precipitation, soil moisture, groundwater depth, and Bowen ratio meteorological data) in the desert–oasis transition zone were continuously monitored from 2015 to 2018 and the ETg was estimated The results showed that the closed degree of Bowen specific energy after data screening was higher, and the annual actual evapotranspiration (ETa) value could be reliably calculated at 260.87 mm. As the main contributor to water consumption in the growing season, latent heat accounted for 70.16~91.86% of the energy balance. Precipitation had no significant impact on water consumption for H. ammodendron vegetation growth, and the precipitation in the main growing season accounted for 59.44% of the ETa. The groundwater depth in the study area decreased yearly and had a significant impact on the growth of H. ammodendron vegetation. Although the groundwater depth in the study area was greater than 9 m, the ETg, as an important part of the water balance, was found to participate in the evapotranspiration process brought about by H. ammodendron due to the strong root system and supporting capillary water in the soil. The actual evapotranspiration ETa for H. ammodendron in the main growing season was 244.32 mm, and the contribution rate for ETg was as high as 74.78% or approximately 182.35 mm. After the ETg was verified using the water balance method and WTF, R was greater than 0.96, the RMSE range was 1.5931~4.5706, the bias range was −0.15~0.11, and the IOA value was greater than 0.95. The accuracy of the estimation model was high, and the results were relatively accurate. The model can be applied in the desert–oasis transition zone to obtain accurate ETg estimations and provide theoretical guidance and a scientific basis for local water resource management and ecological protection.