Energy fluxes and evapotranspiration over irrigated maize field in an arid area with shallow groundwater

Abstract

Precise estimations of energy fluxes and evapotranspiration for cropland in arid areas with shallow groundwater are necessary but limited. There is also a need to better understand the contribution of groundwater for meeting crop water consumptive use. In this study, considering the environmental conditions, such as meteorological variables, crop growth and water regimes, energy fluxes and actual evapotranspiration (ETa) in an irrigated maize field with shallow groundwater were examined using eddy covariance system during two growing seasons. Due to the energy imbalance of the eddy covariance method, latent heat (LE) was adjusted using the Bowen ratio preservation of energy principle and subsequently very close (R2 = 0.94) to that measured by the Bowen-ratio-energy-balance system. For seasonal and diurnal variations, LE accounted for the highest proportion of net radiation (Rn) from the jointing stage. While LE/Rn was a little lower in 2018 than that in 2017 which may be because of the deeper groundwater table depth (WTD) in 2018. The negative sensible heat in the afternoon implied the occurrence of heat advection in the irrigated cropland. Energy fluxes and ETa were significantly affected by crop development when leaf area index was less than about 4 m2 m-2. Moreover, the increasing WTD from 1.2 to 2.2 m decreased LE by about 70 % and increased daily average Bowen ratio. From the jointing stage, total ETa was 522.1 and 453.8 mm in 2017 and 2018, respectively. The inter-seasonal variation in ETa was more dependent on water conditions than the potential evaporative demand. The water balance result showed that average WTD of 1.52 and 1.76 m for the growing period contributed to 36.3 % and 26.2 % ETa in 2017 and 2018, respectively. Besides, daily groundwater contribution between two seasons was different because of the differences in irrigation allocation and WTD. These results highlight the key effect of shallow groundwater to energy fluxes and water exchange of agroecosystem in arid areas.