Temporal characteristics and influencing factors of evapotranspiration and water use efficiency on sloping farmlands with purple soil

Evapotranspiration (ET) and water use efficiency (WUE) are important indices of the carbon and water cycles in farmland ecosystems. There are few systematic studies on the water cycle process in the soil-plant-atmosphere continuum in purple soil areas, and analyses of long-term changes in ET and WUE and its influencing factors in agroecosystems are lacking. In this study, the eddy covariance system was used to obtain carbon-water flux data from 2014 to 2018. The dynamic characteristics of ET and WUE in agroecosystems under rain-fed winter wheat-summer corn rotations on sloping farmlands with purple soil and the impact of major environmental factors were analyzed to provide a scientific basis for seasonal drought and rational water responses. The results showed that the diurnal dynamics of ET had a single peak, and the maximum value of each month occurred around 14:00. ET was highest in August and lowest in January. Seasonally, the magnitude of ET diurnal variation was the largest in summer, followed by spring, and there was relatively little variation in winter and autumn. Leaf area index and air temperature were the most important environmental factors affecting ET of the purple soil in sloping farmlands, followed by the vapor pressure deficit. ET had a linear relationship with net radiation and vapor pressure deficit (P < 0.05). ET also had an exponential growth relationship with air and soil temperature (P < 0.05) but tended to decrease with increasing air relative humidity (P < 0.05). The diurnal dynamics of WUE from 9:00 to 17:00 was first decreasing and then increasing. Seasonally, WUE in winter was the largest, and in winter and autumn was higher than in spring and summer. The leaf area index and carbon dioxide (CO₂) flux were the dominant factors affecting WUE of the purple soils in sloping farmland, and air temperature, relative humidity, and vapor pressure deficit were the secondary factors influencing WUE. WUE decreased exponentially with net radiation and soil temperature (P < 0.05) and decreased linearly with the increasing air temperature (P < 0.05). WUE first decreased and then increased with the soil water content. WUE was significantly negatively correlated with the vapor pressure deficit (P < 0.05) and significantly positively correlated with the air relative humidity (P < 0.05). The differences between two hydrological years showed that WUE in the maize planting period in summer may be more sensitive to precipitation, and the soil moisture in winter had a significant effect on ET and WUE in the sloping farmlands. Due to the lack of data series during this study, further study of the detailed dynamics of ET and WUE in the purple soil sloping farmland ecosystem is needed to explore the systematic adaptation strategies of seasonal drought in the local crops during spring and summer.