Changes of cropland evapotranspiration and its driving factors on the loess plateau of China.

Agricultural water resource, mainly consumed through evapotranspiration, plays a critical role in agricultural production of arid and semiarid regions. Quantifying the changes of evapotranspiration in cropland (ETc), and its driving factors, may provide rich information for improving human land-use and water resource management. Here we first investigated the multi-year (2000-2015) changes in the ETc (mm yr) and associated driving factors of the Loess Plateau (LP), using a combination of the Vegetation Interfaces Processes model and a factorial analysis of variance. We found that the ETc of the LP showed a significant upward trend of 0.31 km yr (3.33 mm yr) (p < .05) over the last 16 years, mainly driven by cropland changes (3.77% per year), which combined the contribution of cropland area changes and cropland leaf area index (LAIc) changes. We then examined the changes of the dominant driving factor: cropland, and results indicated that the cropland changes consisted of the decrease in cropland area (net decrease of 10.50 × 10 km) and the increase in LAIc (increased by 10.72%), which suggest the actual contribution of the ETc uptrend was the increasing LAIc. Our further analysis on the causes of the increasing LAIc by correlating the LAIc with land-use management factors revealed that the cropland greening on the LP showed high positive correlations with the increasing inputs of total power of agriculture machinery and farm plastic film, followed by chemical fertilizer. The increase of LAIc was also promoted by the increased ratio of the garden fruits output to total crops output (increased by 67.12%) and multiple cropping (increased by 21.66%). These results suggest that the ETc uptrend can be related to the agricultural intensification. Our study highlights the need for a realistic representation of socio-economic development and human land-use practices in the sustainable optimal allocation of agricultural water resources on the LP.