Drivers of evapotranspiration increase in the Baiyangdian Catchment

The Baiyangdian Catchment is facing a growing shortage of water resources. Identifying the sensitive drivers of evapotranspiration (ET) changes from land and crop management will be critical to understanding the reasons for mountainous runoff reduction and depletion of groundwater resources in the plain. It will also be important for making Xiong’an become a Future Example City for green and sustainable development. In this study, remotely sensed ET data from PML V2 products with a spatial resolution of 500 m was used to analyze the trend of ET at the pixel level and to understand its influence on vegetation such as GPP (Gross Primary Production) and NDVI (Normalized Difference Vegetation Index) under different land-use types for 2002‒2018. Results showed that there was a significant increase in ET in mountain regions and a slight increase in plain regions of the catchment. The spatial pattern of mean annual ET was very much relevant to the changing trend of GPP and NDVI. For the whole catchment, the average increases of ET, GPP, and NDVI were respectively 2.4 mm∙a⁻¹, 9.8 g∙cm⁻²∙a⁻¹, and 0.0021 at an annual rate. In the mountainous region, changes in annual precipitation and vegetation recovery together caused a total increase of ET by 56.5 mm over the period and negatively affected the runoff. In the plain region, there were 3 factors influencing the change of ET. While intensification of urbanization and reduction in the cultivation of wheat, the water consumptive crop, had both resulted in the decrease of ET and water consumption, ET or water consumption in most irrigated fields increased. Since the beneficial effects from urbanization and crop adjustment were not enough to offset the increase of ET in irrigated fields, an overall ET increase of 6.4 mm over the period was found. In conclusion, both in the mountainous and plain regions, ET increased. And therefore, more efforts are needed to control the ET increase in natural vegetation and cropland for a green and sustainable catchment.