Spatial-temporal changes in green water and its driving factors in the Bashang area of Hebei Province

The Bashang area is a semi-arid farming-pastoral zone in Hebei Province and represents an important ecological barrier northwest of the capital. Green water plays an important role in maintaining the stability of the semi-arid ecosystem in Bashang and ensuring the safety of the ecological environment in the Beijing-Tianjin-Hebei region. However, a quantitative understanding of the temporal and spatial distribution of green water in this area and underlying driving factors is lacking. This study used Global Land Surface Satellite (GLASS) evapotranspiration products to quantitatively analyze the spatio-temporal changes in green water and evaluated the effect on changes in green water of temperature, precipitation, net radiation (R_n), gross primary productivity (GPP), and land use at the county level (Kangbao, Guyuan, Shangyi, and Zhangbei Counties) from 2001 to 2015. These results provide a scientific basis for the construction of the "Capital Water Conservation Functional Zone and Ecological Environment Support Zone" and more efficient use of the water resources in the Bashang area. This study used the linear trend method to assess the changing trends of green water, and based on the pixel and correlation coefficient methods to analyze the driving factors of green water variation. Multiple regression analysis was used to calculate the contribution rate of each driving factor. The results showed that: 1) the amount of green water in the four counties showed an overall insignificant downward trend from 2001 to 2015. The maximum value was in 2003 (415.34 mm), and the minimum value was in 2009 (322.35 mm). The annual average green water amount was 371.11 mm. The seasonal variation of green water amount was obvious. Summer had the highest amount of green water, followed by spring, autumn, and winter. 2) The amount of green water increased from northwest to southeast regions, most of which showed a decreasing trend. Guyuan County had the highest amount of green water, and Kangbao County had the least amount. Across the regions, 21.2% showed an increasing trend in the volume changes of green water, and 78.8% of the regions showed a decreasing trend from 2001 to 2015. 3) From 2001 to 2015, the precipitation, temperature, and GPP showed an overall increasing trend, whereas R_n showed an insignificant downward trend. Green water was positively correlated with precipitation and GPP and negatively correlated with temperature and R_n. The contribution rate of each influencing factor was in the following order: GPP > temperature > precipitation > R_n. The GPP contribution rate was as high as 51%. The above-mentioned factors primarily affected temporal changes in green water. 4) The highest amount of green water was in the forest land, followed by grassland, cultivated land, construction land, and unused land. There were no significant differences in the amount of green water among land-use types, but the trend of green water changes was significantly affected by land-use changes. Land use directly affected the spatial distribution of green water.