Assessment Of Spatial And Temporal Variability Of Green And Blue Water Flows In The Heihe River Basin,China

Ensuring sufficient water supply is essential for the survival and sustenance of humans and ecosystems. In the long term, insufficient water availability for essential ecosystem functions and services can lead to ecosystem degradation with consequent impacts on overall water scarcity and human well-being. Water resources assessment and management often emphasize on blue water, ignoring green water. Conceptually, water can be divided into green water and blue water. Blue water is the water in rivers, lakes, wetland and shallow aquifers, while green water is precipitation water stored in unsaturated soil, and later used for evapotranspiration. Although green water is often ignored, it plays an essential role in crop production and other ecosystem services. The green/blue water research has become a hot topic of water resources research.The concepts of green and blue water make the scientific community start a new way of thinking and understanding of water resources and their assessment framework. In particular in arid and semi-arid regions, water use competition is intense between human and ecosystems:hence, a comprehensive assessment of green and blue water resources in a spatially and temporal explicit way is a key to deepening the understanding of the renewable water endowments as well as to enhancing water management towards sustainable, efficient and equitable use of limited water resources. In this study, we selected China’s second-largest inland river, the Heihe River, as a case study for the analysis of green/blue water.Our results show that:blue water flow (the sum of surface runoff, lateral water and groundwater runoff) and green water flow (the actual evapotranspiration) are larger in upstream sub-basins than in midstream and downstream sub-basins. The green water coefficient (the proportion of green water flow in total blue and green water flows) of Heihe river basin was more than88%. This implies that more than88%of water resources in the Heihe river basin are green water. Human activities can influence the green/blue water variability and distribution. In midstream, the urbanization increased blue water flow and decreased green water flow; in contrast, irrigation increased green water flow and decreased blue water flow. The blue water flow and total blue and green water flows of the entire basin increased significantly during the study period, while the proportion of green water decreased significantly. The three flows (green water flow, blue water flow, and the total flows) increased significantly in the upstream and midstream basins, but did not change significantly in the downstream basin. Blue water flows changed abruptly in1963, and total flows changed abruptly in1963and1978. Green/blue water flows in the Heihe river basin will continue to increase in future, but the proportion of green water will decrease. The results for typical reference years showed the green/blue water flows in a typical wet year (1998,25.27billion m3) were more than in a typical dry year (1978,16.77billion m3). The green water coefficient was greater in a typical dry year (90.30%) than in a typical wet year (85.41%). Therefore, the percent of green water in total water resources was higher in a typical dry year or dry region (mid-and downstream) than in a typical wet year or wet region (upstream). This study provides insights into green and blue water endowments for different sub-basins in the entire Heihe river basin. The results are helpful to benchmark the green and blue water assessment at a river basin level and to improve water resources management in the inland river basins of China.