Trend analysis for the flows of green and blue water in the Heihe River basin, northwestern China

Climate change will affect the availability of water resources and the sustainability of their management. Despite the progress made in understanding trends of runoff and potential evapotranspiration, the historical and future trends for flows of green and blue water remain poorly understood. In this study, we selected China’s second-largest inland river, the Heihe River, as a case study for analysis of these trends for the entire river basin; for the upstream, midstream, and downstream basins; and for all 32 sub-basins. We assessed the significance of simulation results from the Soil and Water Assessment Tool for both flows, for the total flow, and for the proportion of the total accounted for by green water from 1960 to 2010, and we predicted future trends. The blue water and total flow of the entire basin increased significantly during the study period, while the proportion of green water decreased significantly. Blue water flow increased by 0.21 × 109 m3 per decade and total flow increased by 0.44 × 109 m3 per decade. The three flows (green water, blue water, and total flow) increased significantly in the upstream and midstream basins, but did not change significantly in the downstream basin. Blue water flows changed abruptly in 1963, and total flow changed abruptly in 1963 and 1978. Future flows in the Heihe River will continue increasing, but the proportion of green water will decrease. However, there was considerable spatial variation among the 32 sub-basins in past and future trends. Regional variability in precipitation and temperature appears to be the main reason for the variability of these flows. Our results summarize the status of the basin’s water resources, demonstrate the importance of analyses at different scales, and provide a reference for water resources management in other inland river basins.