Irrigation is the greatest human interference with the terrestrial water cycle. Detailed knowledge on irrigation is required to better manage water resources and to increase water use efficiency (WUE). This study brings forward a novel framework to quantify net irrigation at monthly timescale at a spatial resolution of 1 kmproviding unprecedented spatial and temporal detail. Net irrigation refers to the evaporative loss of irrigation water. The study is conducted in the Haihe River Basin (HRB) in China encompassing the North China Plain (NCP), a global hotspot of groundwater depletion. Net irrigation is estimated based on the systematic evapotranspiration (ET) residuals between a remote sensing based model and a hydrologic model that does not include an irrigation scheme. The results suggest an average annual net irrigation of 126 mm (15.2 km) for NCP and 108 mm (18.6 km) for HRB. It is found that net irrigation can be estimated with higher fidelity for winter crops than for summer crops. The simulated water balance of the HRB was evaluated with GRACE data and it was found that the net irrigation estimates could close the water balance gap. Annual winter wheat classifications reveal an increasing crop area with a trend of 2200 km yr. This trend is not accompanied by a likewise increasing trend in irrigation, which suggests an increased WUE in the NCP. The proposed framework can easily be scaled up or transferred to other regions and support decision makers to tackle irrigation induced water crises and support sustainable water management.

The Beijing-Tianjin-Hebei Metropolitan Region is the biggest urbanized megalopolis in North China. The region has one the lowest water resources availability per capita in China and around the world. Rapid economic development in the past 40 years has resulted in substantially increased water consumption in the region which led to various water security problems including huge burden on water supply, groundwater overdraft, serious damage of river and lake ecosystems etc. Water scarcity has now become one of the largest limiting factors that hinders the further development of the region. The objective of water resources management in this region is thus to ensure healthy balances between the natural and the societal water circulation. This means we need to protect the natural hydrological flow to maintain water's service functions, and in the meantime fulfill the increasing water demand for economic growth. In the present study, we inspected past evolution and future trend of the region's water budget, and subsequently proposed methods for restricting water use and promoting multi-source water supply optimization. This research also reviewed some new concepts and technologies which can potentially help us overcome the water security challenges in the Beijing-Tianjin-Hebei region.