This paper is distinct from existing studies on water resources carrying capacity which usually use dimensionless data to represent trend and status of water resources carrying capacity. Here, on the grounds of the most stringent water resource management system and following the principles of water determining population, water determining city scale, water determining production and so on, water resources carrying capacity prediction model was established. The water resources carrying capacity was represented by population, which can directly reflect the status of water resources. Under the rigid constraints of water use quantity and water use efficiency, six scenarios were set to predict China's maximum population in 2030. The results demonstrated that the maximum population in each scenario is close to 1.45 billion of National Population Development Plan. It means water resources rigid constraints can support population and economic growth at the socio-economic development current pace and path. Total water use quantity will not break through the limit of 800–900 billion m3 when achieving the expected goals of social and economic development, not even more than 700 billion m3. Meanwhile, in order to relieve water resources stress, to improve water resources carrying capacity, and to accelerate construction of a water-saving society, some suggestions were put forward.

Water shortage is a global risk that could arguably be mitigated using water more efficiently. However, the profound relationship between water use efficiency and regional economic size has not been empirically tested. The research design employed an exploratory empirical analysis done through non-linear curve function and attempted to analyze the evolution of water use efficiency over economic growth. First, the water use efficiency change was decomposed into pure technical efficiency change, scale efficiency change, and technological advance change. Second, the scale efficiency is generally less than 1, revealing that it is the main reason for the decreased water use efficiency by the empirical analysis of trends across China. Third, the fitting function between water use efficiency and economic development was constructed. The results supported the existence of an inverted-S shape between water use efficiency and regional economic growth. This analysis will be the reference to formulate scenarios for economic and demographic growth coupled with water use, particularly for planning and managing future water provision and demand.

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Ensuring water security means meeting the water demand required to sustain human activity. This study constructs an early-warning model of water resources security based on water resource carrying capacity, which consists of water demand predictions and early-warning standards. Taking China as an example, we compare expected water demand in 2030 with the maximum available water (water use red line) regulated by the government. The results reveal that some regional water demand will surpass their own red line at the provincial level, whereas at the national level, total water use is not expected to exceed the red line control of 700 billion m 3 .This approach provides a valuable method for tracking the future water demands of human activity. An early-warning model for water resources contributes to identifying strategies for expanding water security in the face of drought and other water hazards and is applicable to other regions.early warning, water demand, water security, water use red line | INTRODUCTIONWater security refers to the capacity of a population to safeguard sustainable access to adequate quantities of acceptable quality water for sustaining livelihoods, human well-being, and socio-economic development; for ensuring protection against water-borne pollution and water-related disasters; and for preserving ecosystems in a climate of peace and political stability (UN-water, 2013). Global water insecurity