Translating blue water footprint caps for a river basin to caps per sub-catchment: trade-offs between upstream-downstream uses and the environment

Setting upper limits to water consumption per river basin is crucial for ensuring sustainable water use. A blue water footprint cap is an upper limit to the consumptive use of surface and groundwater. A method to determine monthly blue water footprint caps in a river basin as a whole has been proposed (monthly natural runoff minus environmental flow requirements). However, the question remains how to translate caps for a river basin as a whole to caps for each sub-catchment within the basin. A relevant question, because human interventions in rivers have reduced water scarcity in upstream parts of river basins, but aggravated it in downstream parts. We apply two alternative water allocation scenarios to translate blue water footprint caps for the Yellow River basin to caps per sub-catchment and evaluate their effects on upstream-downstream differences in water scarcity: (i) the population-based scenario takes the relative population size per sub-catchment as the basis for water allocation, which makes sense from the perspective of equity and fair sharing of natural resources; (ii) the demand-based scenario takes the historical water demand as the basis for water allocation, which is an option to consider for river basin managers aiming to reduce environmental flow violations. Both scenarios make use of the fact that blue water can be reserved (not consumed) in an upstream sub-catchment for consumption further downstream. We measure the effects of the scenarios against a base case in which sub-catchments are allowed to consume all available water after considering environmental flow requirements without the consideration of downstream uses: the use-what-is-there principle. We find that blue water scarcity increases from upstream to downstream under the use-what-is-there principle. Both the population- and demand-based scenarios reduce upstream-downstream differences in the degree of blue water scarcity. The demand-based scenario is most effective in this respect. On the other hand, the population-based scenario leads to the smallest upstream-downstream differences in water availability per capita. The results feed into a discussion on how to translate upper-limits to water consumption from the river basin to the sub-catchment level which needs to take place for cap-setting to become a practical instrument in river basin management.