Abstract
The amount of groundwater and surface water consumed in a river basin constitutes the blue water footprint (BWF). To safeguard the environment, it has been suggested to set a cap to the BWF based on blue water availability (BWA). BWA depends on the precipitation that becomes runoff and the need to reserve environmental flow requirements. Previous studies determined BWF caps based on the use-what-is-there principle, which assumes that all BWA in a sub-catchment may be consumed locally, without the need to reserve water for downstream. However, BWA in an upstream sub-catchment does not have to be consumed locally but could be consumed downstream instead, thereby mitigating blue water scarcity in downstream areas. Therefore, this study aims to investigate the effect of alternative allocation principles – that account for downstream demands – to set monthly BWF caps per sub-catchment on BWS levels across a large river basin. We take the Yellow River basin for the period 2010 to 2014 as a case study to evaluate four scenarios of BWF cap-setting. We compare the ‘natural’ and ‘reservoir’ scenario that both apply the use-what-is-there principle to determine the effects of reservoirs on BWF caps. We then apply two alternative allocation principles that take relative population size (‘population-based’ scenario) and historic blue water demand (‘demand-based’ scenario) as a basis to determine BWF caps per sub-catchment and compare the effects against the ‘reservoir’ scenario. Our results confirm previous findings on the effects of reservoirs on caps. We further 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 BWA per capita. The results feed into a discussion on alternative approaches to set BWF caps in a large river basin which needs to take place for BWF caps to become effective and practical concepts in policy making.
Highlights
Freshwater scarcity, the competition between different human ac tivities and ecosystems over limited freshwater flows, occurs across the globe
The natural runoff represents the hydrology of the Yellow River Basin (YRB) under the current land use conditions without human water abstractions and acts as a reference in evaluating the effects of blue water footprint (BWF) caps on spatiotemporal blue water allocation
We have investigated alternative allocation scenarios to determine monthly blue water footprint cap (BWF cap) per sub-catchment of the YRB and evaluated the effects on blue water scarcity ratio (BWS) levels experienced across the YRB
Summary
Freshwater scarcity, the competition between different human ac tivities and ecosystems over limited freshwater flows, occurs across the globe. The ultimate source of freshwater availability is precipitation over land, which subsequently feeds rivers, recharges groundwater (blue water flows), recharges soil moisture and evaporates from the (vege tated) land surface (green water flows). Water scar city can lead to a lack of freshwater for basic human needs and cause harm to aquatic ecosystems (Vorosmarty et al, 2010). Human interventions in river systems have helped to mitigate water scarcity to some extent (compared to scenarios without interventions), but they caused downstream areas to be more affected by water scarcity compared to upstream areas (Veldkamp et al, 2017)
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