Water is a naturally circulating resource that is constantly recharged. Therefore, even though the stocks of water in natural and artificial reservoirs are helpful to increase the available water resources for human society, the flow of water should be the main focus in water resources assessments. The climate system puts an upper limit on the circulation rate of available renewable freshwater resources (RFWR). Although current global withdrawals are well below the upper limit, more than two billion people live in highly water-stressed areas because of the uneven distribution of RFWR in time and space. Climate change is expected to accelerate water cycles and thereby increase the available RFWR. This would slow down the increase of people living under water stress; however, changes in seasonal patterns and increasing probability of extreme events may offset this effect. Reducing current vulnerability will be the first step to prepare for such anticipated changes.

Water scarcity has become a major constraint to socio‐economic development and a threat to livelihood in increasing parts of the world. Since the late 1980s, water scarcity research has attracted much political and public attention. We here review a variety of indicators that have been developed to capture different characteristics of water scarcity. Population, water availability, and water use are the key elements of these indicators. Most of the progress made in the last few decades has been on the quantification of water availability and use by applying spatially explicit models. However, challenges remain on appropriate incorporation of green water (soil moisture), water quality, environmental flow requirements, globalization, and virtual water trade in water scarcity assessment. Meanwhile, inter‐ and intra‐annual variability of water availability and use also calls for assessing the temporal dimension of water scarcity. It requires concerted efforts of hydrologists, economists, social scientists, and environmental scientists to develop integrated approaches to capture the multi‐faceted nature of water scarcity.

Life cycle assessment (LCA) has been used to assess freshwater-related impacts according to a new water footprint framework formalized in the ISO 14046 standard. To date, no consensus-based approach exists for applying this standard and results are not always comparable when different scarcity or stress indicators are used for characterization of impacts. This paper presents the outcome of a 2-year consensus building process by the Water Use in Life Cycle Assessment (WULCA), a working group of the UNEP-SETAC Life Cycle Initiative, on a water scarcity midpoint method for use in LCA and for water scarcity footprint assessments.

(2008). The implications of projected climate change for freshwater resources and their management. Hydrological Sciences Journal: Vol. 53, No. 1, pp. 3-10.

Water scarcity is rapidly increasing in many regions. In a novel, multi-model assessment, we examine how human interventions (HI: land use and land cover change, man-made reservoirs and human water use) affected monthly river water availability and water scarcity over the period 1971–2010. Here we show that HI drastically change the critical dimensions of water scarcity, aggravating water scarcity for 8.8% (7.4–16.5%) of the global population but alleviating it for another 8.3% (6.4–15.8%). Positive impacts of HI mostly occur upstream, whereas HI aggravate water scarcity downstream; HI cause water scarcity to travel downstream. Attribution of water scarcity changes to HI components is complex and varies among the hydrological models. Seasonal variation in impacts and dominant HI components is also substantial. A thorough consideration of the spatially and temporally varying interactions among HI components and of uncertainties is therefore crucial for the success of water scarcity adaptation by HI.

As a first step toward designing a comprehensive model for validating land surface hydrology and river flow in Earth system models, a global river channel network has been prepared at 1° latitude × 1° longitude resolution. The end product is the Total Runoff Integrating Pathways (TRIP) network. The aim of TRIP is to provide information of lateral water movement over land following the paths of river channels. Flow directions were determined from vector data of river channels and river pathways available in two recent atlases; however, an automatic procedure using a digital elevation map of the corresponding horizontal resolution was used as a first guess. In this way, a template to convert the river discharge data into mean runoff per unit area of the basin has been obtained. One hundred eighty major rivers are identified and adequately resolved; they cover 63% of land, excluding Antarctica and Greenland. Most of the river basin sizes are well within a 20% difference of published values, with a r... Abstract As a first step toward designing a comprehensive model for validating land surface hydrology and river flow in Earth system models, a global river channel network has been prepared at 1° latitude × 1° longitude resolution. The end product is the Total Runoff Integrating Pathways (TRIP) network. The aim of TRIP is to provide information of lateral water movement over land following the paths of river channels. Flow directions were determined from vector data of river channels and river pathways available in two recent atlases; however, an automatic procedure using a digital elevation map of the corresponding horizontal resolution was used as a first guess. In this way, a template to convert the river discharge data into mean runoff per unit area of the basin has been obtained. One hundred eighty major rivers are identified and adequately resolved; they cover 63% of land, excluding Antarctica and Greenland. Most of the river basin sizes are well within a 20% difference of published values, with a r...