But how can a local water community actually begin quantifying the notion of “sufficiency”? In the examples below we offer insights into how one might calculate sufficiency with respect to water quantity (i.e. water availability) concerns associated with rivers and aquifers. However, the notion of sufficiency can also be applied to the other two components of water stress depicted in Figure 1 (i.e., water quality and accessibility), as well as a range of possible other non-water-stress-related factors, such as flooding and climate resilience. Indeed, the concept of sufficiency as it relates to water quality is akin to the definition of Total Maximum Daily Loads or TMDLs, as is done in the United States under the Clean Water Act. In the future, the authors aim to develop similar examples for water quality and water access issues, but for now we focus on water quantity. Developing a quantitative estimate of the volume of water use that must be reduced, or the volume of water that would need to be added/imported in any particular watershed or aquifer will requirpreparation of a “water budget.” A water budget generally accounts for the volume of renewable water available for human uses and to support the environment; the volume of water being withdrawn and consumptively used; and the water remaining in the watershed or aquifer. A variety of global water models exist that can be used to obtain an initial accounting for the water budget of a watershed or aquifer. For example, the global WaterGAP model developed at the University of Kassel in Germany can be used to develop an initial estimate of water availability and use (by sector) for more than 140,000 watersheds globally (see Table 1). Table 1: Water Budget for the Jiaojiang Watershed in China (based on output from the global WaterGAP3 model, summarized here as annual average values. Monthly values are also available. MCM=million cubic meters) Total Renewable Water Available in Watershed = 630 MCM Agricultural consumption = 160 MCM Domestic consumption = 11 MCM Manufacturing consumption = 77 MCM Electricity consumption = 0.58 MCM Livestock consumption = 3.3 MCM Total consumption in watershed = 252 MCM Total flow remaining in watershed = 378 MCM These WaterGAP3 outputs can provide insight into the proportion of water that is being consumed by agriculture, manufacturing, domestic use, power production, and other uses. These model outputs also include an estimate of how much water remains in a river or aquifer for environmental support. This information can be used by water users and other stakeholders as a starting point for discussions about potential water savings that might be attained in each water-use sector, and for estimating how much rebalancing of water use among each sector and the environment may be necessary or sufficient to meet the community’s needs and goals. In recent research conducted by Brauman and others using the WaterGAP3 model, the investigators found that water flow or aquifer depletion is quite minimal (<5% of available, renewable water supply) in 2/3 of all global watersheds. This suggests that water scarcity is not an immediate risk in most of the watersheds of the world, and there appears to be adequate water available to meet both environmental and human needs (assuming adequate access is provided and quality assured) in these lightly-used watersheds. However, serious challenges exist in the remaining 1/3 of watersheds. Unfortunately, nearly 3/4 of all irrigated cropland and half of the global population are situated in these scarce watersheds. Though methods, datasets, and others resources for doing such an assessment for water quality or community access to water and sanitation services are not available to the same extent as those for water quantity, the same overarching concepts can be applied. For example, if a shared water resource is experiencing particularly high levels of a specific pollutant (or pollutants), the local water community might seek to take account of current levels of each of those contaminants relative to levels considered sufficient for human and ecological health (or otherwise defined as acceptable by the local community). This would provide a sound basis for understanding what type and how much action is needed, and as a tangible objective for collective action in the region.