Global monthly water scarcity: blue water footprints versus blue water availability

Abstract

Freshwater scarcity is a growing concern, placing considerable importance on the accuracy of indicators used to characterize and map water scarcity worldwide. We improve upon past efforts by using estimates of blue water footprints (consumptive use of ground- and surface water flows) rather than water withdrawals, accounting for the flows needed to sustain critical ecological functions and by considering monthly rather than annual values. We analyzed 405 river basins for the period 1996-2005. In 201 basins with 2.67 billion inhabitants there was severe water scarcity during at least one month of the year. The ecological and economic consequences of increasing degrees of water scarcity--as evidenced by the Rio Grande (Rio Bravo), Indus, and Murray-Darling River Basins--can include complete desiccation during dry seasons, decimation of aquatic biodiversity, and substantial economic disruption.

Figure 1. Monthly blue water footprint in the period 1996–2005.

The data are shown in mm/month on a 5 by 5 arc minute grid. Data per grid cell have been calculated as the water footprint within a grid cell (in m³/month) divided by the area of the grid cell (in 10³ m²).

Figure 2. Monthly water scarcity in the world's major river basins, based on the period of 1996–2005.

In each month that a river basin is colored in some shade of green, the monthly water scarcity is low (blue water footprint is less than net availability). In such cases, the presumed environmental flow requirements are not violated, and river runoff in that month is unmodified or only slightly modified. In each month that a river basin is colored yellow, water scarcity is moderate. Blue water footprint is between 20 and 30% of natural runoff; runoff is hence moderately modified and environmental flow requirements are not fully met. When a river basin is colored orange, water scarcity is significant. Blue water footprint is between 30 and 40% of natural runoff, so monthly runoff is significantly modified. In each month that a river basin is colored red, water scarcity is severe; the blue water footprint exceeds 40% of natural runoff, therefore runoff is seriously modified.

Figure 3. Water scarcity over the year for the Murray-Darling River Basin in Australia (average for the period 1996–2005).

Net available water – that is natural runoff minus environmental flow requirement – is shown in green. From October until May, the blue water footprint exceeds net available water; in these months, the presumptive environmental flow requirement is not met. When the blue water footprint moves into the yellow, orange and red colors, water scarcity is moderate, significant and severe, respectively.

Figure 4. Number of months during the year in which the blue water footprint exceeds blue water availability for the world's major river basins, based on the period of 1996–2005.

Blue water availability refers to natural flows (through rivers and groundwater) minus the presumed environmental flow requirement.