Freshwater Availability, Distribution, & Climate Impacts

While it may seem that the earth has an abundance of water, only 2.5% is freshwater, and a mere 0.3% of that is readily available for human use (the majority stemming from groundwater aquifers, followed by lakes, reservoirs, rivers, and wetlands) [Vörösmarty et al., 2005]. On top of that, water is unequally distributed throughout the world – nine countries – Brazil, Russia, China, Canada, Indonesia, U.S., India, Columbia and the Democratic Republic of Congo – possess 60% of the world’s available freshwater supply [WBCSD, 2005].

Rain and snowfall constantly renew freshwater ecosystems, which in turn provide society with provisioning (i.e. food, water), regulating (i.e. regulation of floods, drought, land degradation, and disease), supporting (i.e. soil formation, nutrient cycling), and cultural (i.e. recreational, spiritual, religious, and other nonmaterial benefits) services when sustainably used ([Vörösmarty et al., 2005]; [TEEB, 2010]). The precipitation absorbed by ecosystems is either processed and transferred back to the atmosphere as ‘‘green water’’ (through evapotranspiration drawn from soils and plant canopies in natural ecosystems and rain-fed agriculture) or runs off as ‘‘blue water,’’ which is what is available to downstream users—both aquatic ecosystems and humans (see (Table 2.2-1) for an overview of available water distribution by ecosystem and region).

The implication of climate change is that all elements of the water cycle, including precipitation, evapo-transpiration, soil moisture, groundwater recharge, and runoff may be modified. Additionally, it may change the timing and intensity of precipitation, snowmelt, and runoff [Vörösmarty et al., 2005]. It has already been observed that mountains are experiencing shortened and earlier snow and ice melt, leading to related changes in flooding [UN WWAP, 2009]. The IPCC estimates that by 2050 annual average runoff will have increased by 10%-40% at high latitudes and decreased by 10%-40% over some dry regions at mid-latitudes and semi- arid low latitudes [Bates et al., 2008]. Globally, the number of great inland flood catastrophes was twice as large per decade between 1996 and 2005 as between 1950 and 1980, and economic losses were five times as great. The dominant drivers of these upward trends are socioeconomic factors, such as population growth, land use change, and greater use of vulnerable areas³ [UN WWAP, 2009].

Table 2.2-1:

Estimates of Renewable Water Supply, Access to Renewable Supplies, and Population Served (taken from [Vörösmarty et al., 2005])

System¹ or Region	Area (million km^²)	Total precipitation	Total Renewable Water Supply, Blue Water Flows (thousand km²/yr) [% global runoff]	Renewable Water Supply, Blue Water Flows, Accessible to Humans² [% of Total Renewable Water Supply]	Population Served by Renewable Ressource³ (billion) [% of World Population]
Forests	41,6	49,7	22,4 [57]	16,0 [71]	4,62 [76]
Mountains	32,9	25,0	11,0 [28]	8,6 [78]	3,95 [65]
Drylands	61,6	24,7	3,2 [8]	2,8 [88]	1,9 [31]
Cultivated⁴	22,1	20,9	6,3 [16]	6,1 [97]	4,83 [80]
Islands	8,6	12,2	5,9 [15]	5,2 [87]	0,79 [13]
Coastal	7,4	8,4	3,3 [8]	3,0 [91]	1,53 [25]
Inland Water	9,7	8,5	3,8 [10]	2,7 [71]	3,98 [66]
Polar	9,3	3,6	1,8 [5]	0,3 [17]	0,01 [0,20]
Urban	0,3	0,22	0,062 [0,2]	0,062 [100]	4,3 [71]
Asia	20,9	21,6	9,8 [25]	9,3 [95]	2,56 [42]
Former Soviet Union	21,9	9,2	4,0 [10]	1,8 [45]	0,27 [4]
Latin America	20,7	30,6	13,2 [33]	8,7 [66]	0,43 [7]
North Africa / Middle East	11,8	1,8	0,25 [1]	0,24 [96]	0,22 [4]
Sub-Saharan Africa	24,3	19,9	4,4 [11]	4,1 [93]	0,57 [9]
OECD	33,8	22,4	8,1 [20]	5,6 [69]	0,87 [14]
World Total	133	106	39,6 [100]	29,7 [75]	4,92 [81]
¹ Note double-counting for ecosystems under the MA definitions. ² Potentially available supply without downstream loss. ³ Population from Vorosmarty et al. 2000. 4 For cultivated systems, estimates are based on cropland extent from Ramankutty and Foley 1999 within this MA reporting system.

³ i.e. increased construction in floodplain areas.