Pressure indicators

Population growth

Human populations living in watersheds affect water resources by extracting water for human consumption, irrigation and industrial development, but also as a result of the construction of infrastructure, such as dams and hydroelectric facilities (UN-WWAP, 2006). Human impact on the environment, particularly on water resources, depends on population size, consumption patterns and technologies used in processes that involve the use of water. The indicator Total, urban and rural population provides a framework for developing policies for the sustainable management of water resources and shows the size of the population demanding water, food and services, as well as its growth trends.

Total volume allocated for consumptive use

Water can be devoted to either: 1) uses away from the water body (called consumptive uses), in which water is transported to the site of use and none or only part of it returns to the water body, and 2) uses in the water body itself (non-consumptive uses), in which water is used in the water body or with a minimum diversion, as in the case of hydroelectric facilities (Conagua, 2008a and 2008c).

The major consumptive uses of water, in terms of the total volume extracted, are those made by the agricultural and industry sectors, as well as those for public supply. Agriculture is the major user of fresh water, accounting for about two thirds of the total volume extracted worldwide (Fry and Rast, 1998; WRI, 1999; UN, 2005). Excessive use of water for irrigation has led in many areas to the lowering of water tables, overexploitation of aquifers (Carabias and Landa, 2005), salinization of groundwater in coastal areas, reduced rivers flows and the disappearance of springs, rivers and wetlands (Hinrichsen, 2003; UN, 2003a). Water extraction beyond a sustainable level threatens future water availability, the integrity of freshwater environments and the environmental services they provide (UN, 2003a). Total volume allocated for consumptive use shows the pressure imposed on this resource by the major users competing for water available. This indicator is used by the United Nations, the OECD and the European Environment Agency (OECD, 2008).

GROUNDWATER VOLUME ALLOCATED FOR CONSUMPTIVE USES

Groundwater is used for the supply of potable water, agricultural irrigation, livestock ranches, industry, leisure and energy generation (Zektser and Everett, 2004). In some arid regions, or where no surface water bodies occur, groundwater is the only source of fresh water available. Intensive use of groundwater is becoming increasingly common, especially in arid and semiarid areas, small islands and coastal areas. Excessive extraction, pollution or mismanagement of groundwater may deprive future generations of this resource; this may also lead to the depletion of wells and, consequently, to land subsidence (WMO, 1998). Groundwater overexploitation threatens riparian ecosystems in arid and semiarid areas worldwide. Groundwater exploitation requires proper management to achieve a sustainable use, for which a reliable monitoring and reporting of the volumes extracted/available and water quality is necessary (Llamas and Custodio, 2002). The indicator Groundwater volume allocated for consumptive uses shows the pressure that water demands impose on the country's aquifers. This information, together with that of aquifer recharge, allows assessing the extent of pressure on aquifers.

PER CAPITA PUBLIC WATER SUPPLY USE

Increased urbanization, population growth and rising living standards have all contributed to the increased urban use of water in the past century. The amount of water used by this sector depends on climate, efficiency and organization of water supply services, consumption habits of the population, technological advances and economic instruments. Building the infrastructure to meet the water needs of urban populations has had significant impacts on the integrity of freshwater ecosystems and their ecosystem services. The indicator Per capita public water supply use shows the trends of water consumption in urban areas relative to the total number of inhabitants, and can be used to assess the efficiency of water use by the urban population; it also shows the intensity of per capita pressure on water resources in a given region (EEA, 2003).