Introduction

Life on Earth depends on the continuous flow of materials and energy through air, water, soil and the biosphere’s food webs. There is a strong relationship between energy flow and material cycles in the environment. Any process, either natural or anthropogenic, that modifies the flow of energy and the water cycle can have major effects on ecosystem functions (Ryszkowski, 2001). For example, the evaporation of seawater supplies an estimated volume of 110.000 km³ of rain and snow that fall on continental areas (Jackson et al., 2001) which determines, to a large extent, inland water availability and, consequently, the presence and functioning of ecosystems. The amount of water available at each location depends primarily on temperature and precipitation. Mexico receives 760 mm of rain annually (average for years 1971-2000), which is considered abundant (OECD, 1998), although with a high spatial and temporal heterogeneity. Northern states comprise about 45% of the country’s area but on average receive only a little over 26% of the rain falling on the territory. By contrast, southern states, with only 27.6% of the total area, receive 48.7% of rainfall (Semarnat, 2009).

Total mean annual precipitation is estimated at 1,488 km³ of water, of which nearly three quarters return to the atmosphere by evapotranspiration. The remaining volume, 378.4 km³, corresponds to mean annual natural runoff, and 79.6 km³ to groundwater recharge (Conagua, 2008a). In Mexico, hydro-meteorological phenomena with significant effects occur every year. About 24 tropical cyclones occur per year, mostly from May to November, which generate heavy rainfall over short periods of time. When the water brought in by these phenomena drains rapidly down to the sea, often causes floods and damages to populations living near the coast (CNA, 2001).

Water availability is commonly expressed in terms of either the total or the per capita volume available, although it is worth noticing that water availability should not be seen only as what a person requires for its daily activities, but it should be sufficient to meet the needs of farming, industry and freshwater ecosystems in addition to those of the direct consumption by human populations. In less than 60 years, water availability in Mexico dropped from 18,035 to 4,312 m³ per person per year in 2007 (Conagua, 2008a and 2008c).

References

CNA. Programa Nacional Hidráulico 2001-2006. México. 2001.

Conagua. Estadísticas del Agua en México. 2008a.

Conagua. Programa Nacional Hídrico 2007-2012. México 2008c.

Jackson, R. B., S. R. Carpenter, C.N. Dahm, D. M. McKnight, R. J. Naiman, S. L. Postel y S. W. Running. Water in a Changing World. Issues in Ecology No. 9. Spring, 2001.

OECD. Análisis del Desempeño Ambiental. México. France. 1998.

Ryszkowski, L. (Ed.) Landscape Ecology in Agroecosystems Management. CRC Press. Boca Ratón, Florida. 2001.

Semarnat. Informe de la situación del medio ambiente en México. Compendio de Estadísticas Ambientales. Edición 2008. México. 2009.