¿Sabía que…? Hechos y cifras sobre el ciclo hidrológico

Hemos extraído del boletín de la UNESCO estos datos que nos parecen importantes compartir.

¿Sabía que …?

• El ciclo hidrológico de la Tierra es el mecanismo global que transfiere agua desde los océanos a la superficie y desde la superficie, o subsuperficie, y las plantas a la atmósfera que envuelve nuestro planeta;


• Los principales componentes naturales de los procesos del ciclo hidrológico son: precipitación, infiltración, escorrentía, evaporación y transpiración.


• Las actividades humanas (asentamientos, industria y desarrollos agrícolas) pueden alterar los componentes del ciclo natural mediante desviaciones del uso del suelo y a través de la utilización, reutilización y vertido de residuos en los recorridos naturales de los recursos hídricos superficiales y subterráneos.


• La atmósfera terrestre contiene unos 13.000 km3 de agua. Esta cantidad representa el 10% de los recursos de agua dulce del planeta que no se encuentran en las aguas subterráneas, en los casquetes polares ni en el permafrost. Sin embargo, de mayor importante es el hecho de que este vapor circula en la atmósfera en un "envoltura dinámica global", que contiene un volumen anual considerable y recurrente, que se estima entre los 113.500 km3 y los 120.000 km3. Estos volúmenes ilustran el rol esencial que desempeñan las precipitaciones en la renovación de nuestros recursos hídricos naturales, sobre todo aquéllos que se utilizan para abastecer los ecosistemas naturales y los cultivos de secano.


• Cuando la precipitación atmosférica alcanza el suelo, se divide en varias partes que prosiguen la fase terrestre del ciclo hidrológico siguiendo diferentes caminos. De la cantidad total anual de 110.000 km3 de precipitación sobre la superficie terrestre, cerca de 40.000 km3 se convierten en escorrentía superficial y recarga de acuíferos (agua azul) y se estima que unos 70.000 km3 quedan almacenados en el suelo y más tarde vuelven a la atmósfera a través de la evaporación y la transpiración de las plantas (agua verde).


• Los procesos de evaporación y transpiración (evapotranspiración) están estrechamente relacionados con el agua presente en la humedad del suelo; estos procesos actúan como fuerzas motrices sobre el agua que se transfiere en el ciclo hidrológico. El movimiento a través del suelo y la vegetación es considerable y representa el 62% del agua dulce que se renueva cada año a nivel global.


• Cerca del 40% del agua que cae en forma de precipitación sobre el suelo proviene del vapor derivado del océano. El 60% restante proviene de fuentes terrestres. · En un clima templado, el 33% de la precipitación total generalmente vuelve a la atmósfera por evaporación o evapotranspiración, el 33 % se convierte en agua superficial a través de la escorrentía y el 33% recarga las aguas subterráneas. 


• En un clima semiárido, el 50% del total de las precipitaciones generalmente vuelve a la atmósfera por evaporación o evapotranspiración, el 30 % se convierte en agua superficial a través de la escorrentía y el 20% recarga las aguas subterráneas.


• En un clima árido, el 70% del total de la precipitación generalmente vuelve a la atmósfera por evaporación o evapotranspiración, el 29 % se convierte en agua superficial a través de la escorrentía y sólo el 1% recarga las aguas subterráneas.

Water
a shared responsibility

The United Nations

World Water Development Report 2

PREFACE

In the three years since the launch of the first World Water
Development Report (2003), the world has witnessed considerable
change. There have been many instances of major water-related
disasters: the 2004 Indian Ocean tsunami; the 2004 and 2005
hurricanes in the Caribbean, the west Pacific and the United States; the
2005 floods in Central and Eastern Europe as well as in many other
regions; and the extensive droughts in Niger, Mali, Spain and Portugal.

These are reminders of both the destructive power of water and the
misery deriving from lack of it in so many regions of the world.
These extreme events are only the most prominent illustrations of
fundamental changes that are affecting water resources worldwide. In
many cases, this evolution is most probably linked to slow but
persistent changes in global climates, a phenomenon that is supported
by a growing body of evidence: the combination of lower precipitation
and higher evaporation in many regions is diminishing water quantities
in rivers, lakes and groundwater storage; while increased pollution is
damaging ecosystems and the health, lives and livelihoods of those
without access to adequate safe drinking water and basic sanitation.

Major demographic changes are also seriously affecting the quality and
quantity of available freshwater on the planet. While the more
developed countries enjoy relatively stable populations, the lessdeveloped
regions of the world are generally experiencing rapid growth
and population shifts, particularly in towns, small cities and megacities.
In many rapidly growing urban areas, it is proving impossible to
build the infrastructure necessary to deliver water supply and
sanitation facilities to service the population, leading to poor health,
low quality of life and, in many cases, to social unrest. To the urban
demands for water must be added the increasing demands on water for
food production, energy creation and industrial uses.

Large shifts in the geographic distribution of populations occur in
various contexts, often adding to water supply problems and social
tension. In areas, such as Darfur, there are both internally displaced
persons and transboundary refugees. Legal and illegal economic
migrants are swelling populations in parts of the United States and
Western Europe, as elsewhere. Tourism to many holiday destinations of
the world often exerts a strain on the water supplies of these regions.
Whether it is the result of continued unrest and warfare, terrorist
activities or economic instability, population movement is a factor that
has a substantial impact on water availability in the world.

It is against these changes in the global situation - some rapid and very noticeable, others insidious and yet
persistent - that the governance of water resources must be assessed. This second Report, Water, A Shared
Responsibility, sets water issues against this evolving background and places greater emphasis on governance

issues.

It is proving extremely difficult for many governments to effectively confront the many and intertwined issues
concerning water. Not only is it difficult for departments within national governments to collaborate effectively,
but problems are compounded when many management decisions have to be taken at sub- national and
community levels, as the linkage and cooperation between different levels of government is often tenuous at
best. The challenges for government agencies to link to NGOs and the private sector for resolving water issues
further complicate management and decision-making. The task of managing water becomes even more complex
when rivers flow from one country to another. The building of cooperative upstream-downstream relationships is
becoming increasingly important with close to half of the world"s people living in river basins or above aquifers
that cross international borders.

An important goal of the World Water Assessment Programme - founded in 2000 at the request of governments
within the Commission on Sustainable Development - is therefore to assist governments in developing and
implementing their national water management plans. Thus, a number of case studies have been developed and
included in the Report. In the first Report, 7 case studies involving 12 countries were included to illustrate the
variety of circumstance in different regions of the world. Since then, the number of case studies has grown to
17 involving 42 countries. In a single volume, it is not possible to describe all case studies in detail. Thus we have
chosen to summarize the case studies in the Report and publish the details of each study on our website. This
strategy also allows us to make necessary updates as new data and information become available.

As we move through the International Decade for Action, "Water for Life", 2005-2015, the World Water
Development Reports will provide a series of assessments to facilitate the monitoring of change in the water
sector, both on a global basis and within a growing number of case-study countries and river basins. The purpose
of the Decade is to focus on the implementation of water-related programmes and projects, while striving to
ensure cooperation at all levels, including the participation and involvement of women, to achieve the
internationally-agreed water-related goals (Agenda 21, the UN Millennium Declaration and the JPOI as well as
those of the 12th and 13th sessions of the Commission on Sustainable Development). A number of issues
identified by UN-Water as priorities for the Decade include coping with water scarcity, access to drinking water,
sanitation and hygiene, and disaster risk reduction, particularly in Africa.
The triennial World Water Development Reports will provide substantive content for the Decade"s agenda
(subsequent editions of the Report are scheduled for production in 2009, 2012 and 2015) and lay the foundation
for a continuous, global monitoring system, pooling the unique perspectives and expertise of the 24 UN agencies
that comprise UN-Water, in partnership with governments and other entities concerned with freshwater issues.

We trust that you will find this and future Reports both informative and stimulating.