Watersheds and Environmental Change

As we consider global environmental change, it is clear that we live in an integrated ecosystem in which there are no clear boundaries.  Every habitat is connected and we cannot hope to manage, conserve and protect natural areas in isolation.  Water is a very important medium for material (and energy) transfer between ecosystems and water itself is an important resource.  The hydrological cycle includes terrestrial, subterranean, coastal, oceanic and atmospheric components.  Although the cycle is well known, there are still wide gaps in our understanding of how human activities affect water quality and hence the watershed and the receiving water bodies. 

The watershed is the basic hydrological unit on land and includes both surface and subterranean components.  Watersheds in Costa Rica tend to be very dynamic due to the very mountainous terrain (73% of the country) and generally high rainfall (average 3,300 mm/year, with maxima around 7500).  This means that rain water has very high erosion potential.  Over the past half century Costa Rica has suffered significant deforestation which has severely compromised the integrity of the country’s watersheds.  From 1940 to 1986, Costa Rica lost over 50% of its forest cover; by the 1980’s, it had the world’s highest rate of deforestation.  With a loss in forest cover, erosion and landslides become significant risks.  Hurricane Mitch in October 1998 and the low pressure over Venezuela in December of 1999 stand as two grizzly reminders of the importance of forest cover and control of development within a watershed.  With high slopes, watersheds cannot recharge their aquifers as fast as in shallower slopes.  In Costa Rica, this is especially important when one considers that over 70% of the inhabitants live in 4% of the surface area: demand for potable water is very concentrated.  Although Costa Rica benefits from ample rainfall, the seasonality and demography can result in problems of water supply.   Water supply is also an issue for the large agricultural sector, which can compound the problem through water contamination.  Runoff from croplands can cause sediment pollution, in addition to contamination by agrochemicals and organic matter.  Many aquifers are also potentially at risk.  Examples will be given of some water supply and water quality issues, including some from Palo Verde and La Selva.

Given the importance of watershed management and water supply and quality, water resources need to be managed in an integrated way from a watershed perspective.  Unfortunately, Costa Rica is plagued by a whole host of governmental agencies which each have various jurisdictions over water resources.  The result is often overlapping responsibilities and inefficiencies.  Several issues involved in water resources management will be discussed.

Near the end of the watershed continuum, wetlands provide very valuable ecological and even economic functions.  Wetlands provide buffers for changing water discharges and storm impacts, process nutrients and receive sediments, thereby “filtering” the water, provide essential habitat for estuarine animals such as fish and birds, and constitute an important global carbon sink.  Human impacts to these ecosystems have been direct and indirect.  Indirect impacts include habitat deterioration such as caused by poor water quality (above). Direct impacts include habitat alteration and destruction.  As an example of the latter, current trends in Costa Rica include conversion of bottomland and wetland areas for rice production.  In addition, mangrove forests are under pressure from aquaculture developers and local inhabitants who harvest the wood for fuel and building material.  Efforts to sustainably harvest mangroves have been largely unsuccessful due to the lack of economy of scale.  Wetlands are also sensitive to any alterations in the hydrology of the watershed (e.g the Tempisque River basin, Palo Verde).  Again, integrated watershed management needs to consider all components of the watershed and the importance of natural flows of materials and energy between them.  Several short research projects can be designed to evaluate some of the impacts of human activities in wetlands (Palo Verde).  These can be a very instructional way to understand the complexities involved in natural resource management.