Ecosystem Studies & Management

Feedbacks between Water and Deforestation in Tropical South America

Tropical South America contains the world’s largest continuous tropical forest and savannah ecosystems (Figure 1). This region is environmentally important not only because of traditional ecological measures, such as its high biodiversity, but also because it generates more than ¼ of the world’s river discharge. This region has undergone explosive development and deforestation in the last 50 years as national and international demand for cattle feed (mostly soy), beef, and other agricultural commodities, such as sugar cane for ethanol, have increased. Already about 60 percent of the savanna and about 10 percent of the rainforest in this large region have been converted to cattle pasture and agriculture.

 

Figure 1. South America has the world's largest extent of rainforest and savanna (locally called Cerrado) environments, encompassing more than 7 million km2 and producing more than 1/4 of the world's river discharge.

Local and Regional Runoff

Deforestation and forest degradation result in a complex set of changes to streams of all sizes. When forests are removed, the new vegetation, which generally has fewer leaves and shallower roots, uses less water than the forest it replaces. Therefore, less water evaporates from the land surface and is returned to the atmosphere, more water runs off of the land, and stream flow is increased (Figure 2a). How much change occurs depends on many local conditions including the amount of rainfall, how much of a watershed is deforested, topography, soils, and the land use after deforestation, but observations indicate little effect with less than 20 percent of a basin deforested and a large increase with 50 to 100 percent of a basin deforested. These changes occur at the local scale, but rivers of all sizes are affected when deforestation is extensive.

Figure 2. Schematic diagram of the influence of deforestation on the local and regional water cycle. a) Annually, deforestation reduces evaporation (E) from the land surface and increases the amount of water that runs off the land into the rivers (R). As a result, total river discharge increases with deforestation. b) However, when large areas are deforested a feedback between the deforested landscape and the atmosphere can result in decreased precipitation. As a result, total river discharge may decrease significantly if a feedback occurs.

 

Local and Regional Climate

In addition to the local effects, tropical forests may also have a strong influence on regional climate, one that is unrelated to atmospheric CO2 change. Observations and computer modeling studies suggest that replacing large areas of forest with grass or seasonal crops cause changes in the atmosphere that lead to decreased water recycling, higher temperatures, and decreased rainfall (Figure 2b). These changes may in some cases create a new climate that will not sustain tropical forests. Therefore, deforestation and the related climate changes could threaten the existence of tropical forests in general, including those in protected areas distant from where deforestation is occurring.

Taken together, these two changes – a local decrease in evaporation and a large-scale decrease in rainfall – may have profound effects on human and environmental water needs, such as agriculture, hydropower, drinking water, aquatic and terrestrial biodiversity, and fisheries.

Center scientists are leading projects in the Amazon to better understand the consequences of these changes. This work, in which a global climate model was coupled to a land-surface ecosystem model, illustrates the complex nature of the effects of deforestation on regional water resources and climate. In that study we investigated the potential effect of two scenarios of future deforestation (Figure 3):

 

Figure 3. Scenarios of Amazonian deforestation for the year 2050. A scenario with strong governance of deforestation (left) results in about 30% of the basin being deforested by 2050. While, relatively weak governance (right) results in about 55% of the basin being deforested by 2050. Tropical evergreen forests (green), Cerrado (beige), and agriculture (orange) are shown. The deforestation scenarios are from Soares-Filho et al., 2006.

1) a “governance” scenario in which no deforestation takes place on protected areas, new protected lands are created, and at least 50 percent of all private lands remain in forest. and 2) a “business-as-usual” scenario, which assumes that current deforestation trends will continue -- all scheduled roads will be paved on time, compliance with laws regarding protected lands (public and private) is low, and no new protected areas are created. In the governance scenario, about 30 percent of the Amazon evergreen forest is deforested by 2050. In the business-as-usual simulation, almost 50% of the tropical evergreen forests of the basin are removed by 2050.

The results of these simulations suggest several important points about the coupled climate and land surface: 1) if deforestation does not cause decreased rainfall via atmospheric feedbacks, discharge will likely be significantly increased throughout the entire southern Amazon; 2) if rainfall does decrease via atmospheric feedbacks the resulting decrease in river discharge may be greater than the changes without feedbacks, and 3) changes in the water resources caused by atmospheric feedbacks will not be limited to those basins where deforestation has occurred but will be spread unevenly throughout the basin by atmospheric circulation. Therefore, the changes to discharge and aquatic and terrestrial environments with future deforestation in the tropics will likely be a complex function of local and non-local deforestation patterns -- how much vegetation has been removed from a particular watershed and how much has been removed from the larger tropical forest region.

Effective land stewardship programs could avoid or minimize these negative consequences of deforestation by reducing total deforestation and promoting management practices that reduce forest degradation, such as maintenance or reforestation of riparian zone forests, promotion of native species over exotics or plantations, and management of fire in flammable landscapes. Protection of existing forests and in particular those around riparian zones will avoid addition of CO2 to the atmosphere and also avoid immediate damage to aquatic environments, preserving their ecosystem benefits such as, clean drinking water, waste removal, flood protection, fish production, river transportation, hydroelectric generation, and potentially future climate. Furthermore, reforestation may, in addition to sequestering carbon, restore many environmental benefits in a relatively short time period—as little as two decades.

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