Controls on Freshwater Supplies in Drying Caribbean Watersheds

forested watershed
Image Credit: Pixabay/Jan Bartel

Puerto Rico’s government is tackling their decade-long economic crisis by clearing 582,000 acres of tropical montane forest to expand the island’s agricultural sector. But forests play a major role in regulating water cycles—a critical ecosystem service for a region experiencing extended drought. A study by Luquillo Long-Term Ecological Research Program (LUQ-LTER) researchers found that such extensive land clearing would likely increase freshwater supply, but also increase flooding risk.

Using the Soil Water Assessment Tool (SWAT), the researchers simulated monthly and annual average stream water discharge in eight tropical montane watersheds in Puerto Rico, based on dominant land cover types from 1977, 1991, and 2000. From 1977 to 1991, abandoned agriculture and pasture land in the central mountains of Puerto Rico was largely reforested. After 1991, reforestation efforts slowed and forest reclamation declined slightly until 2000. The researchers expected these changes in land cover and configuration would impact stream discharge and freshwater supply from tropical montane watersheds, offering a good opportunity to test and calibrate the simulation.

The researchers found that landscape composition and configuration explained 64% of the changes in Puerto Rico’s freshwater supply. During periods of reforestation, increased forest cover significantly reduced the average stream discharge and freshwater supply as deep roots intercepted ground flow and forest transpiration drove losses to the atmosphere. Reforestation also reduced large-scale water discharge, flooding, and landslide risk following large rain events. When reforestation efforts declined, there was a significant increase in the average stream discharge in the tropical watersheds.

Deforestation tended to increase hydrological connectivity in the watersheds. Land fragmentation, in contrast, decreased average water discharge because of an enhanced edge effect, which exposed more forest edges to solar radiation, increased evapotranspiration, and increased interception of lateral surface and subsurface water flows.

As Puerto Rico experiences more deforestation and land fragmentation, combined with increased climate drying in the Caribbean, these conditions could lead to insufficient water supplies on an already impoverished and economically strained region. The findings of this study help scientists, land use planners, and decision makers understand the impacts of land modification on the hydrological services of tropical ecosystems.

Original article in PLOS ONE

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