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Tracing water supply to climate patterns

This article is part of the Spring 2015 issue of the Earth Science Matters Newsletter.

Forested mountain watersheds provide an important water source for humans and ecosystems, which may be vulnerable to changes in precipitation patterns related to regional or local changes in atmospheric dynamics. Although climate modeling scenarios suggest that precipitation in the Caribbean Basin is likely to decrease over the next century, we have lacked the hydrologic data needed to understand the relative importance of two major climate patterns to streamflow and groundwater:  mountain-influenced (orographic) precipitation from trade winds (including rain and cloud water) and deep convective precipitation (including tropical storms and hurricanes).  

orographic clouds over El Yunque National Forest
Pico del Este and the Rio Mameyes, El Yunque National Forest, Puerto Rico. Orographic clouds (as shown) are a significant source of annual precipitation and streamflow.

In a comprehensive study at the USGS Water, Energy, and Biogeochemical Budgets (WEBB) site in the Luquillo Mountains of Puerto Rico, measurements of hydrogen and oxygen isotopes from precipitation were matched to weather type and cloud height to determine the distinctive “isotopic signature” of each climate pattern.   Isotope signatures of streams and groundwater were also generated and compared to those of the weather type and cloud height in order to determine the source of these important components of regional water supply.  

The result of this study was counterintuitive – despite being only 25-35% by volume, mountainous precipitation originating from trade winds comprised 59% of stream baseflow and groundwater.  Explanations for this result included: 1) frequent low-intensity mountain rain events filled the available space in low-permeability tropical clay soils, so that intense rain from large convective storms flowed rapidly into streams instead of infiltrating into the soil; and 2) high contributions of cloud water deposition, which is not measured in standard rain gages and therefore is not accounted for in traditional water balance methods, may play a more significant role in the local hydrology than expected.

In another part of this study, researchers evaluated whether the precipitation patterns in the Luquillo Mountains have been influenced by regional climate changes. The scientists found that rainfall amount and intensity increased over the past 20 years, contradicting global climate model projections for drying in the Caribbean, but agreeing with projections of increased rainfall intensity.  Global climate models do not simulate precipitation on relatively small mountain ranges.  Given the importance of orographic precipitation found in this study, regional climate models may be a better approach to developing climate change projections for water management planning in mountainous watersheds. 

This research also shows that measuring rainfall totals in the tropics is not enough to determine water availability; rather, it is equally important to understand the interaction between rainfall source and the permeability and storage properties of tropical soils.  The methods developed here can be applied to hydrology studies elsewhere in the tropics, because atmospheric temperature gradients and climate controls on rain isotopic signatures are similar.

The paper, published in Water Resources Research in 2014, can be found at: https://pubs.er.usgs.gov/publication/70118356

The supplemental water stable isotope data are published in: https://doi.org/10.3133/ofr20141101

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