Precipitation isotopes link regional climate patterns to water supply in a tropical mountain forest, eastern Puerto Rico

Abstract

Like many mountainous areas in the tropics, watersheds in the Luquillo Mountains of eastern Puerto Rico have abundant rainfall and stream discharge and provide much of the water supply for the densely populated metropolitan areas nearby. Projected changes in regional temperature and atmospheric dynamics as a result of global warming suggest that water availability will be affected by changes in rainfall patterns. It is essential to understand the relative importance of different weather systems to water supply to determine how changes in rainfall patterns, interacting with geology and vegetation, will affect the water balance. To help determine the links between climate and water availability, stable isotope signatures of precipitation from different weather systems were established to identify those that are most important in maintaining streamflow and groundwater recharge. Temporal isotope values exhibit a reverse seasonality from those observed in higher latitude continental watersheds, with higher isotopic values in the winter and lower values in the summer. Despite the higher volume of convective and low-pressure system rainfall, stable isotope analyses indicated that under the current rainfall regime, frequent trade -wind orographic showers contribute much of the groundwater recharge and stream base flow. Analysis of rain events using 20 years of 15 -minute resolution data at a mountain station (643 m) showed an increasing trend in rainfall amount, in agreement with increased precipitable water in the atmosphere, but differing from climate model projections of drying in the region. The mean intensity of rain events also showed an increasing trend. The determination of recharge sources from stable isotope tracers indicates that water supply will be affected if regional atmospheric dynamics change trade-wind orographic rainfall patterns in the Caribbean.

Stable-isotope and solute-chemistry approaches to flow characterization in a forested tropical watershed, Luquillo Mountains, Puerto Rico

“…In addition to an abundant clean water supply, the watershed provides habitat for threatened species, biodiversity, recreation, and erosion control. Though global climate models have projected increased drying in the Caribbean (overview in Scholl and Murphy, 2014), Beck et al (2013), Scholl and Murphy (2014) and Van Beusekom et al (2015) have noted increases in rainfall amount, rain intensity, and minimum temperatures over the past two decades in eastern Puerto Rico and the Luquillo Mountains. This discrepancy between projections and observations of rainfall amount indicates that further research is needed to determine the effects of changes in temperature and rainfall patterns on watershed runoff response.…”

“…At a regional scale, Scholl et al (2009) and Scholl and Murphy (2014) characterized seasonal and climatic controls on the stable isotope composition of rainfall in eastern Puerto Rico, and utilized stable isotopes to determine sources of groundwater recharge and stream baseflow.…”

“…Shanley et al (2011) using solute-chemistry methods and Scholl and Murphy (2014) using isotopic methods both noted that the flashy response of stream flow to rainfall that is characteristic of the Luquillo Mountain watersheds could be explained by saturation excess, because of the high frequency of rainfall and resulting high antecedent soil moisture, combined with low-permeability, highclay content soils. Streamflow recession curve analysis also indicated relatively low storage (i.e.…”

Stable isotope (δ¹⁸O and δ²H) data for precipitation, stream water, and groundwater in Puerto Rico

“…The highest single-event rainfall totals tend to be associated with tropical storms, hurricanes, and cold fronts, although frequent low-intensity orographic showers occur throughout the year in the mountains. The stable isotope signatures of rainfall ( 2 H and  18 O) are broadly correlated with the weather type that produced the rainfall (Scholl and others, 2009;Scholl and Murphy, 2014). …”

Analyzing cloud base at local and regional scales to understand tropical montane cloud forest vulnerability to climate change

“…Experiments and long-term monitoring in the Luquillo Mountains showed that orographic precipitation events occur consistently throughout the year (from the trade winds and thermal lifting over the mountains), providing 25 % of the rainfall by volume (Scholl and Murphy, 2014) as well as additional, unmeasured cloud water. Low clouds at the mountains have been hypothesized to originate with consistent winds carrying oceanic clouds to the mountains too rapidly for dissipation (Raga et al, 2016), and additional condensation may occur with topographic lifting of moist air up the slopes.…”