Date Published: March 11, 2019
Publisher: Public Library of Science
Author(s): Jason Christian, Joel Martin, S. Kyle McKay, Jessica Chappell, Catherine M. Pringle, Andrés Viña.
Here we provide an empirical hydrologic foundation to inform water management decisions in the El Yunque National Forest (EYNF) in eastern Puerto Rico. Tropical watershed hydrology has proven difficult to quantify due to high rainfall variability, high evapotranspiration rates, variation in forest canopy interception and storage, and uncertain hydrologic inputs from fog condensation in cloud forests. We developed mass-balance and observation-based water budgets for nine local watersheds within the EYNF using a novel assemblage of remotely sensed rainfall data, gaged streamflow observations, and municipal water withdrawal rates. It is important to note that, while prior budgets considered large water withdrawals outside (downstream) of EYNF boundaries, our current budget is confined to within EYNF boundaries. Here, we also base our estimates of water withdrawal volume on operational data, in contrast to prior water budgets that estimated volume based on either the capacity of known water intakes or regulatory permit limits. This resulted in more conservative and realistic estimates of withdrawals from within the EYNF. Finally, we also discuss the ecological importance of considering the effects of water withdrawals not only at an average monthly scale, but also on the basis of exceedance probability to avoid over-abstraction for the protection of native migratory fishes and shrimps. This analysis highlights a number of unique challenges associated with developing hydrologic foundations for water management in tropical ecosystems.
Freshwater availability has emerged as a global problem, given that more than four billion people currently experience some periods of severe water scarcity . Water security challenges are further exacerbated when biodiversity risks and declines in freshwater ecosystem services are considered, with more than 80% of global citizens affected . Balancing competing anthropogenic and ecological demands on finite water resources is a crucial issue for water and land managers [3–4], and frameworks are emerging to address the complex trade-offs associated with water management decision-making. Recent studies have emphasized the critical role of establishing a hydrologic foundation to inform water management decisions–including “Ecological Limits of Hydrologic Alteration, ELOHA”  and “Eco-Engineering Design Scaling, EEDS” .
A computed monthly water budget for each of the nine focal watersheds is generated using observed data covering the period 2005–2013 (Table 2). Annual precipitation was characterized with a forest area-weighted average of 280 cm per year. However, precipitation was temporally variable within the annual cycle with all nine watersheds exhibiting ranges over 20 cm between the wettest and driest months. Rainfall was also spatially variable given that average annual rainfall varied from 209–378 cm across the Canovanas and Mameyes watersheds, respectively.
Approximate decadal water budgets have been compiled for the EYNF and adjacent downstream regions outside of the EYNF [29–30,48]. While all budgets have relied on the mass balance equation (i.e., Eq 1), different methods were used to estimate parameter values (i.e., P, RO, W, and ET) with each successive analysis. Improved temporal and spatial resolution is now possible as additional observational and remotely sensed data have become available Table 3 provides a comparison of the current water budget presented here with two prior budgets. Although methods vary substantially between budgets, estimates of precipitation, evapotranspiration, and runoff are generally similar in that they do not differ more than 20% between individual budget estimates.