Insights into the water mean transit time in a high-elevation tropical ecosystem

Giovanny M Mosquera,Patricio Crespo,Kellie B Vaché,Catalina Segura,David Windhorst,Lutz Breuer

doi:10.5194/hess-20-2987-2016

Giovanny M Mosquera, Patricio Crespo + Show 4 more

Open Access

https://doi.org/10.5194/hess-20-2987-2016

Copy DOI

Abstract

Abstract. This study focuses on the investigation of the mean transit time (MTT) of water and its spatial variability in a tropical high-elevation ecosystem (wet Andean páramo). The study site is the Zhurucay River Ecohydrological Observatory (7.53 km2) located in southern Ecuador. A lumped parameter model considering five transit time distribution (TTD) functions was used to estimate MTTs under steady-state conditions (i.e., baseflow MTT). We used a unique data set of the δ18O isotopic composition of rainfall and streamflow water samples collected for 3 years (May 2011 to May 2014) in a nested monitoring system of streams. Linear regression between MTT and landscape (soil and vegetation cover, geology, and topography) and hydrometric (runoff coefficient and specific discharge rates) variables was used to explore controls on MTT variability, as well as mean electrical conductivity (MEC) as a possible proxy for MTT. Results revealed that the exponential TTD function best describes the hydrology of the site, indicating a relatively simple transition from rainfall water to the streams through the organic horizon of the wet páramo soils. MTT of the streams is relatively short (0.15–0.73 years, 53–264 days). Regression analysis revealed a negative correlation between the catchment's average slope and MTT (R2 = 0.78, p < 0.05). MTT showed no significant correlation with hydrometric variables, whereas MEC increases with MTT (R2 = 0.89, p < 0.001). Overall, we conclude that (1) baseflow MTT confirms that the hydrology of the ecosystem is dominated by shallow subsurface flow; (2) the interplay between the high storage capacity of the wet páramo soils and the slope of the catchments provides the ecosystem with high regulation capacity; and (3) MEC is an efficient predictor of MTT variability in this system of catchments with relatively homogeneous geology.

Highlights

Investigating ecohydrological processes through the identification of fundamental catchment descriptors, such as the mean transit time (MTT), specific discharge, evapotranspiration to precipitation ratios, and others, is fundamental in order to (1) advance global hydrological, ecological, and geochemical process understanding and (2) improve the management of water resources
Mean electrical conductivity (MEC) was found to be significantly correlated with MTT using all catchments of the nested system in the basin (Fig. 7)
The regression analysis showed strong correlation, with mean electrical conductivity (MEC) increasing as MTT increases

Summary

Introduction

Investigating ecohydrological processes through the identification of fundamental catchment descriptors, such as the mean transit time (MTT), specific discharge, evapotranspiration to precipitation ratios, and others, is fundamental in order to (1) advance global hydrological, ecological, and geochemical process understanding and (2) improve the management of water resources. This is critical in high-elevation tropical environments, such as the wet Andean páramo (further referred to as “páramo”), in which hydrological knowledge remains limited, despite its importance as the major water provider for millions of people in the region (De Bièvre and Calle, 2011; IUCN, 2002). We focus on the MTT of water, which we define as the average time elapsed since a water molecule enters a catchment as recharge to when it exits it at some discharge point (Bethke and Johnson, 2002; Etcheverry and Perrochet, 2000; Rodhe et al, 1996)

Methods

Results

Discussion

Conclusion