Factors influencing stream baseflow transit times in tropical montane watersheds

Abstract

Stream water mean transit time (MTT) is a fundamental hydrologic parameter that integrates the distribution of sources, flow paths, and storages present in catchments. However, in the tropics little MTT work has been carried out, despite its usefulness for providing important information on watershed functioning at different spatial scales in (largely) ungauged basins. In particular, very few studies have quantified stream MTTs or have related these to catchment characteristics in tropical montane regions. Here we examined topographic, land use/cover and soil hydraulic controls on baseflow transit times for nested catchments (0.1-34 km 2 ) within a humid mountainous region, underlain by volcanic soil (Andisols) in central Veracruz (eastern Mexico). We used a 2-year record of bi-weekly isotopic composition of precipitation and stream baseflow data to estimate MTT. Land use/cover and topographic parameters (catchment area and form, drainage density, slope gradient and length) were derived from geographic information system (GIS) analysis. Soil water retention characteristics, and depth and permeability of the soil-bedrock interface were obtained from intensive field measurements and laboratory analysis. Results showed that baseflow MTTs ranged between 1.2 and 2.7 years across the 12 study catchments. Overall, MTTs across scales were mainly controlled by catchment slope and the permeability observed at the soil-bedrock interface. In association with topography, catchment form and the depth to the soil-bedrock interface were also identified as important features influencing baseflow MTTs. The greatest dif-ferences in MTTs were found both within groups of small (0.1-1.5 km 2 ) and large (14-34 km 2 ) catchments. Interestingly, the longest stream MTTs were found in the headwater cloud forest catchments.

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

“…It is clear, however, that given the mathematical limitations in representing systems' non-stationarities (Duvert et al, 2016;Seeger and Weiler, 2014), the high temporal resolution of tracer data (Harman, 2015;Heidbüchel et al, 2012), and the general unavailability of long-term tracer records (Hrachowitz et al, 2010;Klaus et al, 2015) required for hydrological modeling under non-stationary conditions, the LCA remains a useful methodology for MTT estimation. This holds true not only in understudied regions such as the tropics (e.g., Farrick and Branfireun, 2015;Muñoz-Villers et al, 2016;Timbe et al, 2014), but also elsewhere (e.g., Duvert et al, 2016;Hu et al, 2015;Seeger and Weiler, 2014).…”

“…Despite the importance of tropical biomes as natural sources and regulators of streamflow, there are very few studies of MTT in tropical environments (e.g., Farrick and Branfireun, 2015;Muñoz-Villers et al, 2016;Roa-García and Weiler, 2010;Timbe et al, 2014). The majority of MTT studies have been conducted in catchments with strong climate seasonality, i.e., located in the Northern and Southern hemispheres (e.g., Lyon et al, 2010;McGlynn and McDonnell, 2003;McGuire et al, 2005), and considerably less attention has been devoted to tropical environments.…”

Interactions between payments for hydrologic services, landowner decisions, and ecohydrological consequences: synergies and disconnection in the cloud forest zone of central Veracruz, Mexico

“…Caballero et al (2013) present results from a semidistributed water balance model that indicated that a cloud forest-dominated watershed had greater groundwater recharge than catchments without cloud forest, which resulted in a four-fold greater stream discharge from the cloud forestdominated watershed. In the case of the Los Gavilanes catchment within our Veracruz study, we estimated that the mean stream water residence times across different catchment areas ranged between 1.2 and 2.7 years, which suggests the presence of deep and presumably long subsurface flow paths that contribute to sustain baseflows, particularly during dry periods (Muñoz-Villers et al 2016). Although these findings suggest that the upland areas -which coincide with pine-oak and cloud forests and highest rainfall totals-are the most important for groundwater recharge and downstream discharge, this study did not spatially identify recharge zones or evaluate linkages between PHS payments and recharge zones.…”

Land use alters dominant water sources and flow paths in tropical montane catchments in East Africa

“…such as surface runoff. Evidence from other studies suggests that the role of vegetation cover in water storage and MTT could be suppressed by geomorphology (Timbe et al, 2017) or soil hydraulic properties (Geris et al, 2015;Mueller et al, 2013;Muñoz-Villers et al, 2016). The latter, however, can also be influenced by land use.…”

“…Transit time also has implications for water quality, since the contact time between water and the soil will affect the chemical composition of the water that finally enters 25 the stream through biogeochemical processes (McGuire and McDonnell, 2006). MTT can be influenced by catchment soil cover (Capell et al, 2012;Rodgers et al, 2005;Soulsby et al, 2006), soil depth, hydraulic conductivity and topographic parameters, such as slope (Heidbüchel et al, 2013;Mosquera et al, 2016b;Muñoz-Villers et al, 2016) or a combination of these factors (Hrachowitz et al, 2009). Changes in vegetation cover and especially soil hydraulic properties as consequence of changes in land management can also modify MTT.…”