Hydrologic Transit Times in Tropical Montane Watersheds: Catchment Scale and Landscape Influences

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 work has been carried out on MTT, despite its usefulness for providing important information about watershed hydrological functioning at different spatial scales in (largely) ungauged basins. In particular, very few studies have quantified stream MTTs and related to catchment characteristics in tropical montane regions. Here we examined topographic, land use/cover and soil hydraulic controls on baseflow MTT for nested watersheds (0.1–34 km²) within a humid mountainous region, underlain by volcanic soil (Andisols) in central Veracruz (eastern Mexico). To estimate MTTs, we used a 2 year record of bi-weekly isotopic composition of precipitation and stream baseflow data. Land use/cover and topographic parameters were derived from GIS analysis. Soil profile hydraulic properties and permeability at the soil-bedrock interface were obtained from intensive field measurements and laboratory analysis. Estimates of baseflow MTT ranged between 1.2 and 2.7 years across the 12 study catchments. Major differences in MTTs were found at the small (0.1–1.5 km²) and at the large scales (14–34 km²), related mostly to catchment slope and morphology and, to much lesser extent, to land cover. Interestingly, longest stream MTTs were found in the cloud forest headwater catchments. Overall, MTTs were mainly controlled by depth to bedrock associated with topography, and permeability at the soil-bedrock interface. Mid and ridge hillslope positions appeared to be the main contributing areas for catchment recharge and runoff. The present study is the first step towards to understand the hydrology and subsurface processes across scales in this tropical environment, with the aim to support decisions for local and regional management water supply under increasing land use and climate change pressures.