On the role of aspect to quantify water transit times in small mountainous catchments

In the current empirical study, we provide evidence about how the hydrologic responses of small mountain catchments are related to aspect ( slope direction and exposure) at Redondo Peak, located in the Valles Caldera, New Mexico, United States. Specifically, we test the hypothesis that the transit time of water is related to the catchment aspect. Aspect is an easily measurable and transferable topographic characteristic that is related to the amount of direct solar radiation a particular catchment receives, and therefore, different catchments with different aspects have different rates of snow ablation, evapotranspiration, and water cycling in general. Transit times, which describe the time between when water enters the catchment as precipitation and when it leaves as stream flow, captures many hydrologic features such as flow path variability and the combined effects of water storage and water fluxes. We have designed an experiment that involves field data collection, isotopic analysis of stream and precipitation samples, and the estimation of transit times using lumped-parameter convolution for 15 sites in small (1-15 km(2)) catchments that drain different aspects of Redondo Peak. Our data suggest that isotopic variability and estimated transit times are both related to aspect. Other potential relationships between topographical features (such as flow path length, slope gradient, and elevation) and isotopic measurements of stream water suggest that landscape and hydrological features are interconnected at Redondo Peak, but these links are not conclusive, suggesting that these topographic indicators do not fully explain the variability of water cycling in these small mountain catchments.