Water residence time and runoff generation in the western Cascades of Oregon.

摘要

The age, or residence time of water is a fundamental descriptor of catchment hydrology, revealing information about the storage, flow pathways and source of water in a single integrated measure. While there has been tremendous recent interest in residence time to characterize catchments, there are few studies that quantify residence time at the catchment scale or explore the process controls on the distribution of residence times. The objective of this study is to determine the controls on catchment-scale residence time using hydrometric, tracer, and modeling approaches at hillslope to multiple catchment scales. Topographic controls on residence time are examined for seven catchments at the H. J. Andrews Experimental Forest that range in basin area from 0.085 to 62.4 km2 representing diverse geologic and geomorphic conditions. Residence times are estimated using stable isotope tracers and convolution integral models. Baseflow mean residence time results range from 0.8 to 3.3 years. There is no correlation between residence time and basin area; however, mean residence time is correlated to the catchment-scale median flowpath distance and flowpath gradient to the stream network, suggesting that topography is a first-order control on catchment-scale transport. The examination of detailed hydrological processes at the hillslope scale through a wet-up period, provide the basis for a dynamic conceptual model of runoff generation and residence time, which are controlled by moisture thresholds and expanding subsurface saturated areas. The residence time of runoff during storms is a dynamic amalgamation of various components, each with their own characteristic shape, mixing behavior, and timescale, which range from 6 to 27 hours for event water and 10 to 30 days for soil water. A coupled hydrologic-tracer model at the hillslope scale indicates that the combination of storm event and between-event processes is necessary for the representation of realistic residence time distributions at hillslope and catchment scales. This study demonstrates that water residence time provides insight to hydrological processes from hillslope to large catchment scales.