On the Link Between Streamflow Transit Time and Hydrograph Recession

In catchment hydrology, new interest has recently focused on estimation of transit time of water as a diagnostic characteristic of a watershed. Regarded as a fundamental control on water chemistry and a key descriptor of storage, estimates of transit time also offer an additional form of data with which to test watershed models. However, as identified in a recent literature review, many challenges in the estimation of a mean transit time (MTT) as well as its statistical distribution remain. While recent papers have addressed the assumptions implicit in the mathematical lumped parameter flow models for tracers like δ18O, few studies have tried to estimate MTT using other more easily available methods. As a result, measurement of MTT remains difficult with today's technologies. In this paper, we further test a method of baseflow hydrograph recession analysis to estimate MTT for the well characterized H.J. Andrews watersheds, a Long Term Ecological Research (LTER) station located in Oregon's Cascade Mountains, USA. The six H.J. Andrews watersheds range in size from 0.102 to 62.4 km2. We compared the simplified baseflow recession-based method and its assumptions against the stable isotope δ18O tracer-based convolution model previously applied to these catchments. The baseflow recession method estimates MTT values ranging from 1.6 to 2.4 years, within the evaluated uncertainty of the convolution models for five of the six watersheds. The analysis presented in this briefing provides encouraging evidence that the less costly baseflow hydrograph recession analysis may be potentially as useful in predicting this fundamental diagnostic of hydrologic behavior for some catchments.