Reevaluation of transit time distributions, mean transit times and their relation to catchment topography

Abstract

Abstract. The transit time of water is a fundamental property of catchments, revealing information about the flow pathways, source of water and storage in a single integrated measure. While several studies have investigated the relationship between catchment topography and transit times, few studies expanded the analysis to a wide range of catchment properties and assessed the influence of the selected transfer function (TF) model. We used stable water isotopes from mostly baseflow samples with lumped convolution models of time-invariant TFs to estimate the transit time distributions of 24 meso-scale catchments covering different geomorphic and geologic regions in Switzerland. The sparse network of 13 precipitation isotope sampling sites required the development of a new spatial interpolation method for the monthly isotopic composition of precipitation. A pointenergy-balance based snow model was adapted to account for the seasonal water isotope storage in snow dominated catchments. Transit time distributions were estimated with three established TFs (exponential, gamma distribution and two parallel linear reservoirs). While the exponential TF proved to be less suitable to simulate the isotopic signal in most of the catchments, the gamma distribution and the two parallel linear reservoirs transfer function reached similarly good model fits to the fortnightly observed isotopic compositions in discharge, although in many catchments the transit time distributions implied by equally well fitted models differed markedly from each other and in extreme cases, the resulting mean transit time (MTT) differed by orders of magnitude. A more thorough comparison showed that equally suited models corresponded to agreeing values of cumulated transit time distributions only between 3 and 6 months. The short-term ( < 30 days) component of the transit time distributions did not play a role because of the limited temporal resolution of the available input data. The long-term component (> 3 years) could hardly be assessed by means of stable water isotopes, resulting in ambiguous MTT and hence questioning the relevance of an MTT determined with stable isotopes. Finally we investigated the relation between MTT estimates based on the three different TF types as well as other transit time properties and a range of topographical catchment characteristics. Depending on the selected transfer model, we found a weak correlation between transit time properties and the ratio between flow path length over the flow gradient, drainage density and the mean discharge. The catchment storage derived from MTTs and mean discharge did not show a clear relation to any catchment properties, indicating that in many studies the mean annual discharge may bias the MTT estimates.

Time series of tritium, stable isotopes and chloride reveal short-term variations in groundwater contribution to a stream

“…It is increasingly recognised that stable isotopes cannot provide realistic estimates of longer TT waters, regardless of the lumped model used Seeger and Weiler, 2014;Kirchner, 2015). In this study, it is very likely that older water (i.e.…”

“…In an earlier study, Stewart et al (2007) reported differences of up to an order of magnitude between the TTs determined using stable isotopes as compared to those determined using tritium ( 3 H). Later works by Seeger and Weiler (2014) and Kirchner (2015) reinforced the point that "stable isotopes are effectively blind to the long tails of TT distributions" (Kirchner, 2015). The effects of older groundwater contributions to streamflow have largely been ignored until recently (Smerdon et al, 2012;Frisbee et al, 2013), and according to Stewart et al (2012), new research efforts need to be focused on relating deeper groundwater flow processes to catchment response.…”

“…Concurrently to these recent advances in catchment hydrology, groundwater scientists have also developed new theoretical bases for the incorporation of transient conditions in RT distribution functions (Massoudieh, 2013;Leray et al, 2014). Nonetheless, the determination of time-variant TT and RT distributions requires data-intensive computing, which still largely limits their use in applied studies (Seeger and Weiler, 2014).…”

“…Maloszewski et al, 1992;Burns et al, 1998;Soulsby et al, 2000;Rodgers et al, 2005;Dunn et al, 2010). Attempts have been made to correlate the TTs to catchment characteristics such as topography (McGuire et al, 2005;Mueller et al, 2013;Seeger and Weiler, 2014), geology (Katsuyama et al, 2010) or soil type (Tetzlaff et al, 2009(Tetzlaff et al, , 2011Timbe et al, 2014). Assessment of the relationship between groundwater RT and catchment TT has also been undertaken occasionally (Matsutani et al, 1993;Herrmann et al, 1999;Reddy et al, 2006).…”

Constitution of a catchment virtual observatory for sharing flow and transport models outputs

“…Several distribution types have been used in catchment studies, including the dispersion model (Kirchner et al, 2001;McGlynn et al, 8 2003;McGuire et al, 2002), the piston flow model (McGlynn et al, 2003), and exponentialpiston flow (Maloszewski and Zuber, 1996;McGlynn et al, 2003;McGuire et al, 2002;Timbe et al, 2014). The exponential distribution has been the most widely used (more than 60% of the studies in several recent reviews; McGuire and McDonnell, 2006;Mueller et al, 2013;Roa-Garcia and Weiler, 2010;Seeger and Weiler, 2014), though more recently, the gamma function has been recognized as more conceptually and mathematically suitable to represent catchment behaviour and mixing processes due to its short breakthrough time and long tail (Birkel et al, 2012;Dunn et al, 2010;Heidbüchel et al, 2012;Hrachowitz, 2011;Hrachowitz et al, 2010;Kirchner et al, 2001Kirchner et al, , 2000Soulsby et al, 2011).…”