Articles | Volume 21, issue 10
Hydrol. Earth Syst. Sci., 21, 5089–5110, 2017
Hydrol. Earth Syst. Sci., 21, 5089–5110, 2017

Research article 09 Oct 2017

Research article | 09 Oct 2017

Using isotopes to constrain water flux and age estimates in snow-influenced catchments using the STARR (Spatially distributed Tracer-Aided Rainfall–Runoff) model

Pertti Ala-aho1, Doerthe Tetzlaff1, James P. McNamara2, Hjalmar Laudon3, and Chris Soulsby1 Pertti Ala-aho et al.
  • 1Northern Rivers Institute, School of Geosciences, University of Aberdeen, AB24 3UF, UK
  • 2Department of Geosciences, Boise State University, Boise, ID 83725, USA
  • 3Department of Forest, Ecology and Management, Swedish University of Agricultural Sciences, 90183 Umeå, Sweden

Abstract. Tracer-aided hydrological models are increasingly used to reveal fundamentals of runoff generation processes and water travel times in catchments. Modelling studies integrating stable water isotopes as tracers are mostly based in temperate and warm climates, leaving catchments with strong snow influences underrepresented in the literature. Such catchments are challenging, as the isotopic tracer signals in water entering the catchments as snowmelt are typically distorted from incoming precipitation due to fractionation processes in seasonal snowpack.

We used the Spatially distributed Tracer-Aided Rainfall–Runoff (STARR) model to simulate fluxes, storage, and mixing of water and tracers, as well as estimating water ages in three long-term experimental catchments with varying degrees of snow influence and contrasting landscape characteristics. In the context of northern catchments the sites have exceptionally long and rich data sets of hydrometric data and – most importantly – stable water isotopes for both rain and snow conditions. To adapt the STARR model for sites with strong snow influence, we used a novel parsimonious calculation scheme that takes into account the isotopic fractionation through snow sublimation and snowmelt.

The modified STARR setup simulated the streamflows, isotope ratios, and snow pack dynamics quite well in all three catchments. From this, our simulations indicated contrasting median water ages and water age distributions between catchments brought about mainly by differences in topography and soil characteristics. However, the variable degree of snow influence in catchments also had a major influence on the stream hydrograph, storage dynamics, and water age distributions, which was captured by the model. Our study suggested that snow sublimation fractionation processes can be important to include in tracer-aided modelling for catchments with seasonal snowpack, while the influence of fractionation during snowmelt could not be unequivocally shown. Our work showed the utility of isotopes to provide a proof of concept for our modelling framework in snow-influenced catchments.

Short summary
We used the Spatially Distributed Tracer-Aided Rainfall-Runoff model (STARR) to simulate streamflows, stable water isotope ratios, snowpack dynamics, and water ages in three snow-influenced experimental catchments with exceptionally long and rich datasets. Our simulations reproduced the hydrological observations in all three catchments, suggested contrasting stream water age distributions between catchments, and demonstrated the importance of snow isotope processes in tracer-aided modelling.