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 Soulsby11Northern Rivers Institute, School of Geosciences, University of Aberdeen, UK, AB24 3UF 2Department of Geosciences, Boise State University, Boise, ID 83725, USA 3Department of Forest, Ecology and Management, Swedish University of Agricultural Sciences, 90183 Umeå, Sweden
Received: 24 Feb 2017 – Accepted for review: 08 Mar 2017 – Discussion started: 09 Mar 2017
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 catchments 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 model (STARR) 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. The sites have exceptionally long and rich datasets 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 developed a novel parsimonious calculation scheme that takes into account the isotopic fractionation through snow evaporation and snow melt.
The modified STARR setup simulated the stream flows, 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, soils and geology. 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 demonstrated the importance of including snow evaporative fractionation processes in tracer-aided modelling for catchments with seasonal snowpack, while the influence of fractionation during snowmelt could not be unequivocally shown. Our work shows the utility of isotopes to provide a proof of concept for our modelling framework in snow influenced catchments.
Ala-aho, P., Tetzlaff, D., McNamara, J. P., Laudon, H., and Soulsby, C.: Using isotopes to constrain water flux and age estimates in snow-influenced catchments using the STARR (Spatially distributed Tracer-Aided Rainfall-Runoff) model, Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2017-106, in review, 2017.