Storage dynamics in hydropedological units control hillslope connectivity, runoff generation, and the evolution of catchment transit time distributions.

D Tetzlaff,C Birkel,J Geris,J Dick,C Soulsby

doi:10.1002/2013wr014147

Abstract

We examined the storage dynamics and isotopic composition of soil water over 12 months in three hydropedological units in order to understand runoff generation in a montane catchment. The units form classic catena sequences from freely draining podzols on steep upper hillslopes through peaty gleys in shallower lower slopes to deeper peats in the riparian zone. The peaty gleys and peats remained saturated throughout the year, while the podzols showed distinct wetting and drying cycles. In this region, most precipitation events are <10 mm in magnitude, and storm runoff is mainly generated from the peats and peaty gleys, with runoff coefficients (RCs) typically <10%. In larger events the podzolic soils become strongly connected to the saturated areas, and RCs can exceed 40%. Isotopic variations in precipitation are significantly damped in the organic-rich soil surface horizons due to mixing with larger volumes of stored water. This damping is accentuated in the deeper soil profile and groundwater. Consequently, the isotopic composition of stream water is also damped, but the dynamics strongly reflect those of the near-surface waters in the riparian peats. “pre-event” water typically accounts for >80% of flow, even in large events, reflecting the displacement of water from the riparian soils that has been stored in the catchment for >2 years. These riparian areas are the key zone where different source waters mix. Our study is novel in showing that they act as “isostats,” not only regulating the isotopic composition of stream water, but also integrating the transit time distribution for the catchment.Key PointsHillslope connectivity is controlled by small storage changes in soil unitsDifferent catchment source waters mix in large riparian wetland storageIsotopes show riparian wetlands set the catchment transit time distribution

Highlights

IntroductionQuantifying the processes of water and tracer transport through catchments remains a key research frontier in hydrology, where new technologies, theoretical developments, and novel modeling approaches provide fresh insights into the dynamic controls on streamflow generation and solute fluxes [e.g., Beven, 2012; McDonnell et al, 2010; McMillan et al, 2012; Rinaldo et al, 2011]
Quantifying the processes of water and tracer transport through catchments remains a key research frontier in hydrology, where new technologies, theoretical developments, and novel modeling approaches provide fresh insights into the dynamic controls on streamflow generation and solute fluxes [e.g., Beven, 2012; McDonnell et al, 2010; McMillan et al, 2012; Rinaldo et al, 2011]. Much of this interest has been stimulated by tracer-based insights into the so-called ‘‘old water paradox’’ whereby short-term rainfall-runoff dynamics controlled by the celerity of hillslope responses mobilize water that has usually been stored in the catchment for much longer periods but constrained by low pore velocities [Kirchner, 2003; McDonnell et al, 2010]
Input-output studies of conservative tracers have provided invaluable insights into the emergent properties of such catchment scale responses [e.g., Kirchner et al, 2000]; they have been useful for catchment intercomparison [e.g., Hrachowitz et al, 2009a; Tetzlaff et al, 2009a] and convenient for hypothesizing dominant flow paths and mixing processes [Hrachowitz et al, 2013]

Summary

Introduction

Quantifying the processes of water and tracer transport through catchments remains a key research frontier in hydrology, where new technologies, theoretical developments, and novel modeling approaches provide fresh insights into the dynamic controls on streamflow generation and solute fluxes [e.g., Beven, 2012; McDonnell et al, 2010; McMillan et al, 2012; Rinaldo et al, 2011]. Nested tracer studies integrating soil profile-hillslope-catchment scales over prolonged periods and combining empirical observations with quantitative modeling are a efficient route to improved understanding of the spatial and temporal dynamics of how water is partitioned, stored, and discharged [Birkel et al, 2011a; Davies et al, 2011; Seibert and McDonnell, 2002]. Recent work [e.g., Lin et al, 2006] has built on earlier studies [e.g., Boorman et al, 1995; Dunne et al, 1975; Western et al, 1999] in emphasizing the ecohydrological importance of soils and their spatial distribution in controlling the catchment hydrological response

Results

Discussion

Conclusion