Doerthe Tetzlaff

A decade of Predictions in Ungauged Basins (PUB)—a review

Abstract The Prediction in Ungauged Basins (PUB) initiative of the International Association of Hydrological Sciences (IAHS), launched in 2003 and concluded by the PUB Symposium 2012 held in Delft (23–25 October 2012), set out to shift the scientific culture of hydrology towards improved scientific understanding of hydrological processes, as well as associated uncertainties and the development of models with increasing realism and predictive power. This paper reviews the work that has been done under the six science themes of the PUB Decade and outlines the challenges ahead for the hydrological sciences community. Editor D. Koutsoyiannis Citation Hrachowitz, M., Savenije, H.H.G., Blöschl, G., McDonnell, J.J., Sivapalan, M., Pomeroy, J.W., Arheimer, B., Blume, T., Clark, M.P., Ehret, U., Fenicia, F., Freer, J.E., Gelfan, A., Gupta, H.V., Hughes, D.A., Hut, R.W., Montanari, A., Pande, S., Tetzlaff, D., Troch, P.A., Uhlenbrook, S., Wagener, T., Winsemius, H.C., Woods, R.A., Zehe, E., and Cudennec, C., 2013. A decade of Predictions in Ungauged Basins (PUB)—a review. Hydrological Sciences Journal, 58 (6), 1198–1255.

Gamma distribution models for transit time estimation in catchments: physical interpretation of parameters and implications for time-variant transit time assessment.

In hydrological tracer studies, the gamma distribution can serve as an appropriate transit time distribution (TTD) as it allows more flexibility to account for nonlinearities in the behavior of catchment systems than the more commonly used exponential distribution. However, it is unclear which physical interpretation can be ascribed to its two parameters (?, ?). In this study, long?term tracer data from three contrasting catchments in the Scottish Highlands were used for a comparative assessment of interannual variability in TTDs and resulting mean transit times (MTT = ??) inferred by the gamma distribution model. In addition, spatial variation in the long?term average TTDs from these and six additional catchments was also assessed. The temporal analysis showed that the ? parameter was controlled by precipitation intensities above catchment?specific thresholds. In contrast, the ? parameter, which showed little temporal variability and no relationship with precipitation intensity, was found to be closely related to catchment landscape organization, notably the hydrological characteristics of the dominant soils and the drainage density. The relationship between ? and precipitation intensity was used to express ? as a time?varying function within the framework of lumped convolution integrals to examine the nonstationarity of TTDs. The resulting time?variant TTDs provided more detailed and potentially useful information about the temporal dynamics and the timing of solute fluxes. It was shown that in the wet, cool climatic conditions of the Scottish Highlands, the transit times from the time?variant TTD were roughly consistent with the variations of MTTs revealed by interannual analysis.

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

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 Points Hillslope connectivity is controlled by small storage changes in soil units Different catchment source waters mix in large riparian wetland storage Isotopes show riparian wetlands set the catchment transit time distribution

Stream water age distributions controlled by storage dynamics and nonlinear hydrologic connectivity: Modeling with high‐resolution isotope data

Abstract To assess the influence of storage dynamics and nonlinearities in hydrological connectivity on time‐variant stream water ages, we used a new long‐term record of daily isotope measurements in precipitation and streamflow to calibrate and test a parsimonious tracer‐aided runoff model. This can track tracers and the ages of water fluxes through and between conceptual stores in steeper hillslopes, dynamically saturated riparian peatlands, and deeper groundwater; these represent the main landscape units involved in runoff generation. Storage volumes are largest in groundwater and on the hillslopes, though most dynamic mixing occurs in the smaller stores in riparian peat. Both streamflow and isotope variations are generally well captured by the model, and the simulated storage and tracer dynamics in the main landscape units are consistent with independent measurements. The model predicts that the average age of stream water is ∼1.8 years. On a daily basis, this varies between ∼1 month in storm events, when younger waters draining the hillslope and riparian peatland dominates, to around 4 years in dry periods when groundwater sustains flow. This variability reflects the integration of differently aged water fluxes from the main landscape units and their mixing in riparian wetlands. The connectivity between these spatial units varies in a nonlinear way with storage that depends upon precipitation characteristics and antecedent conditions. This, in turn, determines the spatial distribution of flow paths and the integration of their contrasting nonstationary ages. This approach is well suited for constraining process‐based modeling in a range of northern temperate and boreal environments.

Do time‐variable tracers aid the evaluation of hydrological model structure? A multimodel approach

[1] In this paper we explore the use of time-variable tracer data as a complementary tool for model structure evaluation. We augment the modular rainfall-runoff modeling framework FUSE (Framework for Understanding Structural Errors) with the ability to track the age distribution of water in all model stores and fluxes. We therefore gain the novel ability to compare tracer/water age signatures measured in a catchment with those predicted using hydrological models built from components based on four existing popular models. Key modeling decisions available in FUSE are evaluated against streamflow tracer dynamics using weekly observations of tracer concentration which reflect the tracer transit time distribution (TTD). Model structure choice is shown to have a significant effect on simulated water age characteristics, even when simulated flow series are very similar. We show that for a Scottish case study catchment, careful selection of model structure enables good predictions of both streamflow and tracer dynamics. We then use FUSE as a hypothesis testing tool to understand how different model characterization of TTDs and mean transit times affect multicriteria model performance. We demonstrate the importance of time variation in TTDs in simulating water movement along fast flow pathways, and investigate sensitivity of the models to assumptions about our ability to sample fast, near-surface flow.

Groundwater–surface water interactions in upland Scottish rivers: hydrological, hydrochemical and ecological implications

Synopsis Contrary to previous hydrogeological assumptions, we now know that drift deposits and fracture systems in crystalline rocks can constitute important aquifers in the Scottish Highlands and other montane environments. Groundwater from these aquifers usually has an important influence on the hydrology, hydrochemistry and ecology of upland river systems. Tracer-based research in the Girnock burn catchment in the Cairngorms revealed that groundwater comprises at least 30% of annual runoff. Groundwater often enters stream channels via drift deposits in valley bottom areas, which appear to be fed from recharge areas on the catchment interfluves. A range of groundwater sources exist in the catchment reflecting the complex solid and drift geology. These account for spatial differences in stream hydrochemistry and the spatial delineation of groundwater discharges to rivers and riparian zones. Areas where groundwaters enter the stream channel directly can have profound ecological implications. Most obvious are low rates of salmonid egg survival where chemically reduced groundwater discharges through the hyporheic zone. However, it is argued that only further research will reveal the full significance of groundwater–surface water interactions to the ecological status of Scottish rivers.

Developing a consistent process‐based conceptualization of catchment functioning using measurements of internal state variables

We use isotope data in addition to discharge and groundwater level data to conceptualize the internal processes of runoff generation and tracer transport in a low parameter coupled flow-tracer model that could predict the runoff response and isotopic composition of an upland stream. We used sensitivity analysis to assess the effect of these data on model calibration in terms of parameter identifiability and the models ability to predict the streams runoff response, isotopic composition and water age. The results showed that the incorporation of tracer data in particular, clearly increased parameter identifiability and improved the predictive power of models for simulating both streamflow and isotopes. This also resulted in a more consistent process-based conceptualization of catchment functioning. We could also show that using models as learning tools can guide sampling campaigns toward measurements with increased information content for further modeling. We conclude that this is a promising approach for assessing dominant processes in coupled flow-tracer models. This is of value when such models are being used to test hypotheses about the hydrological functioning of catchments, particularly in relation to pollutant transfers.

Is the composition of dissolved organic carbon changing in upland acidic streams

The quantity and composition of dissolved organic carbon (DOC) exported from upland soils to surface waters is a key link in the global carbon cycle and economically important for treating potable waters. The relationship between ultraviolet (UV) absorbance and DOC concentrations can be used to infer changes in the proportion of hydrophobic (aromatic, recalcitrant) carbon and hence biodegradability of DOC. This study describes a significant change in the relationship between UV absorbance and DOC over 22 years at two upland moorland catchments in Scotland, UK. Despite increases in long-term DOC concentrations, analysis suggests that the proportion of hydrophobic material has declined. A statistical mixed-effect modeling approach was used to examine the likely mechanisms that could explain these observations. Annual nonmarine sulfate load was the only significant forcing factor that could explain the observed long-term trend in the UV absorbance-DOC relationship at both sites. It is hypothesized that enhanced heterotrophic decomposition of organic matter and increased solubility of carbon compounds in soils where sulfate driven acidification is being reversed are the dominant mechanisms behind this change in DOC composition. These trends will impact on carbon substrate dynamics by potentially increasing biodegradability of exported organic matter, influencing carbon cycling in terrestrial and aquatic ecosystems.

Scale-dependent groundwater contributions influence patterns of winter baseflow stream chemistry in boreal catchments

Understanding how the sources of surface water change along river networks is an important challenge, with implications for soil-stream interactions, and our ability to predict hydrological and bio ...

A preliminary assessment of water partitioning and ecohydrological coupling in northern headwaters using stable isotopes and conceptual runoff models.

Abstract We combined a conceptual rainfall‐runoff model and input–output relationships of stable isotopes to understand ecohydrological influences on hydrological partitioning in snow‐influenced northern catchments. Six sites in Sweden (Krycklan), Canada (Wolf Creek; Baker Creek; Dorset), Scotland (Girnock) and the USA (Dry Creek) span moisture and energy gradients found at high latitudes. A meta‐analysis was carried out using the Hydrologiska Byråns Vattenbalansavdelning (HBV) model to estimate the main storage changes characterizing annual water balances. Annual snowpack storage importance was ranked as Wolf Creek > Krycklan > Dorset > Baker Creek > Dry Creek > Girnock. The subsequent rate and longevity of melt were reflected in calibrated parameters that determine partitioning of waters between more rapid and slower flowpaths and associated variations in soil and groundwater storage. Variability of stream water isotopic composition depends on the following: (i) rate and duration of spring snowmelt; (ii) significance of summer/autumn rainfall; and (iii) relative importance of near‐surface and deeper flowpaths in routing water to the stream. Flowpath partitioning also regulates influences of summer evaporation on drainage waters. Deviations of isotope data from the Global Meteoric Water Line showed subtle effects of internal catchment processes on isotopic fractionation most likely through evaporation. Such effects are highly variable among sites and with seasonal differences at some sites. After accounting for climate, evaporative fractionation is strongest at sites where lakes and near‐surface runoff processes in wet riparian soils can mobilize isotopically enriched water during summer and autumn. Given close soil–vegetation coupling, this may result in spatial variability in soil water isotope pools available for plant uptake. We argue that stable isotope studies are crucial in addressing the many open questions on hydrological functioning of northern environments.