Christian Birkel

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.

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.

Water sources and mixing in riparian wetlands revealed by tracers and geospatial analysis

Abstract Mixing of waters within riparian zones has been identified as an important influence on runoff generation and water quality. Improved understanding of the controls on the spatial and temporal variability of water sources and how they mix in riparian zones is therefore of both fundamental and applied interest. In this study, we have combined topographic indices derived from a high‐resolution Digital Elevation Model (DEM) with repeated spatially high‐resolution synoptic sampling of multiple tracers to investigate such dynamics of source water mixing. We use geostatistics to estimate concentrations of three different tracers (deuterium, alkalinity, and dissolved organic carbon) across an extended riparian zone in a headwater catchment in NE Scotland, to identify spatial and temporal influences on mixing of source waters. The various biogeochemical tracers and stable isotopes helped constrain the sources of runoff and their temporal dynamics. Results show that spatial variability in all three tracers was evident in all sampling campaigns, but more pronounced in warmer dryer periods. The extent of mixing areas within the riparian area reflected strong hydroclimatic controls and showed large degrees of expansion and contraction that was not strongly related to topographic indices. The integrated approach of using multiple tracers, geospatial statistics, and topographic analysis allowed us to classify three main riparian source areas and mixing zones. This study underlines the importance of the riparian zones for mixing soil water and groundwater and introduces a novel approach how this mixing can be quantified and the effect on the downstream chemistry be assessed.

Integrating parsimonious models of hydrological connectivity and soil biogeochemistry to simulate stream DOC dynamics

To improve understanding and prediction of dissolved organic carbon (DOC) sources and fluxes in northern peat-dominated catchments, we present the development and application of a parsimonious tracer-aided rainfall-runoff model coupled with a biogeochemistry subroutine able to concurrently simulate streamflow and DOC dynamics. The modeling approach which included quantitative assessment of associated uncertainties was conditioned by geochemical tracers which discriminate dominant water sources. Integration of DOC was predicated on statistical time series models which identified air temperature and streamflow as the key proxies that capture DOC supply and transport processes in two upland catchments in Scotland, UK. Conceptualizing the nonlinear partitioning of quick near-surface and slower groundwater runoff sources in combination with a DOC mass balance resulted in a coupled, low-parameter mechanistic model. Model tests showed mostly sensitive parameters and reasonable simulation results with seasonally controlled DOC supply and event-based DOC transport. Transport is facilitated even for smaller events by overland flow from saturated histosols connected to the stream network. However, during prolonged dry periods, near-surface runoff “switches off” and stream DOC is dominated by low concentration groundwaters. Furthermore, the model was able to explain subtle differences in DOC dynamics between the two catchments mainly reflecting the distribution of saturated soils and available storage. We conclude that tracers and statistical time series models can successfully guide the development of parsimonious yet structurally consistent water quality models. Parsimonious models provide tools for estimating DOC dynamics and loads with reduced uncertainty and potentially greater transferability.

Modelling the impacts of land-cover change on streamflow dynamics of a tropical rainforest headwater catchment

Abstract A modelling experiment is used to examine different land-use scenarios ranging from extreme deforestation (31% forest cover) to pristine (95% forest cover) conditions and related Payment for Ecosystem Services (PES) schemes to assess whether a change in streamflow dynamics, discharge extremes and mean annual water balance of a 73.4-km2 tropical headwater catchment in Costa Rica could be detected. A semi-distributed, conceptual rainfall–runoff model was adapted to conceptualize the empirically-based, dominant hydrological processes of the study area and was multi-criteria calibrated using different objective functions and empirical constraints on model simulations in a Monte Carlo framework to account for parameter uncertainty. The results suggest that land-use change had relatively little effect on the overall mean annual water yield (<3%). However, streamflow dynamics proved to be sensitive in terms of frequency, timing and magnitude of discharge extremes. For low flows and peak discharges of return periods greater than one year, land use had a minor influence on the runoff response. Below these thresholds (<1-year return period), forest cover potentially decreased runoff peaks and low flows by as much as 10%, and non-forest cover increased runoff peaks and low flows by up to 15%. The study demonstrated the potential for using hydrological modelling to help identify the impact of protection and reforestation efforts on ecosystem services. Editor Z.W. Kundzewicz Citation Birkel, C., Soulsby, C., and Tetzlaff, D., 2012. Modelling the impacts of land-cover change on streamflow dynamics of a tropical rainforest headwater catchment. Hydrological Sciences Journal, 57 (8), 1543–1561.

Tropical precipitation anomalies and d-excess evolution during El Niño 2014-16†

The last 2014-16 El Nino event was among the three strongest episodes on record. El Nino considerably changes annual and seasonal precipitation across the tropics. Here, we present a unique stable isotope data set of daily precipitation collected in Costa Rica prior to, during, and after El Nino 2014-16, in combination with Lagrangian moisture source and precipitation anomaly diagnostics. δ2H composition ranged from -129.4 to +18.1 (‰) while δ18O ranged from -17.3 to +1.0 (‰). No significant difference was observed among δ18O (P = 0.0186) and δ2H (P = 0.664) mean annual compositions. However, mean annual d-excess showed a significant decreasing trend (from +13.3 to +8.7 ‰) (P < 0.001) with values ranging from +26.6 to -13.9 ‰ prior to and during the El Nino evolution. The latter decrease in d-excess can be partly explained by an enhanced moisture flux convergence across the southeastern Caribbean Sea coupled with moisture transport from northern South America by means of an increased Caribbean Low Level Jet regime. During 2014-15, precipitation deficit across the Pacific domain averaged 46% resulting in a very severe drought; while a 94% precipitation surplus was observed in the Caribbean domain. Understanding these regional moisture transport mechanisms during a strong El Nino event may contribute to a) better understanding of precipitation anomalies in the tropics and b) re-evaluate past stable isotope interpretations of ENSO events in paleoclimatic archives within the Central America region. This article is protected by copyright. All rights reserved.

Assessing urbanization impacts on catchment transit times

Stable isotopes have potential for assessing the hydrologic impacts of urbanization, although it is unclear whether established methods of isotope modeling translate to such disturbed environments. We tested two transit time modeling approaches (using a gamma distribution and a two-parallel linear reservoir (TPLR) model) in a rapidly urbanizing catchment. Isotopic variability in precipitation was damped in streams with attenuation inversely correlated with urban cover. The models captured this reasonably well, although the TPLR better represented the integrated dual response of urban and nonurban areas with reduced uncertainty. Percent urban cover influenced the shape of the catchment transit time distribution. Total urban cover reduced the mean transit time to 4 years for nonurban sites.

Nonlinear and threshold‐dominated runoff generation controls DOC export in a small peat catchment

We used a relatively simple two-layer, coupled hydrology-biogeochemistry model to simultaneously simulate streamflow and stream dissolved organic carbon (DOC) concentrations in a small lead and arsenic contaminated upland peat catchment in northwestern Germany. The model procedure was informed by an initial data mining analysis, in combination with regression relationships of discharge, DOC, and element export. We assessed the internal model DOC processing based on stream DOC hysteresis patterns and 3-hourly time step groundwater level and soil DOC data for two consecutive summer periods in 2013 and 2014. The parsimonious model (i.e., few calibrated parameters) showed the importance of nonlinear and rapid near-surface runoff generation mechanisms that caused around 60% of simulated DOC load. The total load was high even though these pathways were only activated during storm events on average 30% of the monitoring time—as also shown by the experimental data. Overall, the drier period 2013 resulted in increased nonlinearity but exported less DOC (115 kg C ha−1 yr−1 ± 11 kg C ha−1 yr−1) compared to the equivalent but wetter period in 2014 (189 kg C ha−1 yr−1 ± 38 kg C ha−1 yr−1). The exceedance of a critical water table threshold (−10 cm) triggered a rapid near-surface runoff response with associated higher DOC transport connecting all available DOC pools and subsequent dilution. We conclude that the combination of detailed experimental work with relatively simple, coupled hydrology-biogeochemistry models not only allowed the model to be internally constrained but also provided important insight into how DOC and tightly coupled pollutants or trace elements are mobilized.

Continuous Dissolved Oxygen Measurements and Modelling Metabolism in Peatland Streams.

Stream water dissolved oxygen was monitored in a 3.2km2 moorland headwater catchment in the Scottish Highlands. The stream consists of three 1st order headwaters and a 2nd order main stem. The stream network is fringed by peat soils with no riparian trees, though dwarf shrubs provide shading in the lower catchment. Dissolved oxygen (DO) is regulated by the balance between atmospheric re-aeration and the metabolic processes of photosynthesis and respiration. DO was continuously measured for >1 year and the data used to calibrate a mass balance model, to estimate primary production, respiration and re-aeration for a 1st order site and in the 2nd order main stem. Results showed that the stream was always heterotrophic at both sites. Sites were most heterotrophic in the summer reflecting higher levels of stream metabolism. The 1st order stream appeared more heterotrophic which was consistent with the evident greater biomass of macrophytes in the 2nd order stream, with resulting higher primary productivity. Comparison between respiration, primary production, re-aeration and potential physical controls revealed only weak relationships. However, the most basic model parameters (e.g. the parameter linking light and photosynthesis) controlling ecosystem processes resulted in significant differences between the sites which seem related to the stream channel geometry.