Pre-event Water Research Articles

Water sources and threshold behaviors of streamflow generation in a mountain headwater catchment

Numerous studies have analyzed the event-scale rainfall-runoff response of hillslopes and catchments. However, there are still knowledge gaps when it comes to understanding the spatiotemporal sources of headwater catchment runoff and the threshold behaviors of multiple runoff components. The aim of this study is to investigate the complex runoff generation mechanisms for 244 rainfall events in a humid headwater catchment of eastern China over five years (2011–2015). We employ a combined approach of isotopic tracing (δ2H and δ18O) and fully-integrated surface/subsurface flow modeling (HydroGeoSphere) to identify streamflow compositions and how the process chain of “rainfall infiltration, mixing with soil water, and subsequent exfiltration to the hillslope surface” impacts streamflow generation in headwater catchments. Our results show that the streamflow consists of event and pre-event water. Event water is composed of direct runoff from rainfall (2 % in the total streamflow) and indirect runoff from infiltrated rainfall (24 %). Pre-event water comprises the exfiltrated shallow soil water of indirect runoff (37 %) and subsurface flow (37 %). The study highlights the significant role of indirect runoff, comprising infiltrated rainfall and pre-event shallow soil water, in streamflow generation, reaching a peak proportion of 81.4 % during rainfall events. In addition, we quantify the threshold behaviors (including the generation threshold, and critical threshold from increasing to decreasing state) for surface and subsurface flow, based on the analysis of the non-linear rainfall-runoff relationships observed in the catchment. Results show that rainfall amount of 33 mm and rainfall intensity of 9 mm/d are generation thresholds for direct runoff and critical thresholds for proportion changes of exfiltrated shallow soil water. Critical thresholds of 55 mm for rainfall amount and 12 mm/d for rainfall intensity are identified for proportion changes of infiltrated rainfall and total indirect runoff. Our findings highlight that the combined analysis of hydrological modeling and isotopic tracing provides an effective solution for quantifying the roles of dominant water components in streamflow generation and elucidate the distinct threshold behaviors during rainfall events. Enhancing our comprehension of streamflow origins and their response mechanisms to rainfall events will improve the understanding of streamflow generation in headwater catchments.

The age and sources of stream water in a boreal forest watershed in the permafrost region: a case study of a watershed in northeast China

Determining the age and sources of stream water is critical for understanding the watershed hydrological processes and biogeochemical cycle. In this study, daily isotope data of rainfall and runoff, as well as continuously monitored conductivity data from June to October in 2019 in-Laoyeling(LYL) watershed located in permafrost region of northeastern China were used to separate streamflow components through the application of two independent methods: isotope-based hydrograph separations (IHS) and the conductivity mass balance (CMB) methods. The results showed that stream water in a boreal forest watershed with permafrost of the Daxing’an Mountains is mainly composed of pre-event water. Although the IHS method is more sensitive and provides more details than the CMB method, the results of both methods show a similar trend. The average value of the young water fractions (Fyw) for those aged less than 65 days is 5.6%, while the mean transit time (MTT) was calculated to be 3.33 years. These findings enhance our understanding of the fundamental characteristics of runoff generation mechanisms and changes in runoff components in permafrost regions. Such knowledge is crucial for effective regional water resource management under the context of climate change, such as construction of water conservancy facilities and prediction of flood and drought disasters.

A novel tool for tracing water sources of streamflow in a mixed land-use catchment

Tracing water sources of streamflow in a mixed land-use catchment is critical for predicting pollutant emissions from various human activities to streams but remains a major challenge. A rain event based field monitoring study was conducted in the Jieliu catchment located in the hilly area of central Sichuan Province, southwest China. The ratio of the maximum fluorescence intensities (Fmax) of the two humic-like dissolved organic matter (DOM) components at excitation/emission wavelengths of 255 (315)/415 nm (component 1; C1) and 260 (375)/480 nm (component 2; C2) was proposed as a tracer for quantifying streamflow water sources. Satisfactory performance of using the Fmax(C1)/Fmax(C2) ratio in hydrograph separation of streamflow at the outlet of a forest sub-catchment was verified by through comparison with the hydrograph separation results based on δ18O data. The Fmax(C1)/Fmax(C2) ratio was then applied to estimate the contributions of rainwater and pre-event water sources under different land use types to the streamflow in an agro-forest sub-catchment and the entire catchment. The hydrograph separation results using the Fmax(C1)/Fmax(C2) ratio can be used to support the optimization of water resource management and the quantification of pollutant loadings from major water sources to streams at the catchment scale.

Applying multivariate techniques to fingerprint water quality impact of the Fundão Dam breach within the Rio Doce basin.

The Fundão Dam breach on 5 November 2015 (the "Event") released tailings, water, soil and/or sediments, and other debris to downstream watercourses. This breach included both direct and indirect impacts from scouring of soils and sediments along and within the affected courses. Multivariate statistical techniques were used to determine the potential of fingerprinting the impact of the breach compared to pre-Event water quality conditions and unaffected watercourses. The selection of key parameters is an important first step for multivariate analyses. Analysis of too many parameters can mask important trends and relationships, while analysis of too few may miss significant water quality indicators. A two-phased selection process was used to identify key parameters that indicated impact from the Event: (a) unbiased, principal component analysis to extract chemically dominant profiles among all measured parameters and (b) comparison of metals' concentrations between unaffected soils and/or sediments and tailings samples. Radar charts of key parameters along with statistical comparisons to pre-Event and not-affected waterways were then aggregated over space and time to assess impact and potential recovery to pre-Event conditions. Nine parameters were identified that characterize tailings-related (direct) and background soil and/or sediment-related (indirect) impacts. Spatially and temporally aggregated radar charts and nonparametric Mann-Whitney U tests were used to assess the statistical significance of these impacts during each wet season since the breach. Indirect parameters, like aluminum and lead, returned to pre-Event levels in the first wet season after the Event. By the 2018/2019 wet season, most of the direct and indirect parameters had returned to pre-Event levels. Integr Environ Assess Manag 2024;20:133-147. © 2023 NewFields Companies, LLC. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Pore doublet micromodel experiments of evaporation influence on pre-event water entrapment and pre-event and event water interaction

The stable isotopic signature of xylem water differs from those of rainfall and river water. Without considering the water exchange through the vapor phase, this phenomenon implies that when the event water (new water) infiltrates the soil, a part of the pre-event water (old water) is isolated. The isolation and immobilization of old water are suspected because of the low matrix potential. Because evaporation is the main process that can reduce the matrix potential after quick gravitational drainage, we conjectured that after evaporation, the old water hides in small pores and increases the likelihood of air entrapment, which separates the new and old water. To examine and visualize the potential effect of evaporation on the interactions between old and new water, we performed a series of wetting–drainage–evaporation–wetting experiments within pore doublet micromodels (PDMs). The experimental results showed that air bubbles are trapped when the menisci of the residual liquid are sufficiently far from the inlet and outlet of the PDM. Evaporation can increase these distances, resulting in the formation of larger air bubbles. The air bubble trapped upstream prevents the invading liquid from quickly flushing the original residual liquid out of the PDM. Nevertheless, the invading wetting liquid is still able to bypass the trapped air bubbles and mix with the residual wetting phase via corner flow. In addition, the remobilization of the upstream bubble enhances the process of removing the old trapped liquid.

Incorporating experimentally derived streamflow contributions into model parameterization to improve discharge prediction

Abstract. Environmental tracers have been used to separate streamflow components for many years. They allow us to quantify the contribution of water originating from different sources, such as direct runoff from precipitation, subsurface storm flow, or groundwater to total streamflow at variable flow conditions. Although previous studies have explored the value of incorporating experimentally derived fractions of event and pre-event water into hydrological models, a thorough analysis of the value of incorporating hydrograph-separation-derived information on multiple streamflow components at varying flow conditions into model parameter estimation has not yet been performed. This study explores the value of such information to achieve more realistic simulations of catchment discharge. We use a modified version of the process-oriented HBV model that simulates catchment discharge through the interplay of hillslope, riparian-zone discharge, and groundwater discharge at a small forested catchment which is located in the mountainous north of South Korea, subject to a monsoon season between June and August. Applying a Monte-Carlo-based parameter estimation scheme and the Kling–Gupta efficiency (KGE) to compare discharge observations and simulations across two seasons (2013 and 2014), we show that the model is able to provide accurate simulations of catchment discharge (KGE ≥ 0.8) but fails to provide robust predictions and realistic estimates of the contribution of the different streamflow components. Using a simple framework that compares simulated and observed contributions of hillslope, riparian zone, and groundwater to total discharge during two sub-periods, we show that the precision of simulated streamflow components can be increased, while remaining with accurate discharge simulations. We further show that the additional information increases the identifiability of all model parameters and results in more robust predictions. Our study shows how tracer-derived information on streamflow contributions can be used to improve the simulation and predictions of streamflow at the catchment scale without adding additional complexity to the model. The complementary use of temporally resolved observations of streamflow components and modeling provides a promising direction to improve discharge prediction by representing model internal dynamics more realistically.

Flow partitioning modelling using high-resolution electrical conductivity data during variable flow conditions in a tropical montane catchment

Tracer-aided hydrological models (TAHMs) are one of the most powerful tools to identify new (event) and old (pre-event) water fractions contributing to stormflow because they account both for streamflow and tracer mixing dynamics in model calibration. Nevertheless, their representativeness of hydrograph dynamics is often limited due to the unavailability of high-resolution conservative tracer data (e.g., water stable isotopes or chloride). Hence, there is a need to identify alternative tracers yielding similar flow partitioning results than “ideal” ones while requiring fewer financial resources for high-frequency monitoring (e.g., sub-hourly). Here, we compare flow partitioning results of a TAHM calibrated using high-frequency electrical conductivity (EC) and water stable isotope (18O) data collected during 37 rainfall-runoff events monitored during variable hydrometeorological conditions in the Zhurucay Ecohydrological Observatory, a tropical alpine catchment located in southern Ecuador. When the model was calibrated using the sampling resolution of stables isotopes (6-hours to 1-hour), no statistically significant differences of pre-event water fractions (PEWFs) using both tracers for model calibration were found. PEWF differences between both tracers for 89% of the events were < 20% regardless of the events’ antecedent moisture and rainfall conditions. Model transfer functions were also similar suggesting that catchment internal processes inferred using both tracers are comparable. Events presenting larger differences (n = 4; up to 27% PEWF difference) had no samples collected during peak flow. Calibration of the model using EC data collected at sub-hourly intervals (every 5-minutes) showed a significant increase in model performance as compared to the frequency of collection of isotopic data. Similarity in flow partitioning results can be attributed to a quasi-conservative nature of EC due to the presence of organic-rich riparian soils (peat-type) overlying compact bedrock across the catchment. Findings also highlight the importance of capturing rapidly occurring catchment mixing processes though high-temporal frequency monitoring of tracer data. Our study encourages the value of assessing the use of alternative tracers, such as EC, to identify fast occurring rainfall-runoff processes, while lowering the costs needed to implement and sustain tracer data collection for long time periods.

Storm event analysis of four forested catchments on the Atlantic coastal plain using a modified SCS-CN rainfall-runoff model

In this study, we calibrated and tested the Soil Conservation Service Curve Number (SCS-CN) based Modified Sahu-Mishra-Eldo (MSME) model for predicting storm event direct runoff (Qtot) and its soil saturation coefficient α as a threshold antecedent moisture condition for partitioning into overland surface and shallow subsurface runoff components. The model calibration was performed using 36 storm events from 2008 to 2015 on a 160-ha low-gradient forested watershed (WS80) on poorly drained soil. The model was further validated without calibration using data from 2011 to 2015 on two sites [115 ha (Conifer) and 210 ha (Eccles Church)] and from 2008 to 2011 on a third site, the 100-ha Upper Debidue Creek (UDC), all similar forested watersheds on the Atlantic Coastal Plain, USA. The calibrated MSME model was able to accurately predict the estimated Qtot_pred for the WS80 watershed, with calculated Nash-Sutcliffe efficiency coefficient (NSE), RMSE-standard deviation ratio (RSR), and percent bias (PBIAS) of 0.80, 0.44, and 16.7%, respectively. By applying the same calibrated α value of 0.639 from the WS80 to two other similar poorly drained watersheds, the MSME model satisfactorily predicted the estimated Qtot_pred for both the Eccles Church (NSE = 0.64; RSR = 0.57; PBIAS = 28.9%) and Conifer (NSE = 0.60; RSR = 0.58; PBIAS = 21.3%) watersheds, respectively. The MSME model, however, yielded unsatisfactory results (NSE = -0.13, RSR = 2.06, PBIAS = 616.3%) on the UDC watershed with coarse-textured soils, indicating the possible association of the α coefficient with soil subsurface texture. Based on the analysis of event rainfall and pre-event water table elevation, and linking them with the calibrated α coefficient that describes the proportion of saturated depth in a soil profile, it was found that rainfall was the main determining factor for overland runoff generation. These results demonstrate the MSME model’s potential to predict direct runoff in poorly drained forested watersheds, which serve as a reference for urbanizing coastal landscapes in a changing climate.

Soil thickness controls the rainfall-runoff relationship at the karst hillslope critical zone in southwest China

Hydrological processes in the critical zone are closely related to the soil–bedrock structures. However, the effect of soil thickness on the rainfall-runoff relationship on the hillslope with complex topography remains unclear. Surface runoff, lateral subsurface runoff from soil–epikarst interface, and epikarst runoff from the epikarst–bedrock interface were monitored on two adjacent plots with deep and shallow (66.0 vs. 35.4 cm) soil thicknesses from June 2019 to December 2020 in the karst region of southwest China. During the monitoring period, surface and subsurface runoff account for 20% and 37% of the total runoff in the deep-soil plot (DSP), and 3% and 43% in the shallow-soil plot (SSP). This demonstrates that runoff from the soil-epikarst system is predominant compared to the relatively small contribution of surface runoff. In the SSP, the surface topography wetness index (TWI) was highly coupled with bedrock TWI, and the bedrock TWI had a significant negative linear relationship (p < 0.01) with subsurface runoff. Moreover, isotope hydrogen-separation results showed that subsurface and epikarst runoff were dominated by pre-event water, but a higher contribution of event water was observed in the SSP than in the DSP. These findings supported the hypothesis that rainwater could infiltrate the epikarst more easily in shallow soil slopes. Rainfall and surface runoff exhibited a linear relationship in the dry season and a non-linear relationship in the rainy season, indicating the occurrence of threshold rainfall–runoff behavior. The rainfall amount threshold for surface runoff was higher in DSP (44.7 mm) than in SSP (39.5 mm), and the corresponding variation of rainfall intensity interpretation was greater (54% vs. 38%). For subsurface runoff, the rainfall amount threshold was higher in the DSP than in the SSP (91.0 vs 79.4 mm), and the corresponding variation of soil moisture interpretation was higher (56% vs. 20%). This demonstrated that runoff can be better predicted at deeper soil hillslopes by rainfall and antecedent soil moisture. Accordingly, this study emphasizes the importance of evaluating the spatial heterogeneity of soil thickness in hydrological process research.

Rainfall-induced groundwater ridging and the Lisse effect on tailings storage facilities: A literature review

The failure of tailings storage facilities (TSFs) results in the discharge of significant quantities of hazardous waste material into the natural environment. Research studies relating to slope instability have identified physical mechanisms such as rainfall-induced erosion, liquefaction, and shear failure as the main triggers. The generation of transient pressure waves and the mobilization of pre-event water in the unsaturated zone have been found to trigger shallow landslides in natural hillslopes. In this paper we review these physical mechanisms, known as groundwater ridging (GWR) and the Lisse effect (LE), from other studies. Previous researchers have explained both these phenomena through field and laboratory observations, numerical modelling, as well as conceptual discussions. These case studies demonstrate the impact of rainfall characteristics on the generation of transient pressure waves that rapidly increase the phreatic surface and change pore water suction. Reference is also made to the influence and behaviour of physical porous medium characteristics on the establishment of a continuous water phase that facilitates the transmission of an induced pressure head. However, previous studies fail to recognize the possibility that the pressure increase in pre-event water through pore air propagation could cause slope instability in tailings dams. The authors suggest that the physical properties and hydraulic behaviour of unsaturated porous tailings media make it susceptible to GWR and the LE, resulting in the creation of a potential failure plane.

Investigating flood processes in karst catchments by combining concentration-discharge relationship analysis and lateral flow simulation

Concentration-Discharge (C-Q) relationship analyses allow better understanding catchment water origin and hydrological process variability across time (from storm-event to pluriannual scale). Though, those approaches mostly include integrative methods that consider catchments as a whole. The aim of this article is to assess the potential of a combined approach to investigate the spatio-temporal variability of flood processes within catchments (across river reaches and seasons), focusing on karst areas. The methodology consists in combining C-Q relationship analysis in nested catchments and simulation of lateral exchanges at the reach scale. For that, we analyze high-frequency records of discharge and electrical conductivity (EC) in karst catchments, in which EC informs on water residence time. Contributions of pre-event water (PEW) and event water (EW) during storm events are investigated through hysteresis loop analysis at each gauging station. Combined with lateral EC simulation at the reach scale, they make it possible to infer runoff processes during storms. A unary karst catchment in a temperate/mountainous climate (Loue River) and a binary catchment in a Mediterranean climate (Cèze River) were selected in France, and their conceptual models of water origin and hydrological process variability were drawn. In the Loue catchment, summer and fall storm events are characterized by contributions of PEW through piston-type flows, whereas decreasing EC values in winter and spring storm events indicate the major contribution of EW through surface runoff and fast infiltration. Within the catchment, EW contribution increases downstream, due to the canyon river morphology grading into open valleys. Regarding the Cèze catchment, contributions of EW are higher, indicating that fast infiltration and surface runoff are the dominant processes, associated with a PEW signature in summer and fall. Within the catchment, PEW contribution is found higher in karstified areas. These results show that flood process seasonality is mainly related to karst aquifer saturation rate, while intra-site variability is linked to karst area extension and river morphology. Results are encouraging to extend this approach to a variety of sites, notably those affected by significant surface water-groundwater interaction and groundwater flooding. Such approach, by providing a discretized information on flood processes, could help refining lumped hydrological models, or facilitate the use of semi-distributed ones.

Applicability of Difference in Oxygen-18 and Deuterium of Water Sources and Isotopic Hydrograph Separation in a Bamboo Catchment during Different Rainfall Types

Typhoon storm and plum rain are two typical rainfall types in the lower regions of the Yangtze River Basin, which frequently cause flood disasters in China. New information in stable water isotopes offers the opportunity to advance understanding of runoff mechanisms and water source dynamics in response to these two typical rainfall types. We intensively monitored two representative rainfall events in a small bamboo forestry watershed in 2016. Results showed that precipitation isotopic variations during the event were generally larger than those of other monitored compartments (including throughfall, surface overland water, groundwater and river water) and also larger for the plum rain than for the typhoon event (δ18O varied in 5.2‰ and 3.7‰, respectively). Importantly, the differences of isotopic temporal variation between rainfall and throughfall showed significant impacts on the two-component hydrograph separation for both rainfall types (e.g., if not considered, the pre-event water fractions were 26.6% and 15.3% higher for the typhoon and plum rain events, respectively). Furthermore, we evaluated the role of soil water on the three-component isotopic hydrograph separation model; results revealed that soil water accounted for 10.9% and 28.3% of the total discharge in typhoon and plum rain events, respectively. This underpins the important role of soil water dynamics during the rainy season in this humid region.

Hydrological Separation of Event and Pre-event Soil Water in a Cultivated Landscape

Soil water mixing is a complex process that is dependent on vegetation, soil physiochemical characteristics, and climate. In this research, we utilize natural tracers, δ18O and δ2H, to determine if hydrological separation of recent precipitation event and pre-event water occurred within the soil profile of a maize field in central Ohio. Soil samples were collected at three sites and four depths, and the soil water was extracted via cryogenic distillation. Soil water isotopic composition showed mixing of precipitation event and pre-event water that reflected weighted average precipitation isotopic composition. The range in isotopic signatures also indicated that hydrological mixing within the soil profile was facilitated by soil physical characteristics and effective gravitational drainage of event water. These results support previous work that hydrological separation does not occur in soil water in temperate climates and contribute to ongoing research to conceptualize soil water cycling in monoculture systems.

Numerical dispersion of solute transport in an integrated surface–subsurface hydrological model

Integrated surface–subsurface hydrological models (ISSHMs) are increasingly being used for the assessment of contaminant transport in the environment, in addition to their more common use in water flow applications. However, the subsurface solute transport solvers in these models are prone to numerical dispersion errors. Numerical dispersion is a well-known issue in groundwater modeling, but its impacts on the results of ISSHM simulations are still poorly understood. In this study, the CATchment HYdrology (CATHY) model is used to assess the potential impacts of numerical dispersion on the simulation of coupled surface–subsurface solute transport. We first simulate the subsurface transport of a nonreactive tracer in two soil column test cases (1D and 3D) with known analytical solutions. The subsurface solute transport solver in CATHY adopts a computationally efficient time-splitting technique whereby the advection component of the governing equation is solved on elements and the hydrodynamic dispersion component is solved on nodes. Comparison between simulation results and analytical solutions with different mesh discretizations and different values for the hydrodynamic dispersion coefficients allows for accurate quantification of the numerical dispersion error and yields insights into the parameters and other factors that control it. It is shown that, taken alone, the advection and dispersion solvers are very robust, but their combination can result in significant numerical dispersion, stemming from the exchange of concentration information from elements to nodes and vice versa in the time-splitting procedure. The tests also show that these errors can be kept under control by ensuring that the grid Péclet number is in the range 0.5-1.0 or smaller. We then apply CATHY in a third test case involving two synthetic hillslopes (concave and convex) in fully coupled surface–subsurface mode, in order to examine the impact of this subsurface numerical dispersion on simulated streamflow hydrographs, in particular with reference to pre-event water contributions to runoff. Here as well the results show that the effect of numerical dispersion can be controlled by keeping the grid Péclet number sufficiently small. This work provides a new set of benchmark test cases for integrated surface–subsurface hydrological models, extending to solute transport the flow-only suite of benchmarks recently published in two intercomparison studies.

Characterizing shallow groundwater in hillslope aquifers using isotopic signatures: A case study in the Upper Blue Nile basin, Ethiopia

Study regionRobit-Bata watershed, Upper Blue Nile basin, Ethiopia. Study focusStable isotopes of water (Oxygen-18 and Deuterium) were used as tracers to estimate the contribution of groundwater in shallow hillslope aquifers to streamflow in the Robit-Bata watershed. To assess the spatiotemporal variability of shallow groundwater and develop a hydrograph separation technique, we collected rainfall, shallow groundwater, and streamflow samples and analyzed their δ18O and δ2H isotopic compositions. The local meteoric water line (LMWL) and local evaporative line (LEL) of the study area were determined and compared with the global meteoric water line (GMWL). A standard unweighted two-component isotope-based hydrograph separation model was used to determine the percentage contribution of shallow groundwater to streamflow. New hydrological insights for the regionThe LMWL (δ2H = 8.63·δ18O + 18.2) mostly showed heavy isotopic enrichment relative to GMWL, and the LEL (δ2H = 5.45·δ18O + 6.96) indicated isotopic enrichment compared to Ethiopian lakes. Shallow groundwater responded rapidly to rainfall, with good spatial correlation depending on topographic positions of wells. Pre-event water contributed <50% to peak discharge in July, but >90% when the watershed reached maximum storage. This finding gives insight towards the predominant runoff generation process and has significant implications for sustainable dry season irrigation expansion in the area as the sub-surface flow drains out of the watershed from October onwards reducing water tables in the shallow wells.

Tracing sources of stormflow and groundwater recharge in an urban, semi-arid watershed using stable isotopes

Study regionIn this study, we use stable isotopes of water to quantify the flow pathways delivering water to the tributaries, mainstem river and groundwater basin underlying urban San Diego. Information about sources of stormflow and recharge are necessary to maintain the health of waterways and aquifers, but studies of these processes are scarce in urban, semi-arid regions. Study focusIsotopic methods have been used in experimental or natural watersheds, but also have potential to quantify urban water cycling behaviour and water sources. We sampled baseflow, precipitation, and hourly stormflow from eight events with a range of antecedent conditions, and used end member mixing analysis to determine stormflow and groundwater sources. New hydrological insights for the regionOur results show that hydrologic connectivity controls stormflow sources. After a dry summer, and early in storm events, connectivity of pre-event water with the channel is low, so only new precipitation reaches the river. In wetter conditions, connectivity is higher and pre-event surface water mixes with infiltration-origin groundwater. Deeper groundwater composition mimics stormflow, a mix of stagnated river water and infiltration-origin water. The close connection between streamflow and groundwater implies that improving groundwater quality requires improvements to surface water quality. Average uncertainty in source fractions was ±8.0 %, suggesting that despite complex water pathways in urban, semi-arid environments, isotopic sampling is valuable for quantifying water sources.

Stable isotopes of water and specific conductance reveal complimentary information on streamflow generation in snowmelt-dominated, seasonally arid watersheds

The utility of a particular tracer to perform hydrograph separations depends on the dominant watershed properties combined with meteorological patterns; therefore, drawing conclusions from one tracer can be misleading. Combining information from multiple tracers can reveal complimentary insights that advance our knowledge of runoff generating processes. We performed hydrograph separations during spring snowmelt and for one summer rain event at two seasonally arid, montane watersheds in southeastern Wyoming using two independent tracer systems: Stable water isotopes (18O and 2H; 1–4 h resolution) and specific conductance (SC; 15 min resolution). Event water dominated streamflow generation during snowmelt using both tracers, but much lower uncertainty in hydrograph separations were achieved during this period with SC than with stable isotope tracers; the stable isotopic composition of pre-event and event water were too similar which led to large degrees of uncertainty. Stable isotopes of stream water did not vary as much as SC throughout the year and indicated the dominance of stream water derived from snowmelt. During the main snowmelt period, high frequency stream water isotopic composition was remarkably consistent, despite considerable variability measured in snowpack and snowmelt samples. Stable isotope measurements were useful for partitioning streamflow during a summer rain event when source waters were more isotopically distinct and suggested the dominance of pre-event water at generating streamflow. Despite the inability to partition event and pre-event streamflow during snowmelt, relationships between isotopic composition, SC, and watershed properties helped to understand mechanisms for streamflow generation. For instance, line conditioned excess (lc-excess) on the rising limb of the main snowmelt period was significantly positively correlated to the amount of rapid diurnal snowmelt contributions, the fastest moving compartment of event flow, supporting evidence that freshly melted snow, with relatively higher lc-excess, preferentially contributed to rapid diurnal snowmelt contributions. During fall low flow stream conditions, the isotopic value of streamflow was significantly positively correlated to elevation, where the highest-elevation watersheds had the highest delta values. This relationship was contradictory to what was expected (i.e. “elevation effect”) and may be explained by extensive snowpack fractionation occurring later in the season in deeper snowpacks located at higher elevations, or by a larger amount rainfall, with enriched isotopic composition relative to snowmelt, occurring at higher elevations. Through synthesizing patterns in two different natural tracers in conjunction with watershed properties, insights into the dominant controls on streamflow generation were gained from seasonally arid, snowmelt-dominated, headwater watersheds.

Analysis of Event-based Hydrological Processes at the Hydrohill Catchment Using Hydrochemical and Isotopic Methods

Abstract. Hydrochemical and isotopic techniques have been widely applied in hydrological sciences because isotopic tracers can identify water sources and hydrochemical tracers can discern runoff flow paths. To better understand the hydrological process, we combined hydrochemical and isotopic techniques under controlled experimental conditions to investigate hydrological process from rainfall to runoff in the Hydrohill experiment catchment, a typical artificial catchment in Chuzhou, China. Hydrochemical and isotopic data, i.e., pH, electric conductivity (EC), total dissolved solids (TDS), anions (Cl−, NO3-, SO42- and HCO3-), cations (K+, Na+, Ca2+ and Mg2+) and dissolved Si, 18O and D in water samples were collected during a rainfall event in 2016, and used to determine the hydrochemical and isotopic characteristics of rainfall and runoff components. We applied EC, TDS, SO42-, Ca2+, Mg2+, 18O and D as tracers to investigate rainfall-runoff processes in the experimental catchment. Runoff flow paths could be well identified by the relationship between 18O and EC, TDS, SO42-, Ca2+ and Mg2+. The quantity of flow flux and mass fluxes of main hydrochemical and isotopic tracers gauged at the catchment outlet shows applicable tracers (Ca2+, Mg2+, SO42-, and 18O) are mainly from deep groundwater runoff (from soil layer of 60–100 cm beneath ground surface). Contributions of the event water and pre-event water to the total runoff during the rainfall-runoff process are different. The quantitative results were very encouraging as a basis to develop hydrological models for further study.

Cascading multiscale watershed effects on differential carbon isotopic characteristics and associated hydrological processes

Understanding land-use change accompanied by anthropogenic activities under alterations in watershed size regulations or differential carbon (C) isotope characteristics remain a challenge in C cycling research. In this study, we investigate changes in the export of C composition and its isotopic characteristics at multiple scales in a subtropical cascading watershed in China. Results show that C concentrations in rainfall and dissolved total carbon (DTC), dissolved organic carbon (DOC) and δ13C in runoff seasonally fluctuate at a temporal scale. On average, the δ13C from silicate rock weathering was 31–32%, contributing the largest amount of δ13C in the different watersheds. Moreover, the contribution of isotopic composition from atmospheric deposition to the δ13C fraction increased as watershed size increased, while the corresponding contribution from soil organic matter (SOM) decomposition decreased. On the other hand, anthropogenic activities play a dominant role in the isotopic composition of large watersheds. In addition, the correlation coefficient between C transport via runoff and the δ18O value in rainfall increased as watershed size increased. This indicated that as a source rainfall had an obvious influence on C transport in runoff according to proportional values measured in event and pre-event water.

Precipitation\u2013soil water chemistry relationship: case study of an intensively managed grassland ecosystem in southwest England

Environmental assessments typically require fine- or high-resolution datasets, example of which the Environmental Change Network (ECN) provides. This study has therefore taken advantage of the free access to the datasets to investigate temporal characteristics of precipitation and soil water chemistry, and relationship between the concentration of selected chemical variables in precipitation and soil water at 0–10 cm and below 10 cm depths based on data availability. The aim was to determine whether observations from the ECN data support results from the previous hypothesis on dominant run-off mechanism at the study area. Method of analysis involves inferential statistics and wavelet transform plot of selected physico-chemical variables for six (2010–2016) years. Results showed that most of the selected chemical variables exhibited low coefficient of determination in the relationship of their concentration in precipitation and soil water. Relatively linear relationships, however, occurred in the values of conductivity, Cl−, Ca2+, alkalinity and Mg2+ between the 0–10 cm and below 10 cm depths, indicating mixing of soil water at both soil zones. Inference from the results suggests a possible significant role of pre-event water or geological influence on soil water chemistry. The study affirms the hypothesis that saturation overland flow dominates areas underlain by Halstow soil series in the area.