Testing a spatially distributed tracer‐aided runoff model in a snow‐influenced catchment: Effects of multicriteria calibration on streamwater ages

Integrating stable isotope tracers into rainfall‐runoff models allows investigation of water partitioning and direct estimation of travel times and water ages. Tracer data have valuable information content that can be used to constrain models and, in integration with hydrometric observations, test the conceptualization of catchment processes in model structure and parameterization. There is great potential in using tracer‐aided modelling in snow‐influenced catchments to improve understanding of these catchments' dynamics and sensitivity to environmental change. We used the spatially distributed tracer‐aided rainfall‐runoff (STARR) model to simulate the interactions between water storage, flux, and isotope dynamics in a snow‐influenced, long‐term monitored catchment in Ontario, Canada. Multiple realizations of the model were achieved using a combination of single and multiple objectives as calibration targets. Although good simulations of hydrometric targets such as discharge and snow water equivalent could be achieved by local calibration alone, adequate capture of the stream isotope dynamics was predicated on the inclusion of isotope data in the calibration. Parameter sensitivity was highest, and most local, for single calibration targets. With multiple calibration targets, key sensitive parameters were still identifiable in snow and runoff generation routines. Water ages derived from flux tracking subroutines in the model indicated a catchment where runoff is dominated by younger waters, particularly during spring snowmelt. However, resulting water ages were most sensitive to the partitioning of runoff sources from soil and groundwater sources, which was most realistically achieved when isotopes were included in the calibration. Given the paucity of studies where hydrological models explicitly incorporate tracers in snow‐influenced regions, this study using STARR is an important contribution to satisfactorily simulating snowpack dynamics and runoff generation processes, while simultaneously capturing stable isotope variability in snow‐influenced catchments.     

Using high resolution tracer data to constrain water storage,flux and age estimates in a spatially distributed rainfall‐runoff model

Models simulating stream flow and conservative tracers can provide a representation of flow paths, storage distributions and mixing processes that is advantageous for many predictive purposes. Compared with models that only simulate stream flow, tracer data can be used to investigate the internal consistency of model behaviour and to gain insight into model performance. Here, we examine the strengths and weaknesses of a data‐driven, spatially distributed tracer‐aided rainfall‐runoff model. The model structure allowed us to assess the influence of landscape characteristics on the routing and mixing of water and tracers. The model was applied to a site in the Scottish Highlands with a unique tracer data set; ~4 years of daily isotope ratios in stream water and precipitation were available, as well as 2 years of weekly soil and ground water isotopes. The model structure was based on an empirically based, lumped tracer‐aided model previously developed for the catchment. The best model runs were selected from Monte Carlo simulations based on dual calibration criteria using objective functions for both stream isotopes and discharge at the outlet. Model performance for these criteria was reasonable (Nash–Sutcliffe efficiencies for discharge and isotope ratios were ~0.4–0.6). The model could generally reproduce the variable isotope signals in the soils of the steeper hill slopes where storage was low, and damped isotope responses in valley bottom cells with high storage. The model also allowed us to estimate the age distributions of internal stores, water fluxes and stream flow. Average stream water age was ~1.6 years, integrating older groundwater in the valley bottom and dynamic younger soil waters. By tracking water ages and simulating isotopes, the model captured the changes in connectivity driven by distributed storage dynamics. This has substantially improved the representation of spatio‐temporal process dynamics and gives a more robust framework for projecting environmental change impacts. Copyright © 2016 The Authors Hydrological Processes Published by John Wiley & Sons Ltd.     

Effectivity of dissolved SF6 tracer for clarification of rainfall–runoff processes in a forested headwater catchment

Understanding rainfall‐runoff processes is crucial for prevention and prediction of water‐related natural disasters. Sulfur hexafluoride (SF₆) is a potential tracer, but few researches have applied it for rainfall‐runoff process studies. We observed multiple tracers including SF₆ in spring water at 1‐ to 2‐hr intervals during rainstorm events to investigate the effectivity of SF₆ tracer in rainfall–runoff studies through the clarification of rainfall–runoff process. The target spring is a perennial spring in a forested headwater catchment with an area of 0.045 km² in Fukushima, Japan. The relationship between the SF₆ concentration in spring water and the spring discharge volume was negative trend; the SF₆ concentration in spring water becomes low as the spring discharge volume increases especially during rainstorms. The hydrograph separation using SF₆ and chloride ion tracers was applied for determining the contribution of principal sources on rainfall–runoff water. It suggested more than 60% contribution of bedrock groundwater at the rainfall peak and high percentage contribution continued even in the hydrograph recession phase. Based on observed low SF₆ concentration in groundwater after heavy rainfall, the replacement of groundwater near the spring with bedrock groundwater is indicated as a mechanism for water discharge with low SF₆ concentration during rainfall events. Consequently, rainstorm events play an important role as triggers in discharging water stored in the deeper subsurface area. In addition, SF₆ tracer is concluded as one of the strongest tracers for examining rainfall–runoff process studies. And, therefore, this study provided new insights into the dynamics of groundwater and its responses to rainfall in terms of SF₆ concentration variance in water in headwater regions.     

Role of bedrock groundwater in the rainfall–runoff process in a small headwater catchment underlain by volcanic rock

The role of bedrock groundwater in rainfall–runoff processes is poorly understood. Hydrometric, tracer and subsurface water potential observations were conducted to study the role of bedrock groundwater and subsurface flow in the rainfall–runoff process in a small headwater catchment in Shiranui, Kumamoto prefecture, south‐west Japan. The catchment bedrock consists of a strongly weathered, fractured andesite layer and a relatively fresh continuous layer. Major chemical constituents and stable isotopic ratios of δ¹⁸O and δD were analysed for spring water, rainwater, soil water and bedrock groundwater. Temporal and spatial variation in SiO₂ showed that stream flow under the base flow condition was maintained by bedrock groundwater. Time series of three components of the rainstorm hydrograph (rainwater, soil water and bedrock groundwater) separated by end member mixing analysis showed that each component fluctuated during rainstorm, and their patterns and magnitudes differed between events. During a typical mid‐magnitude storm event, a delayed secondary runoff peak with 1·0 l s⁻¹ was caused by increase in the bedrock groundwater component, whereas during a large rainstorm event the bedrock groundwater component increased to ≈ 2·5 l s⁻¹. This research shows that the contribution of bedrock groundwater and soil water depends strongly on the location of the groundwater table, i.e. whether or not it rises above the soil–bedrock interface. Copyright © 2010 John Wiley & Sons, Ltd.     

Conceptual modelling to assess how the interplay of hydrological connectivity,catchment storage and tracer dynamics controls nonstationary water age estimates

Although catchment storage is an intrinsic control on the rainfall–runoff response of streams, direct measurement remains a major challenge. Coupled models that integrate long‐term hydrometric and isotope tracer data are useful tools that can provide insights into the dynamics of catchment storage and the volumes of water involved. In this study, we use a tracer‐aided hydrological model to characterize catchment storage as a dynamic control on system function related to streamflow generation, which also allows direct estimation of the nonstationarity of water ages. We show that in a wet Scottish upland catchment dominated by runoff generation from riparian peats (histosols) with high water storage, nonstationarity in water age distributions is only clearly detectable during more extreme wet and dry periods. This is explained by the frequency and longevity of hydrological connectivity and the associated relative importance of flow paths contributing younger or older waters to the stream. Generally, these saturated riparian soils represent large mixing zones that buffer the time variance of water age and integrate catchment‐scale partial mixing processes. Although storage simulations depend on model performance, which is influenced by input variability and the degree of isotopic damping in the stream, a longer‐term storage analysis of this model indicates a system that is only sensitive to more extreme hydroclimatic variability. Copyright © 2014 John Wiley & Sons, Ltd.     

Linking tracers,water age and conceptual models to identify dominant runoff processes in a sparsely monitored humid tropical catchment

Assessing catchment runoff response remains a key research frontier because of limitations in current observational techniques to fully characterize water source areas and transit times in diverse geographical environments. Here, we report a study that combines empirical data with modelling to identify dominant runoff processes in a sparsely monitored humid tropical catchment. The analysis integrated isotope tracers into conceptual rainfall–runoff models of varying complexity (from 5 to 11 calibrated parameters) that are able to simulate discharge and tracer concentrations and track the evolving age of stream water exiting the catchment. The model structures can be seen as competing hypotheses of catchment functioning and were simultaneously calibrated against uncertain streamflow gaugings and a 2‐year daily isotope rainfall–runoff record. Comparison of the models was facilitated using global parameter sensitivity analysis and the resulting effect on calibration. We show that a variety of tested model structures reproduced water and tracer dynamics in stream, but the simpler models failed to adequately reproduce both. The resulting water age distributions of the tested models varied significantly with little similarity between the stream water age and stored water age distributions. The sensitivity analysis revealed that only some of the more complex models (from eight parameters) could be better constrained to infer more plausible water age distributions and catchment storage estimates. These models indicated that the age of water stored in the catchment is generally older compared with the age of water fluxes, with evapotranspiration age being younger compared with streamflow. However, the water age distributions followed a similar temporal behaviour dominated by climatic seasonality. Stream water ages increased during the dry season (greater than 1 year) and decreased with increased streamflow (a few weeks old) during the wet season. We further show that the ratios of the streamwater age to stored water age distribution and the water age distribution of actual evapotranspiration to the stored water age distribution from constrained models could potentially serve as useful hydrological indicators of catchment functioning. Copyright © 2016 John Wiley & Sons, Ltd.     

Hydrometric and natural tracer (oxygen-18, silica,tritium and sulphur hexafluoride) evidence for a dominant groundwater contribution to Pukemanga Stream,New Zealand

Pukemanga is a small (3 ha) steep headwater catchment at the Whatawhata Research Station near Hamilton, New Zealand. The water balance (1996–2002) shows average annual rainfall of 1640 mm producing annual runoff of 440 mm (baseflow 326 mm, stormflow 114 mm) and ‘deep seepage’ loss of 450 mm (i.e. 450 mm of water not appearing in the stream). Oxygen-18 (¹⁸O) concentrations were measured at weekly intervals for 8–15 months at six sites, ranging from Pukemanga Stream baseflow through wetland seepage to ephemeral streams and surface runoff. The first two showed no significant ¹⁸O variations. Inferred mean residence times within the catchment ranged from at least 4 years (for the stream baseflow and seepage) to a few weeks (for the ephemeral flows and surface runoff). Silica concentrations could also be used to distinguish deep flowpath water from near-surface flowpath water. Tritium concentrations gave an estimated mean residence time of 9 years for Pukemanga Stream baseflow. Sulphur hexafluoride tended to give younger ages, while the chlorofluorocarbon ages were older, but are not considered as reliable for dating streamflow in this time range. These results show that deep pathways predominate with over 74% of runoff deriving from deep hillslope flowpaths via the wetland, and 87% of total drainage (baseflow and deep seepage) travelling via deep hillslope flowpaths. Our conception of the deep drainage process is that there is a large volume of slowly moving water in the system (above and below the water table), which reaches the wetland and stream via an unconfined groundwater system. Subsurface water equivalents are estimated to be 2·9 m for drainage at the weir and 4·1 m for drainage bypassing the weir, giving a total of 7 m depth over the catchment. The unsaturated zone plays an important role in storing water for long periods (about 4 years), while linking the surface with the groundwater water table to contribute to the fast streamflow response to rainfall. A schematic model of the various pathways with indicative residence times is given. Copyright © 2006 John Wiley & Sons, Ltd.     

Spatial delineation of groundwater–surface water interactions through intensive in‐stream profiling

The dominance of ‘old’ pre‐event water in headwater storm runoff has been recorded in numerous upland catchment studies; however, the mechanisms by which this pre‐event water enters the stream channel are poorly understood. Understanding these processes is fundamental to determining the controls on surface water quality and associated impacts on stream ecology. Previous studies in the upland forested catchment of the Afon Hafren (River Severn) at Plynlimon, mid‐Wales, identified an active bedrock groundwater system that was discharging into the stream channel during storm response. Detailed analysis showed that these discharges were small and could not account for the majority of pre‐event storm water response identified at this site; pre‐event storm runoff had to be sourced predominantly from further upstream. An intensive stream survey was used to determine the spatial nature of groundwater–surface water (GW–SW) interactions in the Hafren Catchment. Detailed physico‐chemical in‐stream profiling identified a marked change in water quality indicating a significant discrete point of bedrock groundwater discharge upstream of the Hafren Transect study site. The in‐stream profiling showed the importance of high spatial resolution sampling as a key to understanding processes of GW–SW interaction and how quick and cost‐effective measurements of specific electrical conductance of stream waters could be used to highlight in‐stream heterogeneity. This approach is recommended for use in headwater catchments for initial characterisation of the stream channel in order to better locate instrumentation and to determine more effective targeted sampling protocols in upland catchment research. Copyright © 2012 John Wiley & Sons, Ltd.     

Linking physiography and evaporation using the isotopic composition of river water in 16 Canadian boreal catchments

Highly seasonal boreal catchments are hydrologically complex and generally data poor and, hence, are ripe for investigation using tracer‐aided hydrologic models. The influence of physiography on isotopic metrics was assessed to identify the catchment characteristics dominating evaporative enrichment. A multiyear stable isotope of water dataset was collected at the outlets of 16 boreal catchments in central Canada ranging in area from 12 to 15,282 km². Physiographic characteristics were obtained through raster analysis of freely available land cover images, stream networks, and digital elevation models. Correlation analysis indicated that as the percentage coverage of open water increased, so too did the evaporative effects observed at the catchment outlet. Correlation to wetland metrics indicated that increasing the percentage coverage of wetlands can reduce or increase evaporative effects observed, depending on the isotopic metric used and the corresponding drainage density, catchment slope, and presence of headwater lakes. The slopes of river evaporative‐mixing lines appear to reflect multifaceted relationships, strongest between catchment slope, headwater lakes, and connected wetlands, whereas mean line‐conditioned excess is more directly linked to physiographic variables. Hence, the slopes of river evaporative‐mixing lines and mean line‐conditioned excess are not interchangeable metrics of evaporative enrichment in a catchment. Relationships identified appear to be independent of catchment scale. These results suggest that adequate inclusion of the distribution of open water throughout a catchment, adequate representation of wetland processes, catchment slope, and drainage density are critical characteristics to include in tracer‐aided hydrologic models in boreal environments in order to minimize structural uncertainty.     

Contributions of bedrock groundwater to the upscaling of storm‐runoff generation processes in weathered granitic headwater catchments

We examined the contributions of bedrock groundwater to the upscaling of storm‐runoff generation processes in weathered granitic headwater catchments by conducting detailed hydrochemical observations in five catchments that ranged from zero to second order. End‐member mixing analysis (EMMA) was performed to identify the geographical sources of stream water. Throughfall, hillslope groundwater, shallow bedrock groundwater, and deep bedrock groundwater were identified as end members. The contribution of each end member to storm runoff differed among the catchments because of the differing quantities of riparian groundwater, which was recharged by the bedrock groundwater prior to rainfall events. Among the five catchments, the contribution of throughfall was highest during both baseflow and storm flow in a zero‐order catchment with little contribution from the bedrock groundwater to the riparian reservoir. In zero‐order catchments with some contribution from bedrock groundwater, stream water was dominated by shallow bedrock groundwater during baseflow, but it was significantly influenced by hillslope groundwater during storms. In the first‐order catchment, stream water was dominated by shallow bedrock groundwater during storms as well as baseflow periods. In the second‐order catchment, deeper bedrock groundwater than that found in the zero‐order and first‐order catchments contributed to stream water in all periods, except during large storm events. These results suggest that bedrock groundwater influences the upscaling of storm‐runoff generation processes by affecting the linkages of geomorphic units such as hillslopes, riparian zones, and stream channels. Our results highlight the need for a three‐dimensional approach that considers bedrock groundwater flow when studying the upscaling of storm‐runoff generation processes. Copyright © 2014 John Wiley & Sons, Ltd.     

Response time and water origin in a steep nested catchment in the Italian Dolomites

In this study, we investigate the surface flow time of rise in response to rainfall and snowmelt events at different spatial scales and the main sources originating channel runoff and spring water in a steep nested headwater catchment (Rio Vauz, Italian Dolomites), characterized by a marked elevation gradient. We monitored precipitation at different elevations and measured water stage/streamflow at the outlet of two rocky subcatchments of the same size, representative of the upper part of the catchment dominated by outcropping bedrock, at the outlet of a soil‐mantled and vegetated subcatchment of similar size but different morphology, and at the outlet of the main catchment. Hydrometric data are coupled with stable isotopes and electrical conductivity sampled from different water sources during five years, and used as tracers in end‐member mixing analysis, application of two component mixing models and analysis of the slope of the dual‐isotope regression line. Results reveal that times of rise are slightly shorter for the two rocky subcatchments, particularly for snowmelt and mixed rainfall/snowmelt events, compared to the soil‐mantled catchment and the entire Rio Vauz Catchment. The highly‐variable tracer signature of the different water sources reflects the geomorphological and geological complexity of the study area. The principal end‐members for channel runoff and spring water are identified in rainfall and snowmelt, which are the dominant water sources in the rocky upper part of the study catchment, and soil water and shallow groundwater, which play a relevant role in originating baseflow and spring water in the soil‐mantled and vegetated lower part of the catchment. Particularly, snowmelt contributes up to 64 ± 8% to spring water in the concave upper parts of the catchment and up to 62 ± 11% to channel runoff in the lower part of the catchment. These results offer new experimental evidences on how Dolomitic catchments capture and store rain water and meltwater, releasing it through a complex network of surface and subsurface flow pathways, and allow for the construction of a preliminary conceptual model on water transmission in snowmelt‐dominated catchments featuring marked elevation gradients.     

Isotope hydrology and water sources in a heavily urbanized stream

Complex networks of both natural and engineered flow paths control the hydrology of streams in major cities through spatio-temporal variations in connection and disconnection of diverse water sources. We used spatially extensive and temporally intensive sampling of water stable isotopes to disentangle the hydrological sources of the heavily urbanized Panke catchment (~220 km²) in the north of Berlin, Germany. The isotopic data enabled us to partition stream water sources across the catchment using a Bayesian mixing analysis. The upper part of the catchment streamflow is dominated by groundwater (~75%) from gravel aquifers. In dry summer periods, streamflow becomes intermittent in the upper catchment, possibly as a result of local groundwater abstractions. Storm drainage dominates the responses to precipitation events. Although such events can dramatically change the isotopic composition of the upper stream network, storm drainage only accounts for 10%–15% of annual streamflow. Moving downstream, subtle changes in sources and isotope signatures occur as catchment characteristics vary and the stream is affected by different tributaries. However, effluents from a wastewater treatment plant (WWTP), serving 700,000 people, dominate stream flow in the lower catchment (~90% of annual runoff) where urbanization effects are more dramatic. The associated increase in sealed surfaces downstream also reduces the relative contribution of groundwater to streamflow. The volume and isotopic composition of storm runoff is again dominated by urban drainage, though in the lower catchment, still only about 10% of annual runoff comes from storm drains. The study shows the potential of stable water isotopes as inexpensive tracers in urban catchments that can provide a more integrated understanding of the complex hydrology of major cities. This offers an important evidence base for guiding the plans to develop and re-develop urban catchments to protect, restore, and enhance their ecological and amenity value.     

Water balance,nutrient and carbon export from a heath forest catchment in central Amazonia,Brazil

Carbon storage values in the Amazon basin have been studied through different approaches in the last decades in order to clarify whether the rainforest ecosystem is likely to act as a sink or source for carbon in the near future. This water balance, dissolved organic carbon (DOC) and nutrient export study were carried out in a micro‐scale heath forest (Campina) catchment in central Amazonia, Brazil. For a 1‐year study period (18 March 2007 until 19 March 2008), rainfall amounted to 3054 mm; of which, 1532 mm was evaporated by the forest (4.1 mm day^?1). Rainfall interception loss amounted to 15.6% of gross rainfall. Surface runoff amounted to 485 mm, whereas another 1071 mm was discharged as regional groundwater outflow. Accumulated DOC exports in surface runoff amounted to 15.3 g m^?2 year^?1, whereas the total carbon exported was 55.9 g m^?2. This is much higher than that observed for a nearby tall rainforest catchment in central Amazonia (DOC export < 20 g m^?2). As Campina heath forest areas cover a significant proportion of the Amazon Basin, these differences in ecosystem hydrological carbon exports should be taken into account in future studies assessing the carbon budget for the Amazon Basin. Macro‐nutrient exports were low, but those of calcium and potassium were higher than those observed for tall rainforest in the Amazon, which may be caused by a lower retention capacity of the heath forest ecosystem. Copyright © 2015 John Wiley & Sons, Ltd.     

Estimation of sub-annual inter-catchment groundwater flow using short-term water balance method

Predicting inter-catchment groundwater flow (IGF) is essential because IGF greatly affects stream water discharge and water chemistry. However, methods for estimating sub-annual IGF and clarifying its mechanisms using minimal data are limited. Thus, we quantified the sub-annual IGF and elucidated its driving factors using the short-term water balance method (STWB) for three forest headwater catchments in Japan (named here catchment A, B and As). Our previous study using the chloride mass balance indicated that annual IGF of catchment A (49.0 ha) can be negligible. Therefore, we calculated the daily evapotranspiration (ET) rate using the Priestley–Taylor expression and the 5-year water balance in catchment A (2010–2014). The sub-annual IGF of the three catchments was then calculated by subtracting the ET rate from the difference between rainfall and stream discharge during the sub-annual water balance periods selected using the STWB. The IGF rates of catchment B (7.0 ha), which is adjacent to catchment A, were positive in most cases, indicating that more groundwater flowed out of the catchment than into it, and exhibited positive linear relationships with rainfall and stream discharge. This suggested that as the catchments became wetter, more groundwater flowed out of catchment B. Conversely, the IGF rates of catchment As (5.3 ha), included in catchment A, were negative in most cases, indicating that more groundwater flowed into the catchment than out from it, and exhibited negative linear relationships with rainfall and stream discharge. Given the topography of the catchments studied, infiltration into the bedrock was the probable reason for the IGF outflow from catchment B. We hypothesized that in catchment As, the discrepancy between the actual hydrological boundary and the surface topographic boundary could have caused an IGF inflow. This study provides a useful tool for determining an IGF model structure to be incorporated into rainfall-runoff models.     

Using high‐resolution isotope data and alternative calibration strategies for a tracer‐aided runoff model in a nested catchment

Testing hydrological models over different spatio‐temporal scales is important for both evaluating diagnostics and aiding process understanding. High‐frequency (6‐hr) stable isotope sampling of rainfall and runoff was undertaken during 3‐week periods in summer and winter within 12 months of daily sampling in a 3.2‐km² catchment in the Scottish Highlands. This was used to calibrate and test a tracer‐aided model to assess the (a) information content of high‐resolution data, (b) effect of different calibration strategies on simulations and inferred processes, and (c) model transferability to <1‐km² subcatchment. The 6‐hourly data were successfully incorporated without loss of model performance, improving the temporal resolution of the modelling, and making it more relevant to the time dynamics of the isotope and hydrometric response. However, this added little new information due to old‐water dominance and riparian mixing in this peatland catchment. Time variant results, from differential split sample testing, highlighted the importance of calibrating to a wide range of hydrological conditions. This also provided insights into the nonstationarity of catchment mixing processes, in relation to storage and water ages, which varied markedly depending on the calibration period. Application to the nested subcatchment produced equivalent parameterization and performance, highlighting similarity in dominant processes. The study highlighted the utility of high‐resolution data in combination with tracer‐aided models, applied at multiple spatial scales, as learning tools to enhance process understanding and evaluation of model behaviour across nonstationary conditions. This helps reveal more fully the catchment response in terms of the different mechanistic controls on both wave celerites and particle velocities.     

Quantifying aggregation and change in runoff source in accordance with catchment area increase in a forested headwater catchment

There has been a great deal of research interest regarding changes in flow path/runoff source with increases in catchment area. However, there have been very few quantitative studies taking subscale variability and convergence of flow path/runoff source into account, especially in relation to headwater catchments. This study was performed to elucidate how the contributions and discharge rates of subsurface water (water in the soil layer) and groundwater (water in fractured bedrock) aggregate and change with catchment area increase, and to elucidate whether the spatial variability of the discharge rate of groundwater determines the spatial variability of stream discharge or groundwater contribution. The study area was a 5‐km² forested headwater catchment in Japan. We measured stream discharge at 113 points and water chemistry at 159 points under base flow conditions. End‐member mixing analysis was used to separate stream water into subsurface water and groundwater. The contributions of both subsurface water and groundwater had large variability below 1 km². The contribution of subsurface water decreased markedly, while that of groundwater increased markedly, with increases in catchment area. The specific discharge of subsurface water showed a large degree of variability and decreased with catchment area below 0.1 km², becoming almost constant above 0.1 km². The specific discharge of groundwater showed large variability below 1 km² and increased with catchment area. These results indicated that the variabilities of stream discharge and groundwater contribution corresponded well with the variability of the discharge rate of groundwater. However, below 0.1 km², it was necessary to consider variations in the discharge rates of both subsurface water and groundwater. Copyright © 2016 John Wiley & Sons, Ltd.     

Insights on stream temperature processes through development of a coupled hydrologic and stream temperature model for forested coastal headwater catchments

Stream temperature controls a number of biological, chemical, and physical processes occurring in aquatic environments. Transient snow cover and advection associated with lateral throughflow inputs can have a dominant influence on stream thermal regimes for headwater catchments in the rain‐on‐snow zone. Most existing stream temperature models lack the ability to properly simulate these processes. We developed and evaluated a conceptual‐parametric catchment‐scale stream temperature model that includes the role of transient snow cover and lateral advection associated with throughflow. The model consists of routines for simulating canopy interception, snow accumulation and melt, hillslope throughflow runoff and temperature, and stream channel energy exchange processes. The model was used to predict discharge and stream temperature for a small forested headwater catchment near Vancouver, Canada, using long‐term (1963–2013) weather data to compute model forcing variables. The model was evaluated against 4 years of observed stream temperature. The model generally predicted daily mean stream temperature accurately (annual RMSE between 0.57 and 1.24 °C) although it overpredicted daily summer stream temperatures by up to 3 °C during extended low streamflow conditions. Model development and testing provided insights on the roles of advection associated with lateral throughflow, channel interception of snow, and surface–subsurface water interactions on stream thermal regimes. This study shows that a relatively simple but process‐based model can provide reasonable stream temperature predictions for forested headwater catchments located in the rain‐on‐snow zone.     

Using TOPMODEL towards identifying and modelling the hydrological patterns within a headwater,humid, tropical catchment

Increasing pressure on the tropical environment requires a more thorough understanding of hydrological processes as part of reconciling the conflicting demands of economic development vis-à-vis sustainable land management. Using TOPMODEL, a physically based semi-distributed topohydrological model, we test its validity in modelling the stream flow dynamics (hydrograph) in a 1 ha tropical rainforest catchment in French Guiana. Another objective is through field validation of TOPMODEL to ascertain possible runoff generation mechanisms. The field validation of the temporal and spatial hydrodynamics across a rainfall–runoff event reveals that TOPMODEL may be suited for applications to this particular tropical rainforest environment; in fact, this is possibly the first successful application of such a model within the humid tropics. The main reasons why the model was successful are the presumed low hydraulic conductivities of the subsoil, coupled with the absence of an additional deep groundwater body, the contribution from which has caused difficulties in application of topographically, ‘physically’ based runoff models elsewhere in the humid tropics. © 1997 John Wiley & Sons, Ltd.     

Stable isotope tracers as diagnostic tools in upscaling flow path understanding and residence time estimates in a mountainous mesoscale catchment

δ¹⁸O measurements of precipitation and stream waters were used as a natural tracer to investigate hydrological pathways and residence times in the River Feshie, a complex mesoscale (231 km²) catchment in the Cairngorm Mountains of Scotland. Precipitation δ¹⁸O exhibited strong seasonal variation over the 2001–02 hydrological year, ranging from −6·9‰ in the summer, to −12·0‰ during winter snowfalls (mean δ¹⁸O −9·59‰). Although damped, this seasonality was reflected in stream water outputs at seven sampling sites in the catchment, allowing δ¹⁸O variations to be used to infer hydrological source areas. Thus, stream water δ¹⁸O was generally controlled by a seasonally variable storm flow end member, mixing with groundwater of more constant isotopic composition. Periodic regression analysis allowed the differences in this mixing process between monitoring subcatchments to be assessed more quantitatively to provide a preliminary estimate of mean stream water residence time. This demonstrated the importance of responsive hydrological pathways associated with peat and shallow alpine soils in the headwater subcatchments in producing seasonally variable runoff with short mean residence times (33–113 days). In contrast, other tributaries with more freely draining soils and larger groundwater storage in shallow aquifers provided more effective mixing of variable precipitation inputs, resulting in longer residence time estimates (178–445 days). The mean residence time of runoff leaving the Feshie catchment reflected an integration of these contrasting influences (110–200 days). These insights from δ¹⁸O measurements extend the hydrological understanding of the Feshie catchment gained from other hydrochemical tracers, and demonstrate the utility of isotope tracers in investigating hydrological processes at the mesoscale. Copyright © 2005 John Wiley & Sons, Ltd.     

Effects of hillslope topography on runoff response in a small catchment in the Fudoji Experimental Watershed,central Japan

We investigated the role of different hillslope units with different topographic characteristics on runoff generation processes based on field observations at two types of hillslopes (0·1 ha): a valley‐head (a convergent hillslope) and a side slope (a planar hillslope), as well as at three small catchments having two types of slopes with different drainage areas ranging from 1·9 to 49·7 ha in the Tanakami Mountains, central Japan. We found that the contribution of the hillslope unit type to small catchment runoff varied with the magnitude of rainfall. When the total amount of rainfall for a single storm event was < 35 mm, runoff in the small catchment was predominantly generated from the side slope. As the amount of rainfall increased (>35 mm), the valley‐head also began to contribute to the catchment runoff, adding to runoff from the side slope. Although the direct runoff from the valley‐head was greater than that from the side slope, the contribution from the side slope was quantitatively greater than that from the valley‐head due to the proportionally larger area occupied by the side slope in the small catchment. The storm runoff responses of the small catchments reflected the change in the runoff components of each hillslope unit as the amount of rainfall increased and rainfall patterns changed. However, similar runoff responses were found for the small catchments with different areas. The similarity of the runoff responses is attributable to overlay effects of different hillslope units and the similar composition ratios of the valley‐head and side slope in the catchments. This study suggests that the relative roles of the valley‐head and side slope are important in runoff generation and solute transport as the catchment size increases from a hillslope/headwater to a small catchment. Copyright © 2011 John Wiley & Sons, Ltd.