Estimating runoff from a glacierized catchment using natural tracers in the semi‐arid Andes cordillera

This paper presents a methodology for hydrograph separation in mountain watersheds, which aims at identifying flow sources among ungauged headwater sub‐catchments through a combination of observed streamflow and data on natural tracers including isotope and dissolved solids. Daily summer and bi‐daily spring season water samples obtained at the outlet of the Juncal River Basin in the Andes of Central Chile were analysed for all major ions as well as stable water isotopes, δ¹⁸O and δD. Additionally, various samples from rain, snow, surface streams and exfiltrating subsurface water (springs) were sampled throughout the catchment. A principal component analysis was performed in order to address cross‐correlation in the tracer dataset, reduce the dimensionality of the problem and uncover patterns of variability. Potential sources were identified in a two‐component U‐space that explains 94% of the observed tracer variability at the catchment outlet. Hydrograph separation was performed through an Informative‐Bayesian model. Our results indicate that the Juncal Norte Glacier headwater sub‐catchment contributed at least 50% of summer flows at the Juncal River Basin outlet during the 2011–2012 water year (a hydrologically dry period in the Region), even though it accounts for only 27% of the basin area. Our study confirms the value of combining solute and isotope information for estimating source contributions in complex hydrologic systems, and provides insights regarding experimental design in high‐elevation semi‐arid catchments. The findings of this study can be useful for evaluating modelling studies of the hydrological consequences of the rapid decrease in glacier cover observed in this region, by providing insights into the origin of river water in basins with little hydrometeorological information. Copyright © 2016 John Wiley & Sons, Ltd.     

Determination of runoff components using path analysis and isotopic measurements in a glacier‐covered alpine catchment (upper Hailuogou Valley) in southwest China

Monitoring of stable water isotopes (δ¹⁸O and δ²H) at the watershed scales can improve our understanding of complex hydrology and hydroclimatology of the watershed, especially in remote regions. Previous studies that used tracers for hydrograph separation are largely based on end‐member mixing approach (EMMA), but one drawback of this approach is that at least two independent tracers are required for multi‐component separation. Here we introduce a new approach—path analysis, in combination with isotopic measurements to investigate the runoff generation in a glacier‐covered alpine catchment (upper Hailuogou Valley) in southwest China. This newly developed method can not only provide a multi‐component hydrograph separation with the aid of only one tracer but also determine the direct and indirect influence of sources on streamflow. Path analysis show that the majority of streamflow is dominated by ice/snow meltwater that represents about 63–78% of the total discharge, whereas precipitation and groundwater contribute approximately 19–39% and 2–4% of the streamflow discharge, respectively. These results are in good agreement with those derived from EMMA (using ¹⁸O and Cl^? as tracers), corroborating that our proposed approach is successful in hydrograph separation of the catchment. This approach may provide new opportunities for the hydrograph separation of catchment with sparse data and be of interest to catchment hydrologists who seek to understand the behaviour of hydrologic systems. Copyright © 2015 John Wiley & Sons, Ltd.     

Surface moisture area during rainfall–run‐off events to understand the hydrological dynamics of a basin in a plain region

The understanding of the hydrology of plain basins may be improved by the combined analysis of rainfall–run‐off records and remote sensed surface moisture data. Our work evaluates the surface moisture area (SMA) produced during rainfall–run‐off events in a plain watershed of the Argentine Pampas Region, and studies which hydrological variables are related to the generated SMA. The study area is located in the upper and middle basins of the Del Azul stream, characterized by the presence of small gently hilly areas surrounded by flat landscapes. Data from 9 rainfall–run‐off events were analysed. MODIS surface reflectance data were processed to calculate SMA subsequent to the peak discharge (post‐SMA), and previous to the rainfall events (prev‐SMA), to consider the antecedent wetness. Rainfall–run‐off data included total precipitation depth (P), maximum intensity of rainfall over 6 hr (I6max), surface run‐off registered between the beginning of the event and the day previous to the analysed MODIS scene (R), peak flow (Qp), and flood intensity (IF). In contrast with other works, post‐SMA showed a negative relationship with the R. Three groups of cases were identified: (a) Events of low I6max, high prev‐SMA, and low R were associated with slow and weakly channelized flow over plain areas, leading to saturated overland flow (SOF), with large SMA; (b) events of high I6max, low prev‐SMA, and medium to high R were rapidly transported along the gentle slopes of the basin, related to Hortonian overland flow (HOF) and low post‐SMA; and (c) events of medium to high I6max and prev‐SMA with medium R were related to heterogeneous input‐antecedent‐run‐off conditions combined: Local spatial conditions may have produced HOF or SOF, leading to an averaged response with medium SMA. The interactions between the geomorphology of the basin, the characteristics of the events, and the antecedent conditions may explain the obtained results. This analysis is relevant for the general knowledge of the hydrology of large plains, whose functioning studies are still in their early stages.     

Rainfall,run‐off,and sediment transport dynamics in a humid mountain badland area: Long‐term results from a small catchment

In this study, we investigated rainfall, run‐off, and sediment transport dynamics (414 run‐off events and 231 events with sediment information) of a humid mountain badland area—the Araguás catchment (Central Pyrenees, Spain)—from October 2005 to September 2016. Use of this long‐term database allows characterization of the hydrological response, which consist of low‐magnitude/high‐frequency events and high‐magnitude/low‐frequency events, and identification of seasonal dynamics and rainfall‐run‐off thresholds. Our results indicate that the Araguás catchment, similarly to other humid badlands, had high hydrological responsiveness (mean annual run‐off coefficient: 0.52), a non‐linear relationship of rainfall with run‐off (common in Mediterranean environments), and seasonal hydrological and sedimentological dynamics. We created and validated a multivariate regression model to characterize the hydrological variables (stormflow and peak discharge) and sedimentological variables (mean and maximum suspended sediment concentrations and total suspended sediment load). In summer and at the beginning of autumn, the response was mainly related to rainfall intensity, suggesting a predomination of Hortonian flows. In contrast, in spring and winter, the responses were mainly related to the antecedent conditions (previous rainfall and baseflow), suggesting the occurrence of saturated excess flow processes, and the contribution of neighbouring vegetated areas. The multivariate analysis also showed that total sediment load is better predicted by a multivariate regression model that integrates pre‐event, rainfall, and run‐off variables. In general, our models provided more accurate predictions of small‐magnitude/high‐frequency events than high‐magnitude/low‐frequency events. This study highlights the high inter‐ and intra‐annual variability response in humid badland areas and that long‐term records are needed to reduce the uncertainty of hydrological and sedimentological responses in Mediterranean badland areas.     

Seasonal changes in runoff generation in a small forested mountain catchment

This study aimed to investigate the seasonal variability of runoff generation processes, the sources of stream water, and the controls on the contribution of event water to streamflow for a small forested catchment in the Italian pre‐Alps. Hydrometric, isotopic, and electrical conductivity data collected between August 2012 and August 2013 revealed a marked seasonal variability in runoff responses. Noticeable differences in runoff coefficients and hydrological dynamics between summer and fall/spring rainfall events were related to antecedent moisture conditions and event size. Two‐component and three‐component hydrograph separation and end‐member mixing analysis showed an increase in event water contributions to streamflow with event size and average rainfall intensity. Event water fractions were larger during dry conditions in the summer, suggesting that stormflow generation in the summer consisted predominantly of direct channel precipitation and some saturated overland flow from the riparian zone. On the contrary, groundwater and hillslope soil water contributions dominated the streamflow response during wet conditions in fall. Seasonal differences were also noted between event water fractions computed based on isotopic and electrical conductivity data, likely because of the dilution effect during the wetter months. Copyright © 2014 John Wiley & Sons, Ltd.     

Glacier retreat and its impact on summertime run‐off in a high‐altitude ungauged catchment

Glaciers are significant freshwater storage systems in western China and contribute substantially to the summertime run‐off of many large rivers in the Tibetan Plateau. Under the scenario of climate change, discussions of glacier variability and melting contributions in alpine basins are important for understanding the run‐off composition and ensuring that water resources are adequately managed and protected in the downstream areas. Based on the multisource spatial data and long‐term ground observation of climatic and hydrologic data, using the remote sensing interpretation, degree‐day model, and ice volume method, we presented a comprehensive study of the glacier changes in number, area, and termini and their impacts on summertime run‐off and water resource in the Tuotuo River basin, located in the source region of the Yangtze River. The results indicated that climate change, especially rising temperature, accelerated the glacier melting and consequently led to hydrological change. From 1969 to 2009, the glacier retreat showed an absolutely dominant tendency with 13 reduced glaciers and lost glacier area of 45.05 km², accompanied by limited growing glaciers in the study area. Meanwhile, it indicated that annual glacial run‐off was averagely 0.38 × 10⁸ m³, accounting for 4.96% of the total summertime run‐off, followed by the supply from precipitation and snowmelt. The reliability of this magnitude was assessed by the classic volume method, which also showed that the water resources from glacier melting in the Tuotuo River basin increased by approximate 17.11 × 10⁸ m³, accounting for about 3.77% of the total run‐off over the whole period of 1969–2009. Findings from this study will serve as a reference for future research about glacier hydrology in regions where observational data are deficient. Also, it can help the planning of future water management strategies in the source region of the Yangtze River.     

Groundwater–surface water interactions in a lowland watershed: source contribution to stream flow

The lower coastal plain of the Southeast USA is undergoing rapid urbanisation as a result of population growth. Land use change has been shown to affect watershed hydrology by altering stream flow and, ultimately, impairing water quality and ecologic health. However, because few long‐term studies have focused on groundwater–surface water interactions in lowland watersheds, it is difficult to establish what the effect of development might be in the coastal plain region. The objective of this study was to use an innovative improvement to end‐member mixing analysis (EMMA) to identify time sequences of hydrologic processes affecting storm flow. Hydrologic and major ion chemical data from groundwater, soil water, precipitation and stream sites were collected over a 2‐year period at a watershed located in USDA Forest Service's Santee Experimental Forest near Charleston, South Carolina, USA. Stream flow was ephemeral and highly dependent on evapotranspiration rates and rainfall amount and intensity. Hydrograph separation for a series of storm events using EMMA allowed us to identify precipitation, riparian groundwater and streambed groundwater as main sources to stream flow, although source contribution varied as a function of antecedent soil moisture condition. Precipitation, as runoff, dominated stream flow during all storm events while riparian and streambed groundwater contributions varied and were mainly dependent on antecedent soil moisture condition. Sensitivity analyses examined the influence of 10% and 50% increases in analyte concentration on EMMA calculations and found that contribution estimates were very sensitive to changes in chemistry. This study has implications on the type of methodology used in traditional forms of EMMA research, particularly in the recognition and use of median end‐member water chemistry in hydrograph separation techniques. Potential effects of urban development on important hydrologic processes (groundwater recharge, interflow, runoff, etc.) that influence stream flow in these lowland watersheds were qualitatively examined. Copyright © 2011 John Wiley & Sons, Ltd.     

A comparison of high‐resolution specific conductance‐based end‐member mixing analysis and a graphical method for baseflow separation of four streams in hydrologically challenging agricultural watersheds

Quantifying the relative contributions of different sources of water to a stream hydrograph is important for understanding the hydrology and water quality dynamics of a given watershed. To compare the performance of two methods of hydrograph separation, a graphical program [baseflow index (BFI)] and an end‐member mixing analysis that used high‐resolution specific conductance measurements (SC‐EMMA) were used to estimate daily and average long‐term slowflow additions of water to four small, primarily agricultural streams with different dominant sources of water (natural groundwater, overland flow, subsurface drain outflow, and groundwater from irrigation). Because the result of hydrograph separation by SC‐EMMA is strongly related to the choice of slowflow and fastflow end‐member values, a sensitivity analysis was conducted based on the various approaches reported in the literature to inform the selection of end‐members. There were substantial discrepancies among the BFI and SC‐EMMA, and neither method produced reasonable results for all four streams. Streams that had a small difference in the SC of slowflow compared with fastflow or did not have a monotonic relationship between streamflow and stream SC posed a challenge to the SC‐EMMA method. The utility of the graphical BFI program was limited in the stream that had only gradual changes in streamflow. The results of this comparison suggest that the two methods may be quantifying different sources of water. Even though both methods are easy to apply, they should be applied with consideration of the streamflow and/or SC characteristics of a stream, especially where anthropogenic water sources (irrigation and subsurface drainage) are present. Copyright © 2014 John Wiley & Sons, Ltd.     

Impact of wildfire on source water contributions in Devil Creek,CA: evidence from end‐member mixing analysis

A geochemical and end‐member mixing analysis (EMMA) is undertaken in Devil Canyon catchment, located in southern California, to further understanding of watershed behaviour and source water contributions after an acute and extensive wildfire. Physical and chemical transformations in post‐fire watersheds are known to increase overland flow and decrease infiltration, mainly due to formation of a hydrophobic layer at, or near, the soil surface. However, less is known about subsurface flow response in burned watersheds. The current study incorporates EMMA to evaluate and quantify source water contributions before, and after, a catchment affected by wildfires in southern California during the fall of 2003. Pre‐ and post‐fire stream water data were available at several sampling sites within the catchment, allowing the identification of contributing water sources at varying spatial scales. Proposed end‐member observations (groundwater, overland flow, shallow subsurface flow) were also collected to constrain and develop the catchment mixing model. Post‐fire source water changes are more evident in the smaller and faster responding sub‐basin (interior sampling point). Early post‐fire storm events are dominated by overland flow with no significant soil water or groundwater flow contribution. Inter‐storm streamwater in this smaller basin shows an increase in groundwater and a decrease in soil water. In the larger, baseflow‐dominated system, source water components appear less affected by fire. A slight increase in lateral flow is observed with only a slight decrease in baseflow. Changes in the post‐fire flow regimes affect nutrient loading and chemical response of the basin. Relatively rapid recovery of the chaparral ecosystem is evidenced, with active re‐growth and evapotranspiration evidenced by the fourth post‐fire rainy season. Copyright © 2008 John Wiley & Sons, Ltd.     

Modelling soil solute release and transport in run‐off on a loessial slope with and without surface stones

Stone covers on loessial slopes can increase the time of infiltration by slowing the velocity of the overland flow, which reduces the transport of solutes, but few mechanistic models have been tested under water‐scouring conditions. We carried out field experiments to test a previously proposed, physically based model of water and solute transport. The area of soil infiltration was calculated from the uncovered surface area, and Richards' equation and the kinematic wave equation were used to describe water infiltration and flow along slopes with stone covers. The transport of chemicals into the run‐off from the surface soil, presumably by diffusion, and their movement in the soil profile could be described by the convection–diffusion equations of the model. The simulated and measured data correlated well. The stones on the soil surface reduced the area available for infiltration but increased the Manning coefficient, eventually leading to increased water infiltration and decreased solute loss with run‐off. Our results indicated that the traditional model of water movement and solute migration could be used to simulate water transport and solute migration for stone‐covered soil on loessial slopes.     

Spatio‐temporal variability of the isotopic input signal in a partly forested catchment: Implications for hydrograph separation

The isotopic composition of precipitation (D and ¹⁸O) has been widely used as an input signal in water tracer studies. Whereas much recent effort has been put into developing methodologies to improve our understanding and modelling of hydrological processes (e.g., transit‐time distributions or young water fractions), less attention has been paid to the spatio‐temporal variability of the isotopic composition of precipitation, used as input signal in these studies. Here, we investigated the uncertainty in isotope‐based hydrograph separation due to the spatio‐temporal variability of the isotopic composition of precipitation. The study was carried out in a Mediterranean headwater catchment (0.56 km²). Rainfall and throughfall samples were collected at three locations across this relatively small catchment, and stream water samples were collected at the outlet. Results showed that throughout an event, the spatial variability of the input signal had a higher impact on hydrograph separation results than its temporal variability. However, differences in isotope‐based hydrograph separation determined preevent water due to the spatio‐temporal variability were different between events and ranged between 1 and 14%. Based on catchment‐scale isoscapes, the most representative sampling location could also be identified. This study confirms that even in small headwater catchments, spatio‐temporal variability can be significant. Therefore, it is important to characterize this variability and identify the best sampling strategy to reduce the uncertainty in our understanding of catchment hydrological processes.     

Spatio‐temporal controls of snowmelt and runoff generation during rain‐on‐snow events in a mid‐latitude mountain catchment

A network of 30 standalone snow monitoring stations was used to investigate the snow cover distribution, snowmelt dynamics, and runoff generation during two rain‐on‐snow (ROS) events in a 40 km² montane catchment in the Black Forest region of southwestern Germany. A multiple linear regression analysis using elevation, aspect, and land cover as predictors for the snow water equivalent (SWE) distribution within the catchment was applied on an hourly basis for two significant ROS flood events that occurred in December 2012. The available snowmelt water, liquid precipitation, as well as the total retention storage of the snow cover were considered in order to estimate the amount of water potentially available for the runoff generation. The study provides a spatially and temporally distributed picture of how the two observed ROS floods developed in the catchment. It became evident that the retention capacity of the snow cover is a crucial mechanism during ROS. It took several hours before water was released from the snowpack during the first ROS event, while retention storage was exceeded within 1 h from the start of the second event. Elevation was the most important terrain feature. South‐facing terrain contributed more water for runoff than north‐facing slopes, and only slightly more runoff was generated at open compared to forested areas. The results highlight the importance of snowmelt together with liquid precipitation for the generation of flood runoff during ROS and the large temporal and spatial variability of the relevant processes. Copyright © 2015 John Wiley & Sons, Ltd.     

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.     

Surface and subsurface water contributions to streamflow from a mesoscale watershed in complex mountain terrain

An understanding of surface and subsurface water contributions to streamflow is essential for accurate predictions of water supply from mountain watersheds that often serve as water towers for downstream communities. As such, this study used the end‐member mixing analysis technique to investigate source water contributions and hydrologic flow paths of the 264 km² Boulder Creek Watershed, which drains the Colorado Front Range, USA. Four conservative hydrochemical tracers were used to describe this watershed as a 3 end‐member system, and tracer concentration reconstruction suggested that the application of end‐member mixing analysis was robust. On average from 2009 to 2011, snowmelt and rainwater from the subalpine zone and groundwater sampled from the upper montane zone contributed 54%, 22%, and 24% of the annual streamflow, respectively. These values demonstrate increased rainwater and decreased snow water contributions to streamflow relative to area‐weighted mean values derived from previous work at the headwater scale. Young water (2.3 ± 0.8 months) fractions of streamflow decreased from 18–22% in the alpine catchment to 8–10% in the lower elevation catchments and the watershed outlet with implications for subsurface storage and hydrological connectivity. These results contribute to a process‐based understanding of the seasonal source water composition of a mesoscale watershed that can be used to extrapolate headwater streamflow generation predictions to larger spatial scales.