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.     

On the importance of river hydrodynamics in simulating groundwater levels and baseflows

Research to date affirmed the key role of stream–aquifer interactions in integrated water resources management. The importance of river hydrodynamics on the spatial and temporal behaviour of groundwater was, however, not yet fully investigated. In contrast to the common approach where topography-based estimates of riverbed elevation may lead to inappropriate discretization and constant river stages, this study couples a fully hydrodynamic and one-dimensional river model to a two-dimensional catchment hydrological model. The surface and subsurface runoff, groundwater, and river components are integrated into a single modelling framework. The coupled model was applied to a medium sized catchment in Belgium with three model setups, in which the level of detail of representation of river hydrodynamics varies. Further model iterations were carried out for the most exhaustive setup to assess the importance of the bi-directional interactions between model components. Results show that higher details of river hydrodynamics help to improve the simulation of time-averaged groundwater levels. However, the impacts were not that clear for the time-varying groundwater levels. Moreover, visual and statistical model performance evaluation indicates a strong enhancement of the coupled models compared to the output from the hydrological model with respect to river discharge observations at catchment outlet and at internal stations. It also reveals the impact of river hydrodynamics on groundwater discharges when the most detailed setting delivered the highest performance among the three coupled models.     

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.     

Interpretation of homogeneity in δ18O signatures of stream water in a nested sub-catchment system in north-east Scotland

The isotope hydrology of a set of nested sub-catchments in the north-east of Scotland has been studied to examine the mixing processes and residence times of water in the catchments. The measured δ¹⁸O in stream waters was found to be exceptionally uniform both temporally and spatially. Hydrochemical mixing analyses showed that groundwater contributes between 62 and 90% of the stream flow in all sub-catchments. Model analysis indicated that the δ¹⁸O in stream water is indicative of a highly mixed system in which near surface runoff appears to be mixed with groundwater, within the soil profile, before being released from the catchment. Small fluctuations in the stream water δ¹⁸O response are generated by a small proportion (<10%) of less-well mixed water in infiltration excess runoff during storm events. A comparative application of the model to a nearby catchment, which has a lower proportion of groundwater runoff, demonstrated contrasting behaviour, with significantly less mixing of waters occurring and a more distinct difference in the age of runoff generated by different flow paths. This highlighted that standard methods for characterization of mixing mechanisms are often insufficient and may not discriminate between systems that have retained quite distinct flow paths throughout catchment transit, and those which have been mixed at some stage. Model sensitivity analysis also indicated that the simulated mean residence time of water varies most strongly in response to different parameters compared with the δ¹⁸O response. This has implications for estimating water residence times from isotope data. Copyright © 2008 John Wiley & Sons, Ltd.     

An overview of hydrometeorological datasets from a small agricultural catchment (Nučice) in the Czech Republic

We introduce the freely available web-based Water in an Agricultural Landscape—NUčice Database (WALNUD) dataset that includes both hydrological and meteorological records at the Nučice experimental catchment (0.53 km²), which is representative of an intensively farmed landscape in the Czech Republic. The Nučice experimental catchment was established in 2011 for the observation of rainfall–runoff processes, soil erosion processes, and water balance of a cultivated landscape. The average altitude is 401 m a.s.l., the mean land slope is 3.9%, and the climate is humid continental (mean annual temperature 7.9°C, annual precipitation 630 mm). The catchment is drained by an artificially straightened stream and consists of three fields covering over 95% of the area which are managed by two different farmers. The typical crops are winter wheat, rapeseed, and alfalfa. The installed equipment includes a standard meteorological station, several rain gauges distributed across the basin, and a flume with an H-type facing that is used to monitor stream discharge, water turbidity, and basic water quality indicators. Additionally, the groundwater level and soil water content at various depths near the stream are recorded. Recently, large-scale soil moisture monitoring efforts have been introduced with the installation of two cosmic-ray neutron sensors for soil moisture monitoring. The datasets consist of observed variables (e.g. measured precipitation, air temperature, stream discharge, and soil moisture) and are available online for public use. The cross-seasonal, open access datasets at this small-scale agricultural catchment will benefit not only hydrologists but also local farmers.     

A test of TOPMODEL'a ability to predict spatially distributed groundwater levels

TOPMODEL was calibrated to a small catchment using precipitation and runoff data. Acceptable fits of simulated and observed runoff were obtained during both the calibration and validation periods. Predictions of groundwater levels using this calibration did not agree well with observations at the 37 points within the catchment where groundwater levels were measured, including three locations with continuous recordings. Groundwater level observations at one single point in time, however, sufficed to calibrate new topographic–soil indices that improved the prediction of the local groundwater levels at the observed tubes. This suggests that spatially distributed calibration data are necessary to exploit reliably TOPMODEL's ability to predict spatially distributed hydrology. The mean or recalibrated transmissivity values at these 37 points differed from the catchment mean as determined by the precipitation–runoff calibration. Thus, while groundwater information can help in predicting groundwater levels at specific locations, increasing the number of local groundwater level measurements is not sufficient to improve the spatially distributed representation of subsurface flow by TOPMODEL for the catchment as a whole, as long as the groundwater information is not integrated in the precipitation–runoff calibration. © 1997 John Wiley & Sons, Ltd.     

Fine scale variations of surface water chemistry in an ephemeral to perennial drainage network

Although temporal variation in headwater stream chemistry has long been used to document baseline conditions and response to environmental drivers, less attention is paid to fine scale spatial variations that could yield clues to processes controlling stream water sources. We documented spatial and temporal variation in water composition in a headwater catchment (41 ha) at the Hubbard Brook Experimental Forest, NH, USA. We sampled every 50 m along an ephemeral to perennial stream network as well as groundwater from seeps and 35 shallow wells across varying flow conditions. Groundwater influences on surface water in this region have not been considered to be important in past studies as relatively coarse soils were assumed to be well drained in steep catchments with flashy runoff response. However, seeps displayed perennial discharge, upslope accumulated areas (UAA) smaller than those for channel initiation sites and higher pH, Ca and Si concentrations than streams, suggesting relatively long groundwater residence time or long subsurface flow paths not bound by topographic divides. Coupled with a large range in groundwater chemistry seen in wells, these results suggest stream chemistry variation reflects the range of connectivity with, and quality of, groundwater controlled by hillslope hydropedological processes. The magnitude of variations of solute concentrations seen in the first order catchment was as broad as that seen at the fifth order Hubbard Brook Valley (3519 ha). Reduction in variation in solute concentrations with increasing UAA suggested a representative elementary area (REA) value of less than 3 ha in the first order catchment, compared with 100 ha for the fifth order basin. Thus, the REA is not necessarily an elementary catchment property. Rather, the partitioning of variation between highly variable upstream sources and relatively homogenous downstream characteristics may have different physical significance depending on the scale and complexity of the catchment under examination. Copyright © 2012 John Wiley & Sons, Ltd.     

Using hydro‐chemograph analyses to reveal runoff generation processes in a Mediterranean catchment

The paper deals with the hydro‐chemical analysis performed in order to reveal processes, sources, paths and timing of the runoff generation in an experimental catchment representative of the hilly, terrigenous and forested watershed in the Mediterranean humid eco‐region of southern Italy. The analysis is based on the data recorded at the outlet of the catchment during 2013–2014. A mixing law procedure was applied on discharge (Q) and electrical conductivity (EC) data, by using the Q–EC end members previously collected at selected groundwater, sub‐surficial and surficial stations. In this way, we found four bound curves delimiting fields in a Q–EC plot, each with hydro‐chemograph value ranges. At annual time scale, the analysis revealed a seasonal behaviour of the hydrological response, different for the wet period, when the aquifer is recharging, and the dry periods, when the aquifer is discharging, despite frequent summer rain showers. At event time scale, the catchment seems to show the behaviour of a typical hydro‐geomorphic threshold system. We interpreted this behaviour as due to a progressive addition of water from distinctive components (i.e. deep aquifer, riparian corridor, hillslope and hollow), each with originally different mechanisms of runoff production (i.e. groundwater, groundwater ridging, saturation excess, infiltration excess and soil pipe exfiltration) and response time. During the event, the contributing areas enlarge upward the riparian corridors and the zero‐order basins, where the aforementioned components become superposed and the mechanisms interact more and more. We hypothesize that the threshold values between different states of the system are defined by the intersections of the boundary curves on the Q–EC plot. Different patterns in the Q–EC hysteretic cycles are prevalently related to the pre‐event soil saturation and groundwater contributions to stormflow and recharge mechanisms. Copyright © 2016 John Wiley & Sons, Ltd.     

Spatially distributed tracer‐aided modelling to explore water and isotope transport,storage and mixing in a pristine,humid tropical catchment

Rapidly transforming headwater catchments in the humid tropics provide important resources for drinking water, irrigation, hydropower, and ecosystem connectivity. However, such resources for downstream use remain unstudied. To improve understanding of the behaviour and influence of pristine rainforests on water and tracer fluxes, we adapted the relatively parsimonious, spatially distributed tracer‐aided rainfall–runoff (STARR) model using event‐based stable isotope data for the 3.2‐km² San Lorencito catchment in Costa Rica. STARR was used to simulate rainforest interception of water and stable isotopes, which showed a significant isotopic enrichment in throughfall compared with gross rainfall. Acceptable concurrent simulations of discharge (Kling–Gupta efficiency [KGE] ~0.8) and stable isotopes in stream water (KGE ~0.6) at high spatial (10 m) and temporal (hourly) resolution indicated a rapidly responding system. Around 90% of average annual streamflow (2,099 mm) was composed of quick, near‐surface runoff components, whereas only ~10% originated from groundwater in deeper layers. Simulated actual evapotranspiration (ET) from interception and soil storage were low (~420 mm/year) due to high relative humidity (average 96%) and cloud cover limiting radiation inputs. Modelling suggested a highly variable groundwater storage (~10 to 500 mm) in this steep, fractured volcanic catchment that sustains dry season baseflows. This groundwater is concentrated in riparian areas as an alluvial–colluvial aquifer connected to the stream. This was supported by rainfall–runoff isotope simulations, showing a “flashy” stream response to rainfall with only a moderate damping effect and a constant isotope signature from deeper groundwater (~400‐mm additional mixing volume) during baseflow. The work serves as a first attempt to apply a spatially distributed tracer‐aided model to a tropical rainforest environment exploring the hydrological functioning of a steep, fractured‐volcanic catchment. We also highlight limitations and propose a roadmap for future data collection and spatially distributed tracer‐aided model development in tropical headwater catchments.     

鄱阳湖湖泊流域系统水文水动力联合模拟

本文以鄱阳湖湖泊流域系统为研究对象,鉴于该湖泊流域系统尺度较大,下垫面自然属性呈现高度空间异质性且具有流域-平原区-湖泊不同机制的水文水动力过程,为了真实描述湖泊流域间的水文水动力联系及反映不同过程间的作用机制,构建了鄱阳湖湖泊流域联合模拟模型.该模型基于自主研发的流域分布式水文模型WATLAC和湖滨平原区产流模型以及水动力模型MIKE 21 3个不同功能子模型的连接来实现该复杂系统的模拟.模型的联合采用输入-输出驱动及子模型的顺序执行进程,即将五大子流域与平原区入湖径流量作为输入条件来驱动湖泊水动力模型,模拟湖泊水位对流域入湖径流量的响应.以2000-2005年鄱阳湖流域6个水文站点的河道径流量、流域基流指数以及湖泊4个站点的水位资料来率定模型,其中各站点日径流量拟合的纳希效率系数E_ns为0.71~0.84,确定性系数R²介于0.70~0.88之间,而湖泊各站点水位拟合的纳希效率系数E_ns变化为0.88~0.98,确定性系数R²为0.96~0.98,均取得令人满意的率定结果.本文提出的鄱阳湖湖泊流域系统水文水动力联合模拟模型能较为理想再现湖泊水位对流域降雨-径流过程的响应.水位模拟结果进一步表明,该联合模型能用来获取重要的水动力空间变化特征.该模型可作为有效工具定量揭示湖泊流域系统水文水动力过程对气候变化和流域人类活动的响应.     

Investigation of runoff generation in a pristine,poorly gauged catchment in the Chilean Andes II: Qualitative and quantitative use of tracers at three spatial scales

Understanding runoff generation processes is important for flood prediction, water management, erosion control, water quality, contaminant transport and the evaluation of impacts of land use change. However, little process research has been carried out in southern Chile. In particular the young volcanic ash soils, which are typical for this area, are not well understood in their hydrologic behaviour. To establish a ‘reference study’ which can then be used for comparison with other (disturbed) sites, this study focuses on the investigation of runoff generation processes in an undisturbed, forested catchment in the Chilean Andes. The paper reports on an investigation of these processes with different tracer methods at different spatial scales. Hydrograph separation with environmental isotopes and geochemical constituents was used on the catchment scale. Thermal energy was used as a tracer to investigate groundwater–surface water interactions at the local stream reach scale and dye tracers were used to study infiltration and percolation characteristics at the plot scale. It was found that pre‐event water dominates the storm hydrograph. In the lower reaches, however, water usually exfiltrates from the stream into the adjacent aquifer. The dye tracer experiments showed that while preferential vertical flow dominates under forest, water infiltrates as a straight horizontal front in the bare volcanic ashes (no vegetation) on the catchment rim. Subsurface flow patterns in the forest differ significantly from summer to winter. All three approaches used in this study suggest an important shift in dominant processes from dry to wet season. Copyright © 2008 John Wiley & Sons, Ltd.     

Effects of bedrock groundwater discharge on spatial variability of dissolved carbon,nitrogen, and phosphorous concentrations in stream water within a forest headwater catchment

The quantitative evaluation of the effects of bedrock groundwater discharge on spatial variability of stream dissolved organic carbon (DOC), dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorous (DIP) concentrations has still been insufficient. We examined the relationships between stream DOC, DIN and DIP concentrations and bedrock groundwater contribution to stream water in forest headwater catchments in warm-humid climate zones. We sampled stream water and bedrock springs at multiple points in September and December 2013 in a 5 km² forest headwater catchment in Japan and sampled groundwater in soil layer in small hillslopes. We assumed that stream water consisted of four end members, groundwater in soil layer and three types of bedrock groundwater, and calculated the contributions of each end member to stream water from mineral-derived solute concentrations. DOC, DIN and DIP concentrations in stream water were compared with the calculated bedrock groundwater contribution. The bedrock groundwater contribution had significant negative linear correlation with stream DOC concentration, no significant correlation with stream DIN concentration, and significant positive linear correlation with stream DIP concentration. These results highlighted the importance of bedrock groundwater discharge in establishing stream DOC and DIP concentrations. In addition, stream DOC and DIP concentrations were higher and lower, respectively, than those expected from end member mixing of groundwater in soil layer and bedrock springs. Spatial heterogeneity of DOC and DIP concentrations in groundwater and/or in-stream DOC production and DIP uptake were the probable reasons for these discrepancies. Our results indicate that the relationships between spatial variability of stream DOC, DIN and DIP concentrations and bedrock groundwater contribution are useful for comparing the processes that affect stream DOC, DIN and DIP concentrations among catchments beyond the spatial heterogeneity of hydrological and biogeochemical processes within a catchment.     

Diel discharge variations in dormant and growing seasons in a headwater catchment suggest potential sources of an evapotranspiration signal

Diel fluctuations in discharge not associated with precipitation have been identified in streams around the world and attributed to evapotranspiration of both hillslope and near-stream vegetation. Several mechanisms have been hypothesized to explain the generation of these signals, but a consensus has not been reached. In order to investigate the origin of these daily discharge fluctuations, we measured discharge and groundwater levels in a 43-ha headwater catchment near Cullowhee, NC, USA. The catchment was logged at least twice in the past 120 years, allowing the evergreen invasive Chinese privet (Ligustrum sinense) to become established in near-stream areas. Stream discharge was measured at the outlet, and stream stage and near-stream groundwater level were measured at a second location higher in the catchment. Daily fluctuations in discharge were quantified and found to increase in magnitude with daily mean discharge. Interestingly, daily discharge fluctuations were observed in the dormant season, and our analysis suggests that their magnitude may increase more with discharge than during the growing season. Two previously hypothesized processes may explain our observations. A decrease in signal dampening due to faster transmission of an evapotranspiration signal at elevated flows may explain the increased diel fluctuation magnitude at higher flows. Additionally, because the primary active vegetation in our catchment in the dormant season is in near-stream areas, we suggest that this indicates most of the evapotranspiration signal in stream discharge is derived from near-stream vegetation, not hillslopes.     

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.     

Hydrogeochemistry of shallow groundwater in an upland Scottish catchment

The hydrogeochemistry of shallow groundwater has been characterized in the Allt a'Mharcaidh catchment in the Scottish Cairngorms in order to: (i) assess the spatial and temporal variation in groundwater chemistry; (ii) identify the hydrogeochemical processes regulating its evolution; and (iii) examine the influence of groundwater on the quality and quantity of stream flow. Shallow groundwater in superficial drift deposits is circumneutral (pH∽7·1) and base cation concentrations are enriched compared with precipitation and drainage water from overlying podzolic soils. Modelling with NETPATH suggests that the dominant geochemical processes that account for this are the dissolution of plagioclase, K-feldspar and biotite. Groundwater emerging as springs from weathered granite underlying high altitude (>900 m) alpine soils shows similar characteristics, though weathering rates are lower, probably as a result of reduced residence times and lower temperatures. Chemical hydrograph separation techniques using acid neutralizing capacity (ANC) and Si as tracers show that groundwater is the dominant source of baseflow in the catchment and also buffers the chemistry of stream water at high flows: groundwater may account for as much as 50–60% of annual runoff in the catchment. Climate and land use in the Cairngorms are vulnerable to future changes, which may have major implications for hydrogeological processes in the area. © 1998 John Wiley & Sons, Ltd.     

Experimental evidence of fast groundwater responses in a hillslope/floodplain area in the Black Forest Mountains,Germany

Analysis of water flow pathways from hillslopes to streams is essential for the optimal protection of water resources as well as for ecohydrological studies. This study addresses runoff generation processes at a hillslope and near‐stream shallow groundwater system in the Black Forest Mountains, southwestern Germany. The changing spatial and temporal flow patterns during differing hydrological situations were examined using a combined hydraulic and hydrochemical approach. Groundwater levels at 10 wells, discharge at a near‐stream saturated area, and several natural tracers (deuterium, dissolved silica, and major anions and cations) were observed at different locations during high and low flows. The importance of the groundwater component during flood formation was clearly demonstrated: its contribution was about 80% during a double peak flood event at the saturated area. In addition, a rapid change of the shallow groundwater levels was observed along two transects of groundwater wells in the floodplain. This led to an enhanced groundwater discharge into the saturated area located at the end of one study transect. The amount of groundwater additionally activated during the event was about 30% of total discharge recorded at the outlet of the saturated area. Two alternative hypotheses are discussed to explain this phenomenon: the establishment of locally confined conditions and the development of a pressure wave (hypothesis A), or the significant change of the three‐dimensional groundwater flow lines that caused a large increase of the groundwater catchment at the saturated area during the investigated event (hypothesis B). Even if the exact flow paths and mechanisms could not be clearly identified, the importance of rapid responding hillslope groundwater was undoubtedly demonstrated by a combination of tracer and hydrometric methods. Copyright © 2004 John Wiley & Sons, Ltd.     

Non-linear quickflow response as indicators of runoff generation mechanisms

Linking quickflow response to subsurface state can improve our understanding of runoff processes that drive emergent catchment behaviour. We investigated the formation of non-linear quickflows in three forested headwater catchments and also explored unsaturated and saturated storage dynamics, and likely runoff generation mechanisms that contributed to threshold formation. Our analyses focused on two reference watersheds at the Coweeta Hydrologic Laboratory (CHL) in western North Carolina, USA, and one reference watershed at the Susquehanna Shale Hills Critical Zone Observatory (SHW) in Central Pennsylvania, USA, with available hourly soil moisture, groundwater, streamflow, and precipitation time series over several years. Our study objectives were to characterise (a) non-linear runoff response as a function of storm characteristics and antecedent conditions, (b) the critical levels of shallow unsaturated and saturated storage that lead to hourly flow response, and (c) runoff mechanisms contributing to rapidly increasing quickflow using measurements of soil moisture and groundwater. We found that maximum hourly rainfall did not significantly contribute to quickflow production in our sites, in contrast to prior studies, due to highly conductive forest soils. Soil moisture and groundwater dynamics measured in hydrologically representative areas of the hillslope showed that variable subsurface states could contribute to non-linear runoff behaviour. Quickflow generation in watersheds at CHL were dominated by both saturated and unsaturated pathways, but the relative contributions of each pathway varied between catchments. In contrast, quickflow was almost entirely related to groundwater fluctuations at SHW. We showed that co-located measurements of soil moisture and groundwater supplement threshold analyses providing stronger prediction and understanding of quickflow generation and indicate dominant runoff processes.     

Comparative analysis of the response of various land covers to an exceptional rainfall event in the central Spanish Pyrenees,October 2012

The hydrological and geomorphological effects of an exceptional rainstorm event that occurred in the central Spanish Pyrenees during 19–21 October 2012 were studied in five experimental catchments under various land covers: (i) subhumid badlands; (ii) dense forest; (iii) an abandoned farmland area recolonized by shrubs and forest patches; and (iv) subalpine grasslands. Hydrographs and sedigraphs demonstrated that vegetation cover is a major factor affecting the control of floods even during exceptional rainstorms, at least at the spatial scale at which the phenomenon was studied (catchment sizes: 0.3–2.8 km2) and under dry catchment conditions. The combined precipitation over the two days (c. 250 mm) was the greatest for any two‐day event recorded since 1950 in the central‐western Pyrenees for all but one of the stations in the study. Five pulses of most intense rainfall were recorded. The forested catchment did not react to the two most intense rainfall pulses, because of the very low antecedent level of the water table. The main peak flow occurred only when at least a part of the catchment was saturated. The abandoned farmland catchment had two small peak discharges at the beginning of the event, which were produced by infiltration excess overland flow from eroded areas close to the main stream. During the third most intense rainfall period a large part of this catchment contributed to runoff and a relatively high peak discharge was produced. The badland catchment reacted immediately from the beginning of the rainstorm, yielding very high discharges accompanied by high suspended sediment concentrations. The subalpine catchment showed a hydrograph mirroring the hyetograph, with brief but intense hydrological responses to increased precipitation, because of the marked gradients and the presence of bare rock in the headwaters. A high volume of bedload was carried during the peak discharge. Copyright © 2013 John Wiley & Sons, Ltd.