Pre-event Water Contributions Research Articles

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

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

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

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

High-Resolution, In Situ Monitoring of Stable Isotopes of Water Revealed Insight into Hydrological Response Behavior

High temporal resolution (20-min intervals) measurements of stable isotopes from groundwater, stream water and precipitation were investigated to understand the hydrological response behavior and control of precipitation and antecedent wetness conditions on runoff generation. Data of 20 precipitation events were collected by a self-sufficient mobile system for in situ measurements over four months in the Schwingbach Environmental Observatory (SEO, temperate climate), Germany. Isotopic hydrograph separation indicated that more than 79% of the runoff consisted of pre-event water. Short response times of maximum event water fractions in stream water and groundwater revealed that shallow subsurface flow pathways rapidly delivered water to the stream. Macropore and soil pipe networks along relatively flat areas in stream banks were likely relevant pathways for the rapid transmission of water. Event water contribution increased with increasing precipitation amount. Pre-event water contribution was moderately affected by precipitation, whereas, the antecedent wetness conditions were not strong enough to influence pre-event water contribution. The response time was controlled by mean precipitation intensity. A two-phase system was identified, at which the response times of stream water and groundwater decreased after reaching a threshold of mean precipitation intensity of 0.5 mm h−1. Our results suggest that high temporal resolution measurements of stable isotopes of multiple water sources combined with hydrometrics improve the understanding of the hydrological response behavior and runoff generation mechanisms.

Understanding of Storm Runoff Generation in a Weathered, Fractured Granitoid Headwater Catchment in Northern China

Few of the classical field studies of streamflow generation in headwater watersheds have been conducted in catchments with thin soils and deeply weathered crystalline silicate bedrock. As such, the role of the (potentially very large) storage capacity of weathered, fractured rock in baseflow and storm event discharge remains poorly characterized. Here we present a study of streamflow generation in an upland semi-humid watershed (Xitaizi Experimental Watershed, XEW, 4.22 km2) dominated by baseflow feeding one of the main water supply reservoirs for the city of Beijing, China. This catchment is relatively dry (625 mm/yr precipitation, 480 mm/yr Evapotranspiration), but has strongly seasonal precipitation that varies in phase with strongly seasonal potential evapotranspiration. The catchment was instrumented with four weather stations and precipitation collectors, 11 deep wells drilled into the bedrock along three hillslopes, and additional soil moisture sensors and water samplers along one hillslope. In six storm events over two years, samples of rainfall, soil water (10–80 cm depth), groundwater, and stream water were collected with high frequency and analyzed for stable water isotopes (δ18O and δ2H). Tracer-based hydrograph separation showed that event water (precipitation) makes up the majority of the hydrograph peak above baseflow, and pre-event water contributions (on average) simply represent the steady release of groundwater. The quantity of event water corresponded to a very small effective contributing area (<0.2% of the catchment) that nevertheless showed a clear dependence on catchment wetness as measured by the streamflow. The streamflow itself was isotopically identical to the deep groundwater in wells. This suggests that the fractured, weathered, bedrock system dominates the production of streamflow in this catchment.

Pre-event water contributions and streamwater residence times in different land use settings of the transboundary mesoscale Lužická Nisa catchment

AbstractThe objective of the study was to evaluate the spatial distribution of peakflow pre-event water contributions and streamwater residence times with emphasis on land use patterns in 38 subcatchments within the 687 km2large mesoscale transboundary catchment Lužická Nisa. Mean residence times between 8 and 27 months and portions of pre-event water between 10 and 97% on a storm event peakflow were determined, using18O data in precipitation and streamwater from a weekly monitoring of nearly two years. Only a small tracer variation buffering effect of the lowland tributaries on the main stem was observed, indicating the dominant impact on the mountainous headwaters on the runoff generation. Longest mean streamwater residence times of 27 months were identified in the nearly natural headwaters of the Jizera Mountains, revealing no ambiguous correlation between the catchment area and altitude and the mean residence time of streamwater. Land use control on the pre-event water portions were determined for three land use categories with percentage of urban areas from 0 to 10%, 10 to 20% and more than 20%. The fraction of pre-event water in the first category decreases from 97% to 65% with the increasing percentage of forest from 76% to 100%, revealing that forests may provide only a limited infiltration of precipitation due to leaf interception and soil water use for transpiration. Fractions of pre-event water of 39–87% in the second (agricultural catchments) and of 10–35% in the third (urbanized catchments) category increase with percentage of non-urban areas.

Spatial variability in the isotopic composition of rainfall in a small headwater catchment and its effect on hydrograph separation

Isotope hydrograph separation (IHS) is a valuable tool to study runoff generation processes. To perform an IHS, samples of baseflow (pre-event water) and streamflow are taken at the catchment outlet. For rainfall (event water) either a bulk sample is collected or it is sampled sequentially during the event. For small headwater catchment studies, event water samples are usually taken at only one sampling location in or near the catchment because the spatial variability in the isotopic composition of rainfall is assumed to be small. However, few studies have tested this assumption. In this study, we investigated the spatiotemporal variability in the isotopic composition of rainfall and its effects on IHS results using detailed measurements from a small pre-alpine headwater catchment in Switzerland. Rainfall was sampled sequentially at eight locations across the 4.3km2 Zwäckentobel catchment and stream water was collected in three subcatchments (0.15, 0.23, and 0.70km2) during ten events. The spatial variability in rainfall amount, average and maximum rainfall intensity and the isotopic composition of rainfall was different for each event. There was no significant relation between the isotopic composition of rainfall and total rainfall amount, rainfall intensity or elevation. For eight of the ten studied events the temporal variability in the isotopic composition of rainfall was larger than the spatial variability in the rainfall isotopic composition. The isotope hydrograph separation results, using only one rain sampler, varied considerably depending on which rain sampler was used to represent the isotopic composition of event water. The calculated minimum pre-event water contributions differed up to 60%. The differences were particularly large for events with a large spatial variability in the isotopic composition of rainfall and a small difference between the event and pre-event water isotopic composition. Our results demonstrate that even in small catchments the spatial variability in the rainfall isotopic composition can be significant and has to be considered for IHS studies. Using data from only one rain sampler can result in significant errors in the estimated pre-event water contributions to streamflow.

Influence of pre-event water on streamflow in a granitic watershed using hydrograph separation

For water resources management, many studies for investigating flow paths from rainfall to subsurface have been conducted for last half century. A hydrograph separation based on end member mixing was carried out to evaluate the importance of the hydrological pathways providing the main sources of a small granitic watershed, Dorim-cheon, Seoul. An analysis of chloride, oxygen-18 and deuterium isotopes from precipitation and stream water during three storms was conducted with high-resolution data using 129 samples. Stream water, collected in advance of rain event, was assumed as a pre-event water (baseflow) component according to its dry condition and isotopic values compared with the values of different time periods. The contribution from vadose water was ignored due to the thin soil layer covering the study area. Using a two-component mixing model, hydrograph separation of the stream water in Dorim-cheon was conducted based on weighted mean values of rain water and pre-event water component with the high-resolution datasets. As a result of the analysis with water isotopes, contribution of groundwater was dominant during the entire study period (73–74%) except of instant dominance of rainfall at the earliest period. Using chloride as a tracer for hydrograph separation, a significant difference for the amount of pre-event water contribution was identified. This might be caused by the large variation of chloride concentration during the rain event and the end member determination.

Solute transport processes in flow-event-driven stream–aquifer interaction

The interaction between streams and groundwater controls key features of the stream hydrograph and chemograph. Since surface runoff is usually less saline than groundwater, flow events are usually accompanied by declines in stream salinity. In this paper, we use numerical modelling to show that, at any particular monitoring location: (i) the increase in stream stage associated with a flow event will precede the decrease in solute concentration (arrival time lag for solutes); and (ii) the decrease in stream stage following the flow peak will usually precede the subsequent return (increase) in solute concentration (return time lag). Both arrival time lag and return time lag increase with increasing wave duration. However, arrival time lag decreases with increasing wave amplitude, whereas return time lag increases. Furthermore, while arrival time lag is most sensitive to parameters that control river velocity (channel roughness and stream slope), return time lag is most sensitive to groundwater parameters (aquifer hydraulic conductivity, recharge rate, and dispersitivity). Additionally, the absolute magnitude of the decrease in river concentration is sensitive to both river and groundwater parameters. Our simulations also show that in-stream mixing is dominated by wave propagation and bank storage processes, and in-stream dispersion has a relatively minor effect on solute concentrations. This has important implications for spreading of contaminants released to streams. Our work also demonstrates that a high contribution of pre-event water (or groundwater) within the flow hydrograph can be caused by the combination of in-stream and bank storage exchange processes, and does not require transport of pre-event water through the catchment.

Pre-event water contributions to runoff events of different magnitude in pre-alpine headwaters

Precipitation and catchment characteristics of mountainous headwaters can vary largely within short distances. It remains unclear how these two factors determine the contribution of event water and pre-event water to stormflow. We investigated this in five neighboring headwaters with high annual precipitation amounts (>2,000 mm y−1) in a steep pre-alpine region in Switzerland. Rainfall and streamwater of 13 different rainstorms were sampled (P: 5 mm intervals, Q: 12 to 51 samples per events) to perform a two-component isotope hydrograph separation. Pre-event water contributions based on δ18O or δ2H computation were similar. The pre-event water contributions of headwaters depended largely on rainfall (amount and intensity) and varied more between events than between catchments, despite clear differences in land cover between the catchments. Furthermore, antecedent wetness was not found to control pre-event water contribution. With increasing rainfall amount, the proportion of rainfall in runoff increased and changed from pre-event to event water dominated. The variable rainfall amount and small active storage (organic soil horizon, 20–50 cm) resulted in a threshold in the upper soil horizon with subsequently more variable pre-event water contribution. Our results show the necessity of sampling in different headwaters and events to better understand controlling factors in runoff generation.

Flowpaths, source water contributions and water residence times in a Mexican tropical dry forest catchment

Summary Runoff in forested tropical catchments has been frequently described in the literature as dominated by the rapid translation of rainfall to runoff through surface and shallow subsurface pathways. However, studies examining runoff generation in tropical catchments with highly permeable soils have received little attention, particularly in tropical dry forests. We present a study focused on identifying the dominant flowpaths, water sources and stream water residence times in a tropical dry forest catchment near the Pacific coast of central Mexico. During the wet season, pre-event water contributions to stormflow ranged from 72% to 97%, with the concentrations of calcium, magnesium, sodium and potassium closely coupling the geochemistry of baseflow and groundwater from the narrow riparian/near-stream zone. Baseflow from the intermittent stream showed a strongly damped isotopic signature and a mean baseflow residence time of 52–110 days was estimated. These findings all suggest that instead of the surface and near-surface subsurface lateral pathways observed over many tropical catchments, runoff is generated through vertical flow processes and the displacement and discharge of stored water from the saturated zone. As the wet season progressed, contributions from the saturated zone persisted; however, the stormflow and baseflow geochemistry suggests that the contributing area of the catchment increased. Our results show that during the early part of the wet season, runoff originated primarily from the headwater portion of the catchment. As the wet season progressed and catchment wetness increased, connectivity among sub-basin was improved, resulting in runoff contributions from across the entire catchment.

Land use change effects on runoff generation in a humid tropical montane cloud forest region

Abstract. While tropical montane cloud forests (TMCF) provide critical hydrological services to downstream regions throughout much of the humid tropics, catchment hydrology and impacts associated with forest conversion in these ecosystems remain poorly understood. Here, we compare the annual, seasonal and event-scale streamflow patterns and runoff generation processes of three neighbouring headwater catchments in central Veracruz (eastern Mexico) with similar pedological and geological characteristics, but different land cover: old-growth TMCF, 20 yr-old naturally regenerating TMCF and a heavily grazed pasture. We used a 2 yr record of high resolution rainfall and stream flow data (2008–2010) in combination with stable isotope and chemical tracer data collected for a series of storms during a 6-week period of increasing antecedent wetness (wetting-up cycle). Our results showed that annual and seasonal streamflow patterns in the mature and secondary forest were similar. In contrast, the pasture showed a 10% higher mean annual streamflow, most likely because of a lower rainfall interception. During the wetting-up cycle, storm runoff ratios increased at all three catchments (from 11 to 54% for the mature forest, 7 to 52% for the secondary forest and 3 to 59% for the pasture). With the increasing antecedent wetness, hydrograph separation analysis showed progressive increases of pre-event water contributions to total stormflow (from 35 to 99% in the mature forest, 26 to 92% in the secondary forest and 64 to 97% in the pasture). At all three sites, rainfall-runoff responses were dominated by subsurface flow generation processes for the majority of storms. However, for the largest and most intense storm (typically occurring once every 2 yr), sampled under wet antecedent conditions, the event water contribution in the pasture (34% on average) was much higher than in the forests (5% on average), indicating that rainfall infiltration capacity of the pasture was exceeded. This result suggests that despite the high permeability of the volcanic soils and underlying substrate in this TMCF environment, the conversion of forest to pasture may lead to important changes in runoff generation processes during large and high intensity storms. On the other hand, our results also showed that 20 yr of natural regeneration may be enough to largely restore the original hydrological conditions of this TMCF.

Use of isotopes to study floodplain wetland and river flow interaction in the White Volta River basin, Ghana

Floodplain wetlands influence the timing and magnitude of stream responses to rainfall. In managing and sustaining the level of water resource usage in any river catchment as well as when modelling hydrological processes, it is essential that the role of floodplain wetlands in stream flows is recognised and understood. Existing studies on hydrology within the Volta River basin have not adequately represented the variability of wetland hydrological processes and their contribution to the sustenance of river flow. In order to quantify the extent of floodwater storage within riparian wetlands and their contribution to subsequent river discharges, a series of complementary studies were conducted by utilising stable isotopes, physical monitoring of groundwater levels and numerical modelling. The water samples were collected near Pwalugu on the White Volta River and at three wetland sites adjacent to the river using the grab sampling technique. These were analysed for 18O and 2H. The analysis provided an estimate of the contribution of pre-event water to overall stream flow. In addition, the variation in the isotopic composition in the river and wetland water samples, respectively, revealed the pattern of flow and exchange of water between the wetlands and the main river system.

Storm pulses of dissolved CO2 in a forested headwater Amazonian stream explored using hydrograph separation

Dissolved CO2 dynamics in stormflow and event water versus preevent water contributions to storm hydrographs were assessed in a forested headwater catchment of the Brazilian Amazon using high‐frequency data. We applied the transfer function hydrograph separation model (TRANSEP) using specific conductance as a conservative tracer, finding preevent water to average 0.79 ± 0.03 of storm discharge (mean ± 1 SE for n = 14 storms). In situ, direct measurements of dissolved CO2 were able to capture new hydrobiogeochemical processes in real time, including CO2 pulses observed on the falling limb of storm hydrographs, the magnitudes of which were inversely related to preevent water fractions (r = −0.97, p < 0.0001).

Using numerical modelling to evaluate the capillary fringe groundwater ridging hypothesis of streamflow generation

The controls on pre-event water contributions to streamflow are still poorly understood, despite a number of proposed processes. One of the most common is the capillary fringe induced groundwater ridging mechanism, identified in many environments as a control on rapid mobilization of groundwater into the channel during events. Nevertheless, despite widespread acceptance, there is little evidence for such a phenomenon outside of particular environments and test cases for which it has been quantified. We use a flow and transport modelling tool to test a number of hypotheses concerning the capillary-fringe groundwater ridging mechanism. The original Abdul and Gilham (Abdul, A.S., Gillham, R.W., 1989. Field studies of the effects of the capillary fringe on streamflow generation. Journal of Hydrology 112, 1–18) laboratory experiment (that is still regarded by those working in the field as the main proof-of-concept) is replicated numerically within a 2D finite element code. An indication of the ‘spaces’ of applicability of the process in the context of the laboratory experiment is determined (soil type, antecedent moisture, riparian volume, slope, rainfall intensity). We show that in only a limited number of cases, high proportions of pre-event water are sustained from this process.

Estimation of denitrification potential in a karst aquifer using the 15N and 18O isotopes of NO 3 −

A confined aquifer in the Malm Karst of the Franconian Alb, South Germany was investigated in order to understand the role of the vadose zone in denitrifiaction processes. The concentrations of chemical tracers Sr2+ and Cl− and concentrations of stable isotope 18O were measured in spring water and precipitation during storm events. Based on these measurements a conceptual model for runoff was constructed. The results indicate that pre-event water, already stored in the system at the beginning of the event, flows downslope on vertical and lateral preferential flow paths. Chemical tracers used in a mixing model for hydrograph separation have shown that the pre-event water contribution is up to 30%. Applying this information to a conceptual runoff generation model, the values of δ15N and δ18O in nitrate could be calculated. Field observations showed the occurence of significant microbial denitrification processes above the soil/bedrock interface before nitrate percolates through to the deeper horizon of the vadose zone. The source of nitrate could be determined and denitrification processes were calculated. Assuming that the nitrate reduction follows a Rayleigh process one could approximate a nitrate input concentration of about 170 mg/l and a residual nitrate concentration of only about 15%. The results of the chemical and isotopic tracers postulate fertilizers as nitrate source with some influence of atmospheric nitrate. The combined application of hydrograph separation and determination of isotope values in δ15N and δ18O of nitrate lead to an improved understanding of microbial processes (nitrification, denitrification) in dynamic systems.

Controls on runoff components on a forested slope and implications for N transport

AbstractTransfer of atmospheric N deposition on shallow‐soil forested basins on the Canadian Shield to receiving water bodies may be enhanced by rapid preferential flow along the soil–bedrock interface (BR runoff) on basin slopes. Controls on BR runoff, partitioning of event and pre‐event water contributions to this flow, and implications of this partitioning for N fluxes in BR runoff were studied under natural and artificial inputs to an instrumented slope. BR runoff as a fraction of water inputs to the slope increased with antecedent soil wetness and input depth. Event water contributions to BR runoff initially increased with antecedent soil wetness, but then declined at large antecedent soil wetness. Export of applied NH4+ from the slope was maximized when event water contributions containing large NH4+ concentrations dominated BR runoff; however, there was no relationship between the fraction of NO3− application transported in BR runoff and either application input or the event water fraction of that runoff. The applicability of our results to other shallow‐soil areas of the Canadian Shield is limited by artificial N inputs to the slope in excess of natural loads and by low rates of N mineralization and negligible nitrification in the slope's soils. Nevertheless, the study reinforces the need to consider how the hydrologic, geometric and pedologic properties of forest slopes interact with biotic and abiotic soil processes to control N transport and transformation. Copyright © 2001 John Wiley & Sons, Ltd.

Runoff Production in a Forested, Shallow Soil, Canadian Shield Basin

Storm flow in forested basins on the Canadian Shield is largely supplied by subsurface water; however, mechanisms by which this water reaches the stream remain unclear. Side slope contributions to storm flow were studied using throughflow trenches on slopes in a headwater basin near Dorset, Ontario. Discharge, soil water content, and chemical and isotopic signatures of subsurface water were monitored at each site. Four hypotheses were tested: (1) most flow occurs at the soil‐bedrock interface on shield slopes with thin soil; (2) a significant fraction of event water moves vertically to bedrock via preferential flow pathways and laterally over the bedrock surface; (3) relative preevent water contribution to subsurface flow on shield slopes is a function of soil thickness; and (4) a significant portion of event water flux in storm flow from forested basins with shallow soil cover is supplied from side slopes via subsurface flow along the soil‐bedrock interface. Hypothesis 1 was confirmed from hydrometric observations during spring and fall rainstorms. Hypotheses 2 and 3 were supported by temporal trends in dissolved organic carbon and 18O in flow at the soil‐bedrock interface and by isotopic hydrograph separations (IHSs) of hillslope runoff. Comparison with the streamflow IHS indicated that event water flux from the basin in excess of that attributable to direct precipitation onto near‐channel saturated areas could be supplied by flow along the bedrock surface (hypothesis 4). Flow at the soil‐bedrock interface on side slopes also contributed ∼25% of preevent water flux from the basin. Much of the event water component of basin storm flow may travel considerable distances via subsurface routes and is not necessarily contributed by surface runoff processes (Horton flow or saturation overland flow). Therefore the assumption that event water undergoes little interaction with the soil during its passage downslope may be unwarranted here.

Changes in soil and stream hydrochemistry during periods of spring snowmelt at a pristine site in mid-Norway

A detailed study of hydrochemical changes during spring snowmelt was undertaken at a pristine site in mid-Norway. The composition of catchment outflow water reflects hydrological pathways through different soil horizons and the contribution of pre-event or long-residence time water. There was evidence of secondary snowpack elution and ionic enrichment of water collected below the organic horizons. The observed time lag in commencement of BC soil horizon flow indicated that during the initial stages of the melt, water did not penetrate the deeper mineral horizons, or that initially the soil was not at field capacity. Increased soil and surface thawing is proposed as an important mechanism affecting the routing and subsequent chemical alteration of soil water leachate. The relative importance of elution, pathways, and dilution processes within the study catchment is discussed and a scenario for the effects of winter anthropogenic inputs at this pristine site is presented.