Pre-event Water Research Articles

Hydrological flow paths during snowmelt: Congruence between hydrometric measurements and oxygen 18 in meltwater, soil water, and runoff

Streamflow generation in boreal catchments remains poorly understood. This is especially true for snowmelt episodes, which are the dominant hydrological event in many seasonally snow covered regions. We examined the spatial and temporal aspects of flow pathways by linking detailed oxygen 18 observations of stream, melt, soil, and groundwater with hydrometric measurements in a small catchment in northern Sweden during the snowmelt period. The results demonstrate that soil horizons below 90 cm were hardly affected by the approximately 200 mm of snowmelt water infiltrating into the soil during the spring. The approximately sixtyfold increase in runoff, from 0.13 mm d−1 to 8 mm d−1, was generated by a 30–40 cm rise of the groundwater level. The total runoff during the snowmelt period from late April to late May was 134 mm, of which 75% was preevent water. Mass balance calculations based on hydrometric and isotopic data independently, both using upscaling of a hillslope transect to the entire 13‐ha catchment, provided similar results of both water storage changes and the amount of event water that was left in the catchment after the snowmelt. In general, groundwater levels and runoff were strongly correlated, but different functional relationships were observed for frozen and unfrozen soil conditions. Although runoff generation in the catchment generally could be explained by the transmissivity feedback concept, the results suggest that there is a temporal variability in the flow pathways during the spring controlled by soil frost during early snowmelt.

How does rainfall become runoff? A combined tracer and runoff transfer function approach

Hydrographs are an enticing focus for hydrologic research: they are readily available hydrological data that integrate the variety of terrestrial runoff generation processes and upstream routing. Notwithstanding, new techniques to glean information from the hydrograph are lacking. After early approaches of graphically separating streamflow components, hydrograph separations in the past two decades have focused on tracers as a more objective means to separate the storm hydrograph. These tracer‐based methods provide process‐based information; however, their implicit assumptions limit their applicability and explanatory power. We present a new method for isotope hydrograph separation that integrates the instantaneous unit hydrograph and embraces the temporal variability of rainfall isotopic composition (one of the largest impediments to the standard use of isotopes as tracers). Our model computes transfer functions for event water and preevent water calculated from a time‐variable event water fraction. The transfer function hydrograph separation model (TRANSEP) provides coupled but constrained representations of transport and hydraulic transfer functions, overcoming limitations of other models. We illustrate the utility of TRANSEP by applying it to two rainfall events from a 17 ha catchment at Maimai in New Zealand, where 18O, rainfall, and runoff data were sampled with a high temporal resolution. We explore which runoff and tracer transfer function (exponential piston flow, gamma distribution, or two parallel linear reservoirs) gave the best results for the proposed model structure and for the example data set. Uncertainty analysis was used to determine if the parameters were identifiable and if the information available for applying TRANSEP was sufficient. The results of the best performing transfer function are considered in detail to identify model performance, illustrate individual event characteristics, and interpret runoff processes in the catchment.

Hydrologic and geochemical influences on the dissolved silica concentration in natural water in a steep headwater catchment

The dissolved silica concentrations in groundwater, springwater, and streamwater were measured on an unchanneled hillslope in the Tanakami Mountains of central Japan. The effects of preferential flowpaths, including lateral and vertical flow in the soil layer and flow through bedrock fractures, on the variation in the dissolved silica concentrations in runoff and groundwater were examined, as were the effects of the mixing of water from geochemically diverse water sources on the dissolved silica concentrations. The mean dissolved silica concentrations in water sampled from 40 cm below the soil surface and in transiently formed groundwater above the soil-bedrock interface during rainfall events were relatively constant, independent of the variation in the mixing ratio of pre-event water and incoming throughfall. These waters were mostly supplied by the vertical infiltration of water in soil. The mean dissolved silica concentrations were similar, regardless of sampling depth, although the mean residence time of the water increased with depth. These results indicated that the dissolved silica concentrations in soil water and transient groundwater were independent of contact time between the water and minerals. The mean dissolved silica concentration in perennially saturated groundwater above the soil-bedrock interface, which was recharged by water infiltrating through soil, and water emerging from bedrock in an area near the spring was more than twice that of transient groundwater, and the variation was relatively large. The mean dissolved silica concentration increased significantly downslope, from perennial groundwater to spring from soil matrix to stream, and the spring and stream concentrations also showed large variations. The dissolved silica concentration was highest in the spring from a bedrock fracture and was relatively constant. The mixing of water from two geochemically diverse water sources, soil and bedrock, controlled the dissolved silica concentrations of the perennial groundwater, the spring from soil matrix, and the stream. Our results demonstrated that in most areas of this headwater catchment, the preferential flowpaths have only a small effect on the dissolved silica concentrations. In a small area, which was < 2% of the total catchment area near the spring, the dissolved silica concentration was controlled by the mixing of water from geochemically diverse water sources.

The use of stream flow routing for direct channel precipitation with isotopically-based hydrograph separations: the role of new water in stormflow generation

Understanding the pathways by which event water contributes to stream stormflow provides insight into stormflow generation mechanisms. We analyze the impact of storm size on the relative contribution of event water to stormflow by using natural variations in the oxygen isotopic composition of precipitation and stream water to separate multiple stormflow hydrographs from a single fourth-order, 1212 ha catchment. We extend previous isotope-based hydrograph separations by independently accounting for the contribution of event water via direct channel precipitation to the stream hydrograph. The direct channel precipitation contribution is determined using a numerical model of stream flow routing though the catchment, taking precipitation and digital elevation data as input variables. For the range of storm sizes sampled, having recurrence intervals ranging from less than a week to ∼4 months, essentially all the event water in stream stormflow can be attributed to direct channel precipitation. Event water not directly falling on the stream channel indirectly contributes to stormflow by increasing the subsurface discharge of pre-event water to the stream. Neither the hydrograph separation data, field observations during the precipitation events, nor experimental observations of flow in a large-scale natural soil column extracted from the watershed are consistent with macropore flow or groundwater ridging as the primary mechanism responsible for increasing subsurface discharge. Results from a series of artificial rain experiments using the unsaturated natural soil column are consistent with a preferential kinematic flow model and indicate that the discharge of pre-event water to the stream during a storm event may be controlled by kinematic flow processes within the watershed soils.

Oxygen 18 fractionation during snowmelt: Implications for spring flood hydrograph separation

Isotopic hydrograph separation (IHS) to define sources of event and preevent water during hydrological episodes has greatly improved the understanding of water, solute, and contaminant transport to streams during recent decades. However, the large variation in snowmelt isotopic composition, caused by fractionation during melting, has impeded an accurate separation of streamflow during spring flood episodes. Here we present a method that greatly improves the separation of event and preevent water during snowmelt by accounting for both the temporal change in the snowmelt isotopic signal and the temporary storage of meltwater in the catchment. Comparison of results of this technique with previous results, using isotopic data from the 1997 spring flood on a small catchment in northern Sweden, suggests that earlier techniques significantly underestimate the preevent contribution. This paper also explores the importance of lateral mixing across the catchment of temporally varying event inputs for the IHS results.

Inverse modeling of the hydrological and the hydrochemical behavior of hydrosystems: Characterization of Karst System Functioning

Inverse modeling of mass transfer characterizes the dynamic processes affecting the function of karst systems and can be used to identify karst properties. An inverse model is proposed to calculate unit hydrographs as well as impulse response of fluxes from rainfall‐runoff or rainfall‐flux data, the purpose of which is hydrograph separation. Contrary to what hydrologists have been doing for years, hydrograph separation is carried out by using transfer functions in their entirety, which enables accurate separation of fluxes, as was explained in the companion paper [Pinault et al., this issue]. The unit hydrograph as well as impulse response of fluxes is decomposed into a quick and a slow component, and, consequently, the effective rainfall is decomposed into two parts, one contributing to the quick flow (or flux) and the other contributing to the slow flow generation. This approach is applied to seven French karstic aquifers located on the Larzac plateau in the Grands Causses area (in the south of France). Both hydrodynamical and hydrogeochemical data have been recorded from these springs over several hydrological cycles. For modeling purposes, karst properties can be represented by the impulse responses of flow and flux of dissolved species. The heterogeneity of aquifers is translated to time‐modulated flow and transport at the outlet. Monitoring these fluxes enables the evaluation of slow and quick components in the hydrograph. The quick component refers to the “flush flow” effect and results from fast infiltration in the karst conduit network when connection is established between the infiltration and phreatic zones, inducing an increase in water head. This component reflects flood events where flow behavior is nonlinear and is described by a very short transfer function, which increases and decreases according to water head. The slow component consists of slow and fast infiltration, underground runoff, storage in annex‐to‐drain systems, and discharge from the saturated zone. These components can be further subdivided by measuring chemical responses at the karst outlet. Using such natural tracers enables the slow component of the unit hydrograph to be separated into preevent water, i.e., water of the reservoir and event water, i.e., water whose origin can be related to a particular rainfall event. These measurements can be used to determine the rate of water renewal. Since the preevent water hydrograph is produced by stored water when pushed by a rainfall event and the event water hydrograph reflects rainwater transfer, separating the two components can yield insights into the characteristics of karst aquifers, the modes of infiltration, and the mechanisms involved in karstification, as well as the degree of organization of the aquifer.

Investigation of the hydrological processes using chemical and isotopic tracers in a small Mediterranean forested catchment during autumn recharge

Flood generation processes are studied in a small mountainous Mediterranean catchment using natural chemical and isotopic water tracing. The study takes place during the autumn of 1996 for three successive floods of uneven magnitude. The flow resumption following the summer period is a prime opportunity to establish the impact of the soil moisture status on the catchment response. A dynamic scheme of the catchment behaviour and mechanisms is obtained. The combined use of different tracers highlights the speed of the water transfer from the hillslope to the catchment outlet as well as the predominant contribution of short residence time water (current water and previous rain water). Study results show that subsurface drainage through the soil macroporosity is one of the dominant processes in the formation of streamflow generation. This is in keeping with the soil's high permeability. Hydrograph separations demonstrate that pre-event water is the major contributor to streamflow. However, with time, a rise in the proportion of direct rainwater is observed, thereby highlighting the impact of the spread of soil saturation on the speed of water transfer and on the expansion of the contributing areas. Thus, for the last flood event (13/15-10), almost a third of the streamwater volume comes from the event rainfall and 50% from the first autumn rain event (19-9). The contributing areas reached 14% of the total catchment surface. A δ 18O simulation in shallow subsurface runoff is carried out, at the slope scale, with the use of a simple model. The results demonstrate that this subsurface runoff came from the mixing of the rain event's successive fractions without involving pre-existent water stored in the upper soil layer. For this autumnal flow resumption, the catchment mechanisms can be explained by a two-reservoir system, each geochemically distinct from the other. One is a surface reservoir primarily containing rainwater and the other is a subsurface reservoir made up of pre-event water from earlier rainfall. The contribution of an underground long residence time water is negligible.

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 &amp; Sons, Ltd.

Portable irrigation system for studying hillslope and wetland runoff generation processes

AbstractThe advantage of an irrigation system is that experiments of varying precipitation duration and intensity can be performed in a controlled situation. An effective portable irrigation system is described for use in experimental plot hillslope and wetland runoff studies. The system consists of four parts:(1) a water pump; (2) a tracer reservoir; (3) a chemical feed pump; and (4) distribution hoses. Relatively uniform water application is achieved by a series of manual valves controlling water flow from the main carrier hose to the distribution hoses. The irrigation system materials are inexpensive and installation and operation costs are minor. The system was used to study runoff generation from a small‐saturated area in a spring‐fed swamp for a range of precipitation intensities and durations. The irrigation system applied a maximum intensity of 14·0mm h−1, for a maximum duration of 180 min, to a 190m2 area. This range of application incorporated all storms up to a one in three year event. The variance of the tracer load was almost three times greater for natural (60%) than with the irrigation system (22%). The irrigation system reduced the uncertainty in the pre‐event water fraction (using a two‐component hydrograph separation) from 3·4% in natural events to only 0·7%. The irrigation system design, operation, calibration and cost are presented. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Hydrograph separation using isotopic, chemical and hydrological approaches (Strengbach catchment, France)

The streamflow components were determined in a small catchment located in Eastern France for a 40 mm rain event using isotopic and chemical tracing with particular focus on the spatial and temporal variations of catchment sources. Precipitation, soil solution, springwater and streamwaters were sampled and analysed for stable water isotopes ( 18O and 2H), major chemical parameters (SO 4 2−, NO 3 −, Cl −, Na +, K +, Ca 2+, Mg 2+, NH 4 +, H +, H 4SiO 4, alkalinity and conductivity), dissolved organic carbon (DOC) and trace elements (Al, Rb, Sr, Ba, Pb and U). 18O, Si, DOC, Ba and U were finally selected to assess the different contributing sources using mass balance equations and end-member mixing diagrams. Isotopic hydrograph separation shows that the pre-event water only contributes to 2% at the beginning of the stormflow to 13% at the main peak flow. DOC associated to Si and U to Ba allow to identify the different contributing areas (upper layers of the saturated areas, deep layers of the hillslope and rainwater). The streamflow (70%) originates from the deep layers of the hillslope, the remaining being supplied by the small saturated areas. The combination of chemical (both trace and major elements) and isotopic tracers allows to identify the origin of water pathways. During the first stage of the storm event, a significant part of the runoff (30–39%) comes from the small extended saturated areas located down part of the basin (overland runoff then groundwater ridging). During the second stage, the contribution of waters from the deep layers of the hillslope in the upper subcatchment becomes more significant. The final state is characterised by a balanced contribution between aquifers located in moraine and downslopes. Indeed, this study demonstrates the interest of combining a variety of hydrometric data, geochemical and isotopic tracers to identify the components of the streamwater in such conditions.

Effects of Riparian Denitrification on Stream Nitrate-Evidence from Isotope Analysis and Extreme Nitrate Leaching during Rainfall-

The effects of riparian denitrification on stream nitrate were investigated by detailed soil water observations and isotope analysis at a small headwater catchment in an urban area near Tokyo, central Japan. In the base flow period, stream nitrate concentration (<100 µM) was comparable with that of riparian ground water which had less nitrate than unsaturated soil water. Nitrogen isotope analysis showed that the consumption of nitrate by denitrification took place in riparian ground water, suggesting that denitrification is an important process to control nitrate leaching to streams. During rainfall, the concentration of stream nitrate increased up to 400 µM, which was comparable with that of pre-event soil water. The fact that soil water nitrate directly leached to streams indicated that the riparian denitrification process did not work during rainfall because of the rapid discharge of water. A decrease of denitrification effects is a possible reason for high stream nitrate concentration during rainfall.

Modelling groundwater–surface water mixing in a headwater wetland: implications for hydrograph separation

Many headwater wetlands contain seasonally or permanently saturated areas adjacent to streams, in small depressions or in regional or local groundwater discharge zones. Environmental isotopes indicate that pre-event water dominates the storm period from these headwater wetlands, however, saturated overland flow usually dominates stormflow at these sites supposedly making groundwater contributions relatively less important as a runoff mechanism. Mixing of event water with surface storage water transported by saturated overland flow has been suggested as an alternative mechanism to account for large volumes of pre-event water during saturated overland flow stormflow. In this paper we present results from various model simulations of groundwater–surface-water mixing in a headwater wetland. In the model we have varied both local and groundwater contributions to the wetland, precipitation intensity and the initial ‘wetness’ of the wetland to better understand the processes controlling both stormflow and water chemistry. Surface water mixing is shown to be an important process during low, moderate and high intensity events. Results from these simulations are used to predict the effect of reduced groundwater input to the wetlands on stormflow and water chemistry. Copyright © 2000 John Wiley & Sons, Ltd.

Contribution of groundwater and overland flows to storm flow generation in a cultivated Mediterranean catchment. Quantification by natural chemical tracing

Little work has up to now been done on the mechanisms of storm flow generation in Mediterranean cultivated environments. The present work analysed such mechanisms by natural chemical tracing in a small Mediterranean wine-growing catchment (0.91km2): Roujan, Hérault, France. Two autumn runoff events with very different characteristics were studied. The first, a minor one (specific peak flow=28l/s/km2), was used to evaluate the sensitivity of the environment to low intensity rainfall. The second was significantly larger (specific peak flow=944l/s/km2) and was used to analyse the response of the catchment to heavy downpours. Tracer concentrations at the catchment outlet, for the groundwater of two distinct geomorphological units (depression and plateau) and in an experimental plot are presented. A mixing model involving three reservoirs and two tracers (chloride and nitrate) is then used to estimate the contributions of the three main storm flow components: (a) the pre-event water deriving from the depression groundwater; (b) the event water of the precipitations; and (c) the pre-event water of the plateau groundwater. The event water end member basically corresponds to infiltration-excess overland flow plus direct precipitation on saturated areas. The imprecision of the calculations was estimated by the Monte Carlo method. During both runoff events, there was little variation in the rate at which the stream was fed by pre-existing water deriving from the groundwater, although the water tables rose rapidly. Overland flow dominated in the rapid storm flow. Its contribution varied between 12 and 82% according to the importance of the event. When the water level rose, particularly in the case of the heavy runoff event, the overland flows concentrated in the man-made network of ditches running down towards the main ditch. This wave of overland flow spread, expelling the pre-event water into the ditches located downstream, which were initially fed by the groundwater. With the rapid rise in the level of water in the ditches, ditchwater infiltrated into the groundwater, and the latter then ceased to contribute to the flow in the ditch network.

Hydrograph separation in a mountainous catchment ? combining hydrochemical and isotopic tracers

Runoff components of the Zastler catchment (18\4 km2, southern Black Forest, Germany) were analysed with hydrograph separations using stable oxygen isotopes and dissolved silica. It was shown that event water and components with low silica contributed only small amounts to total runoff. In addition, comparison of the two-component hydrograph separations showed that the low-silica components are generated by both event water and pre-event water fractions, depending on the state of the system. A modified three-component hydrograph separation method was introduced using dissolved silica and 18O. During storm events an interaction of three runoff components having distinct silica concentrations could be shown. Based on the geological and geomorphological genesis of the study site, it was appropriate to assign (i) the low silica component to the riparian zones and impermeable areas, (ii) the medium silica component to the periglacial debris cover and (iii) the high silica component to the crystalline detritus and crystalline hard rock. Exact quantification of the runoff components remained difficult. However, runoff components with medium silica concentrations reacted very sensitively and intensely. The contribution of this component to total runoff is comparatively large. This shows the important role of the periglacial debris to runoff generation of the study site and emphasizes the importance of runoff generation processes occurring in this reservoir. Copyright © 2000 John Wiley & Sons, Ltd.

Modelling groundwater-surface water mixing in a headwater wetland: implications for hydrograph separation

Many headwater wetlands contain seasonally or permanently saturated areas adjacent to streams, in small depressions or in regional or local groundwater discharge zones. Environmental isotopes indicate that pre-event water dominates the storm period from these headwater wetlands, however, saturated overland flow usually dominates stormflow at these sites supposedly making groundwater contributions relatively less important as a runoff mechanism. Mixing of event water with surface storage water transported by saturated overland flow has been suggested as an alternative mechanism to account for large volumes of pre-event water during saturated overland flow stormflow. In this paper we present results from various model simulations of groundwater- surface-water mixing in a headwater wetland. In the model we have varied both local and groundwater contributions to the wetland, precipitation intensity and the initial wetness' of the wetland to better understand the processes controlling both stormflow and water chemistry. Surface water mixing is shown to be an important process during low, moderate and high intensity events. Results from these simulations are used to predict the effect of reduced groundwater input to the wetlands on stormflow and water chemistry.

Solute concentrations of overland flow water in a cultivated field: spatial variations, intra‐ and inter‐storm trends

Major solute concentrations in overland flow water (OFW) were measured in an agricultural field of Brittany (western France). Two storm events were monitored in detail to examine the short time-scale processes. During one year, samples were taken at different positions on the slope after each storm event to describe the spatial and seasonal variations of OFW chemistry. Although the total dissolved load in OFW is not much higher than in rain water, distinctive features are observed. K+, Ca2+, NH4 , Cl− and SOare the major solutes. The main origin of the elements (sea salts, exchangeable soil complex or fertilizers) determined most of the variations observed. Spatial variations along the slope are mainly seen for exchangeable cations, while seasonal variations are predominant for sea salts. Rainfall intensity and suspended sediment load induce strong differences between the two storm events studied in detail. However, the within-storm variations and the seasonal monitoring show that this relationship is complex. Within-storm variations suggest that, in addition to desorption processes, mixing with pre-event water may occur. The lack of a relationship between sediment load and dissolved load is attributed to the high rate of the exchange processes, which has been checked by a simple experiment in vitro. It is concluded that the conditions of the transit of water on the field (velocity, length, status of the surface, crusted or not) may well play a major role in the chemical changes between rain water and OFW. The results suggest that vegetated buffer strips designed to reduce the sediment load only, and not the amount of overland flow, will have little effect on the transfer of dissolved pollutants to the watercourses. Copyright © 1999 John Wiley & Sons, Ltd.

Solute concentrations of overland flow water in a cultivated field: spatial variations, intra- and inter-storm trends

Major solute concentrations in overland flow water (OFW) were measured in an agricultural field of Brittany (western France). Two storm events were monitored in detail to examine the short time-scale processes. During one year, samples were taken at different positions on the slope after each storm event to describe the spatial and seasonal variations of OFW chemistry. Although the total dissolved load in OFW is not much higher than in rain water, distinctive features are observed. K+, Ca2+, NH4 , Cl− and SOare the major solutes. The main origin of the elements (sea salts, exchangeable soil complex or fertilizers) determined most of the variations observed. Spatial variations along the slope are mainly seen for exchangeable cations, while seasonal variations are predominant for sea salts. Rainfall intensity and suspended sediment load induce strong differences between the two storm events studied in detail. However, the within-storm variations and the seasonal monitoring show that this relationship is complex. Within-storm variations suggest that, in addition to desorption processes, mixing with pre-event water may occur. The lack of a relationship between sediment load and dissolved load is attributed to the high rate of the exchange processes, which has been checked by a simple experiment in vitro. It is concluded that the conditions of the transit of water on the field (velocity, length, status of the surface, crusted or not) may well play a major role in the chemical changes between rain water and OFW. The results suggest that vegetated buffer strips designed to reduce the sediment load only, and not the amount of overland flow, will have little effect on the transfer of dissolved pollutants to the watercourses. Copyright © 1999 John Wiley & Sons, Ltd.

Baseflow recession and recharge as nonlinear storage processes

Discharge in many rivers is often fed by outflow from a shallow groundwater reservoir. It is becoming clear that the outflow from this aquifer is not linearly proportional to storage as is commonly assumed in many algorithms. Numerical analysis of flow recession curves from about 100 river gauging stations instead reveals a nonlinear relationship between baseflow, Q, and storage, S, for which the equation S=aQb was adopted. Values of the exponent b are found by calibration to be between 0 and 1 but with a high concentration around 0·5, which is in accordance with the findings of other studies and theoretical approaches yielding b=0·5 for unconfined aquifers and relating the coefficient a to catchment properties, primarily area and shape of basin, pore volume and transmissivity. This non-linear reservoir function is proposed as a more realistic alternative to the linear reservoir function. The relatively fast response of groundwater flow to rainfall is mainly a result of the increase of hydraulic head of the groundwater reservoir accelerating the exfiltration of ‘old’, pre-event water into the river bed. As fissure and pore volumes communicate hydraulically, it appears physically reasonable to model the system by one non-linear reservoir for catchments, or parts of them, instead of applying independent parallel linear reservoirs. The non-linear reservoir algorithms are supported by an analytical derivation. They are extended for the automatic separation of baseflow from a time-series of daily discharge in rivers and the computation of storage and effective recharge of groundwater in river basins by inverse nonlinear reservoir routing. The time-series obtained allow the identification and quantification of long-term changes to the water balance. Relationships between computed groundwater storage and observed groundwater level can also be established. Copyright © 1999 John Wiley & Sons, Ltd.

Effects of forest thinning on the streamwater chemistry of two forest watersheds in the Bavarian Alps

Two well-buffered mountain forest watersheds were studied with regard to streamwater chemistry and the impact of forest thinning. The areas are located in the Bavarian Alps near lake Tegernsee, 50 km southwest of Munich. In order to get information about the influence of disturbances on the streamwater chemistry 40% of the trees were removed from one watershed after two years of monitoring. The trees were removed by crane to avoid soil damage and erosion. The second watershed was used as a reference. The output at the weirs is characterized, in general, by a strong export of silicate, alkali and alkaline earth cations, and bicarbonate. Acidity and nitrogen accumulate in the watersheds. Both areas are subject to a continuously increasing load of acidity. The acidity is mainly buffered by cation exchange and silicate hydrolysis. This is indicated by continuously decreasing pH values and the dominance of Ca 2+ and silicic acid in the streamwater. Strong changes in the water chemistry along the flowpath indicate that rainwater will be stored in the system for sometime, and water derived from the headwater areas influences the discharge. During precipitation events the new rainwater replaces pre-event water, stored in soils, which had a longer contact time with the biotic and abiotic soil matrix. Acute effects of the selective harvest are leveled in the streamwater by multiple buffer reactions. The discharge of the disturbed watershed was increased by the reduction of interception and transpiration. Increased rates of mineralization and acidity loads caused an increase in ion concentration and solution load in the discharge from the disturbed area. NH + 4 showed the strongest effects compared to the reference area and the pre-event period. The changes in the runoff returned to pre-event conditions about one year after the disturbance.

Component flow processes at four streams in the Catskill Mountains, New York, analysed using episodic concentration/discharge relationships

Plots of solute concentration against discharge have been used to relate stream hydrochemical variations to processes of flow generation, using data collected at four streams in the Catskill Mountains, New York, during the Episodic Response Project of the US Environmental Protection Agency. Results suggest that a two-component system of shallow and deep saturated subsurface flow, in which the two components respond simultaneously during hydrologic events, may be applicable to the study basins. Using a large natural sea-salt sodium input as a tracer for precipitation, it is argued that an additional distinction can be made between pre-event and event water travelling along the shallow subsurface flow path. Pre-event water is thought to be displaced by infiltrating event water, which becomes dominant on the falling limb of the hydrograph. Where, as appears to be the case for sulfate, a solute equilibrates rapidly within the soil, the pre-event–event water distinction is unimportant. However, for some solutes there are clear and consistent compositional differences between water from the two sources, evident as a hysteresis loop in concentration–discharge plots. Nitrate and acidity, in particular, appear to be elevated in event water following percolation through the organic horizon. Consequently, the most acidic, high nitrate conditions during an episode generally occur after peak discharge. A simple conceptual model of episode runoff generation is presented on the basis of these results. Copyright © 1999 John Wiley & Sons, Ltd.