Hydrograph Separation Technique Research Articles

Hydrological response of an Alpine catchment to rainfall and snowmelt events

Alpine catchments are important sources of fresh water but compared to lower altitude catchments our understanding of the hydrological functioning of these catchments during rainfall and snowmelt events is still limited. The objectives of this study were i) to identify the dominant runoff generation mechanisms in the 0.14-km2 Bridge Creek Catchment in the Italian Dolomites during nine rainfall–runoff events and six snowmelt–runoff events in spring, summer and autumn of 2010–2012; and ii) to assess the effect of the selection of the pre-event water sample on the isotope hydrograph separation results. The isotopic composition of the pre-event water was determined by either a stream water sample taken prior to the event or the average of 19 stream water samples taken during baseflow conditions. The hydrograph separation results for the two methods were very similar for the rainfall events but differed for the snowmelt events. Average event water contributions ranged between 4% and 19% or 2% and 20% of the total runoff during rainfall events, and between 7% and 25% or 9% and 38% during snowmelt events, depending on the method used to determine the isotopic composition of pre-event water. Event water contributions were important during large rainfall events, intense rainfall events and late in the snowmelt season, with maximum event water contributions up to 37% and 46%, depending on the method used for determining the pre-event water composition. The electrical conductivity of stream water tended to first decrease and reach a minimum before peak streamflow and then to increase above pre-event values. The results of this study suggest that during dry conditions, direct channel precipitation and overland flow from the permanently saturated part of the riparian zone dominated the runoff response, with limited contributions of riparian or hillslope groundwater. During wet or very wet conditions (large rainfall events or peak snowmelt), saturation overland flow increased due to the expansion of the saturated areas and riparian groundwater and hillslope subsurface flow to streamflow increased as well. On the one hand, this work contributes to a better understanding of runoff generation processes in mountain headwater catchments where rainfall and snowmelt events dominate the hydrological response. On the other hand, this study highlights the sensitivity of the two-component hydrograph separation technique to the selection of the pre-event water sample for snowmelt events. This calls for further studies in snow-dominated catchments to determine the consistency of the isotopic composition of stream water prior to individual snowmelt events and to assess whether the individual melt events during the melt season should be considered part of a single long snowmelt event.

Insights gained from four component hydrograph separation

Traditional hydrograph separation techniques split an observed storm hydrograph into two main components representing ‘storm runoff’ and ‘baseflow’. In this paper a new separation technique is described which makes an initial split into two main components, quickflow and slowflow, which are each then subsequently split into two further subcomponents. The resulting procedure is termed the ‘four component hydrograph separation technique’. Various ways of recombining these four subcomponents to build up a curve that represents the observed storm hydrograph are possible, of which two ways are examined in further detail. If it is assumed that the four component separation technique provides a promising representation of an observed storm hydrograph, these two ways allow theoretical and practical insights to be gained into four existing hydrograph separation techniques. A conclusion, common to all four, is that much more care is required in naming the flow lines separating out each of the suggested subcomponents making up the observed storm hydrograph. This paper also emphasises the key role played by the slowflow storm runoff subcomponent, which has not been given sufficient prominence in existing event-based models in the past. A procedure for estimating each of the four subcomponents is illustrated for an observed event.

Investigation of the potential surface–groundwater relationship using automated base-flow separation techniques and recession curve analysis in Al Zerba region of Aleppo, Syria

The management of water resources requires an adequate understanding of the relationship between the various components of the hydrological cycle. The accelerated urbanization and agricultural development impose the concern about the sustainability of water resources, particularly in arid and semi-arid regions. In such environments, water scarcity and drought often lead to intensive groundwater abstraction which can adversely affect the hydrological system. Severe over-exploitation of groundwater has been observed on the Aleppo basin of the Al Qweek River. The Al Qweek valley constitutes the central part of the study area (southwestern parts of the Aleppo basin). The region is characterized by a semi-arid climate with an average annual precipitation of 325 mm. The intensive exploitation of the upper aquifer in the catchment is responsible for a continuous decline of the piezometric levels with an average of 1.8 m/year. The objective of this study was to investigate the relationship between the upper aquifer and the Al Qweek River within the catchment boundary using streamflow and hydrograph separation techniques by means of daily discharge data for both the Al Qweek and Al Qwaak rivers. HydroOffice was used for the base-flow separation, flow duration curves (FDC), and recession curve displacement analysis. Furthermore, groundwater recharge was estimated by means of RECESS and RORA. This is the first study using the hydrograph separation method in this region to our knowledge. The results indicate a proportional relationship between the surface and groundwater with an average discharge fluctuation that ranged from 0.30 to 0.20 m for the Al Qweek and Al Qwaak rivers, respectively. The results also revealed a dominant base flow in the catchment that ranged from 86.3 to 88.2 % of the total flow with a computed base-flow index that varied from 0.85 to 0.90 in all gauge stations, indicating a stable flow regime in the region. Furthermore, the high base flow is resulting in high permeability conditions of the upper aquifer and low direct surface runoff. The visual interpretation of the FDC suggests sustained base flow from groundwater storage. The low-flow index indicates an average contribution of groundwater storage of 45 to 58 %. The result of recession curve analysis shows a recession constant from 0.8 to 0.9 indicating a dominant interflow and low overland runoff. Furthermore, the results also show that estimation of groundwater recharge using RORA is inappropriate for the region of interest.

강우와 융설의 안정동위원소 변동에 의한 동위원소 수문분리법의 계통오차계산

물안정동위원소를 이용하여 지하수와 강우 또는 융설이 하천에 미치는 영향을 정량적으로 분리하는 방법을 동위원소 수문분리법이라고 하며 지난 30년 동안 사용되어 왔다. 오래된 물(지하수)과 새로운 물(강우 및 융설)의 두 성분이 하천에 영향을 미치는 것으로 가정하고 새로운 물과 오래된 물의 주어진 시간동안의 동위원소를 측정하여 각각의 성분에 대한 비율을 결정할 수 있다. 본 연구에서는 동위원소 수문분리법을 수계에 적용할 때 새로운 물의 시간적인 안정동위원소분화를 고려하지 않고 새로운 물의 평균값을 이용하였을 때 계통오차가 발생함을 보였다. 이러한 표준오차의 크기는 (1) 새로운 물이 하천에 많이 기여할수록, (2) 사용된 평균값과 분화된 새로운 물의 안정동위원소 값과 차이가 클수록, 마지막으로 (3) 오래된 물과 새로운 물의 안정동위원소 값의 차이가 작을수록 커진다. 집중 호우로 유출이 증가하거나 봄철에 지면이 아직 녹지 않아 융설이 유출이 되는 경우 새로운 물이 하천에 미치는 영향이 커지게 되어 상대오차 역시 증가한다. 이러한 오차를 줄이기 위해서 각각의 새로운 물, 오래된 물, 하천의 안정동위원소를 같은 시간 간격으로 측정하여 새로운 물이 분화되는 것을 고려한 수문분리법을 수행하는 것을 고려하여야 한다. An isotopic hydrograph separation technique has been able to determine the contribution of new water (event water such as rain or snowmelt) and old water (pre-event water like groundwater) to a stream hydrograph for last several decades using stable water isotopes. It is based on the assumption that the isotopic compositions of both new water and old water at a given instant in time are known and the stream water is a mixture of the two waters. In this study, we show that there is a systematic error (standard error in the new water fraction) in the isotopic hydrograph separation if the average isotopic compositions of new water were used ignoring the temporal variations of those of new water. The standard error in the new water fraction is caused by: (1) the isotopic difference between the average value and temporal variations of new water; (2) the new water fraction as runoff contributing to the stream during rainfall or spring melt; and (3) the isotopic differences between new and old water (inversely). The standard error is large, in particular, when new water dominates the stream flow, such as runoff during intense rainfall and in areas of low infiltration during spring melt. To reduce the error in the isotopic hydrograph separation, incorporation of fractionation in the isotopic composition of new water observed at a point should be considered with simultaneous sampling of new water, old water and stream water.

Optimal hydrograph separation filter to evaluate transport routines of hydrological models

Hydrograph separation (HS) using recursive digital filter approaches focuses on trying to distinguish between the rapidly occurring discharge components like surface runoff, and the slowly changing discharge originating from interflow and groundwater. Filter approaches are mathematical procedures, which perform the HS using a set of separation parameters. The first goal of this study is to minimize the subjective influence that a user of the filter technique exerts on the results by the choice of such filter parameters. A simple optimal HS (OHS) technique for the estimation of the separation parameters was introduced, relying on measured stream hydrochemistry. The second goal is to use the OHS parameters to benchmark the performance of process-based hydro-geochemical (HG) models. The new HG routine can be used to quantify the degree of knowledge that the stream flow time series itself contributes to the HG analysis, using newly developed benchmark geochemistry efficiency (BGE). Results of the OHS show that the two HS fractions (“rapid” and “slow”) differ according to the HG substances which were selected. The BFImax parameter (long-term ratio of baseflow to total streamflow) ranged from 0.26 to 0.94 for SO4−2 and total suspended solids, TSS, respectively. Then, predictions of SO4−2 transport from a process-based hydrological model were benchmarked with the proposed HG routine, in order to evaluate the significance of the HG routines in the process-based model. This comparison provides valuable quality test that would not be obvious when using the traditional measures like r2 or the NSE (Nash–Sutcliffe efficiency). The process-based model resulted in r2=0.65 and NSE=0.65, while the benchmark routine results were slightly lower with r2=0.61 and NSE=0.58. However, the comparison between the two model resulted in obvious advantage for the process-based model with BGE=0.15.

Groundwater–surface water interactions, vegetation dependencies and implications for water resources management in the semi-arid Hailiutu River catchment, China – a synthesis

Abstract. During the last decades, large-scale land use changes took place in the Hailiutu River catchment, a semi-arid area in northwest China. These changes had significant impacts on the water resources in the area. Insights into groundwater and surface water interactions and vegetation-water dependencies help to understand these impacts and formulate sustainable water resources management policies. In this study, groundwater and surface water interactions were identified using the baseflow index at the catchment scale, and hydraulic and water temperature methods as well as event hydrograph separation techniques at the sub-catchment scale. The results show that almost 90% of the river discharge consists of groundwater. Vegetation dependencies on groundwater were analysed from the relationship between the Normalized Difference Vegetation Index (NDVI) and groundwater depth at the catchment scale and along an ecohydrogeological cross-section, and by measuring the sap flow of different plants, soil water contents and groundwater levels at different research sites. The results show that all vegetation types, i.e. trees (willow (Salix matsudana) and poplar (Populus simonii), bushes (salix – Salix psammophila), and agricultural crops (maize – Zea mays)), depend largely on groundwater as the source for transpiration. The comparative analysis indicates that maize crops use the largest amount of water, followed by poplar trees, salix bushes, and willow trees. For sustainable water use with the objective of satisfying the water demand for socio-economical development and to prevent desertification and ecological impacts on streams, more water-use-efficient crops such as sorghum, barley or millet should be promoted to reduce the consumptive water use. Willow trees should be used as wind-breaks in croplands and along roads, and drought-resistant and less water-use intensive plants (for instance native bushes) should be used to vegetate sand dunes.

Developing an integrated hydrograph separation and lumped modelling approach to quantifying hydrological pathways in Irish river catchments

Summary An appreciation of the quantity of streamflow derived from the main hydrological pathways involved in transporting diffuse contaminants is critical when addressing a wide range of water resource management issues. In order to assess hydrological pathway contributions to streams, it is necessary to provide feasible upper and lower bounds for flows in each pathway. An important first step in this process is to provide reliable estimates of the slower responding groundwater pathways and subsequently the quicker overland and interflow pathways. This paper investigates the effectiveness of a multi-faceted approach applying different hydrograph separation techniques, supplemented by lumped hydrological modelling, for calculating the Baseflow Index (BFI), for the development of an integrated approach to hydrograph separation. A semi-distributed, lumped and deterministic rainfall runoff model known as NAM has been applied to ten catchments (ranging from 5 to 699 km 2 ). While this modelling approach is useful as a validation method, NAM itself is also an important tool for investigation. These separation techniques provide a large variation in BFI, a difference of 0.741 predicted for BFI in a catchment with the less reliable fixed and sliding interval methods and local minima turning point methods included. This variation is reduced to 0.167 with these methods omitted. The Boughton and Eckhardt algorithms, while quite subjective in their use, provide quick and easily implemented approaches for obtaining physically realistic hydrograph separations. It is observed that while the different separation techniques give varying BFI values for each of the catchments, a recharge coefficient approach developed in Ireland, when applied in conjunction with the Master Recession Curve tabulation method, predict estimates in agreement with those obtained using the NAM model, and these estimates are also consistent with the study catchments’ geology. These two separation methods, in conjunction with the NAM model, were selected to form an integrated approach to assessing BFI in catchments.

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

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

Quantificação dos efeitos da heterogeneidade subsuperficial na escorrência em vertentes, através de uma abordagem estocástica

The role of heterogeneity and uncertainty in hydraulic conductivity on hillslope runoff production was evaluated using the fully integrated hydrologic model ParFlow. Simulations were generated using idealized high-resolution hillslopes configured both with a deep water table and a water table equal to the outlet to isolate surface and subsurface flow, respectively. Heterogeneous, correlated random fields were used to create spatial variability in the hydraulic conductivity. Ensembles, generated by multiple realizations of hydraulic conductivity, were used to evaluate how this uncertainty propagates to runoff. Ensemble averages were used to determine the effective runoff for a given hillslope as a function of rainfall rate and degree of subsurface heterogeneity. Cases where the water table is initialized at the outlet show runoff behavior with little sensitivity to variance in hydraulic conductivity. A technique is presented that explicitly interrogates individual realizations at every simulation timestep to partition overland and subsurface flow contributions. This hydrograph separation technique shows that the degree of heterogeneity can play a role in determining proportions of surface and subsurface flow, even when effective hillslope outflow is seen. This method is also used to evaluate current hydrograph separation techniques and demonstrates that recursive filters can accurately proportion overland and base-flow for certain cases.

The role of storm flows in concentration of pesticides associated with particulate and dissolved fractions as a threat to aquatic ecosystems - Case study: the agricultural watershed of Save river (Southwest of France)

Key-words: pesticide, TSM, DOC, hysteresis patterns, flood flow Measurement of the fluxes of pesticides was carried out for a year, ending in March 2009, in the Save catchment, in the vicinity of Toulouse. The hydrograph separation technique was used to evaluate the respective contribution of stormflow and baseflow in transport of 12 pesticide molecules. Transport of over 59% of pesticides and their controlling factors such as total suspended matter (TSM), particulate organic carbon (POC) and dissolved organic carbon (DOC) occurred during storm periods. Hysteresis patterns could be observed in the concentration-discharge relationships only for some molecules between rising and falling periods of the storm hydrograph. Clockwise hysteresis was noticed for low to moderately soluble pesticide molecules and for particulate fractions, which explains the role of surface runoff in pesticide displacement. In contrast, anticlockwise hysteresis was registered for soluble molecules and dissolved fractions, explaining the role of subsurface flows and soil leaching processes. The important role of TSM, POC and DOC in pesticide transport was clearly established. We also came to the conclusion that the role of stormy periods in pesticide movement and their controlling factors worked as a threat to aquatic ecosystems. And there was a positive relation between riverine TSM, POC, DOC and pesticides according to pesticide properties.

The role of midsummer urban aquifer recharge in stormflow generation using isotopic and chemical hydrograph separation techniques

Results dispute the traditional view that groundwater contribution to urban streams during storm events is too small to significantly influence stream flow rates. The study used a two-component hydrograph separation technique involving oxygen-18 and electrical conductivity to quantify event and pre-event components of stream flow associated with a midsummer precipitation event in a highly urbanized, formerly glaciated catchment. Isotopic separation results showed that only about 15% of the rainfall amount contributed to overland flow, with groundwater accounting for 22% of the total stormflow. Elevated electrical conductivity values, possibly associated with the flushing of urban contaminants during the storm, suggested a slightly higher (30%) groundwater contribution to total stormflow. It was also demonstrated that nearly 6% of total rainfall contributed to aquifer recharge representing significant summer recharge in the urban catchment. The findings are contrary to the prevailing assumption that stormflow in highly urban catchments is largely overland flow.

Groundwater Recharge through Conservation Structures – A Study in Ghat Area of Orissa

Groundwater recharge from two different water conservation structures under humid conditions of eastern ghat high land (EGHL) zone of Orissa were estimated by employing water table fluctuation procedure. A power function relationship best fitted the recharge and water storage depths. Contribution of base flow was segregated from the recharge values using base flow hydrograph separation technique. Potential groundwater recharge based on four years (2004-07) was estimated in the range between 3.5 - 11.0 % of the annual rainfall while considering the runoff contributing area of the conservation structures. Temporal groundwater contour maps generated using grid based graphics SURFER package (Version # 8) and GPSMAP (Version # 76CSX) for the study watershed revealed that there was a total net increase of 8.12 m and decrease of 2.61 m water table depths during wet and dry periods, respectively, resulting in net increase of water table by 1.38 m per year due to the influence of the water conservation structures. Groundwater quality of the wells in the watershed was found to be safe for irrigation and domestic consumption purposes.

Constraining groundwater discharge in a large watershed: Integrated isotopic, hydraulic, and thermal data from the Canadian shield

The objective of this study is to evaluate the pattern and rate of groundwater discharge in a large, regulated fractured rock watershed using novel and standard methods that are independent of base flow recession. Understanding the rate and pattern of groundwater discharge to surface water bodies is critical for watershed budgets, as a proxy for recharge rates, and for protecting the ecological integrity of lake and river ecosystems. The Tay River is a low‐gradient, warm‐water river that flows over exposed and fractured bedrock or a thin veneer of coarse‐grained sediments. Natural conservative (δ2H, δ18O, Cl, and specific conductance), radioactive (222Rn), and thermal tracers are integrated with streamflow measurements and a steady state advective model to delimit the discharge locations and quantify the discharge fluxes to lakes, wetlands, creeks, and the Tay River. The groundwater discharge rates to most surface water body types are low, indicating that the groundwater and surface water system may be largely decoupled in this watershed compared to watersheds underlain by porous media. Groundwater discharge is distributed across the watershed rather than localized around lineaments or high‐density zones of exposed brittle fractures. The results improve our understanding of the rate, localization, and conceptualization of discharge in a large, fractured rock watershed. Applying hydraulic, isotopic, or chemical hydrograph separation techniques would be difficult because the groundwater discharge “signal” is small compared to the “background” surface water inflows or volumes of the surface water bodies. Although this study focuses on a large watershed underlain by fractured bedrock, the methodology developed is transferable to any large regulated or unregulated watershed. The low groundwater discharge rates have significant implications for the ecology, sustainability, and management of large, crystalline watersheds.

Macropore flow estimations under no-till and till systems

Abstract The processes associated with water movement through silt loam soils involve both the flow through macropores as preferential flow or macropore flow and flow through the micropore as matrix flow. Macropore and matrix flow components were separated from total flow by a hydrograph-separation technique which used the assumption of dual porosity and a tracer mass balance. A mixture of potassium bromide was applied through a rain simulator to four plots in northern Mississippi in two rain events at 12.7 mm/h lasting 5 and 3 h separated by 6 h. The plots were either tilled or no-tilled with drains installed by two methods at the surface of the fragipan. The magnitude of water and bromide (Br − ) transported by macropore flow to a drain line were estimated and the resulting hydrographs provided an indication of the potential significance of macropore flow in transporting water and non-reactive chemicals through macropores to the shallow groundwater system. Matrix flow appears to contribute the majority of the water moving to the drains even during the early stages of the drain flow hydrographs. The no-till plots produced more macropore flow than the tilled plots, independent of how the drains were installed. Macropore flow in the drainage at any time was small as compared to the matrix flow; however it contributed a disproportionate amount of Br − tracer. These data support the concept that models used to predict mass balances using only the matrix (Darcian) flow will underestimate those chemicals that move like bromide into the soil profile.

Stream nitrate and DOC dynamics during three spring storms across land uses in glaciated landscapes of the Midwest

Many studies have investigated nitrate and dissolved organic carbon (DOC) export mechanisms during storms in forested mountainous to hilly catchments. However, the number of studies across land uses in artificially drained landscapes of the Midwest remains relatively small despite the importance of nitrate and carbon losses to streams on water quality in this region. This study investigates water, nitrate, and DOC delivery (timing) to streams during storms, and the mechanisms (flowpaths) affecting nitrate and DOC flushing trajectories during three spring storms in an agricultural watershed and a mixed land use watershed in glaciated landscape of the Midwest. Hydrograph separation techniques using oxygen-18 isotopes of water and the analysis of major cation flushing trajectories were used to determine the flowpath associated with the export of nitrate and DOC in each watershed. Higher anthropogenic N inputs in the agricultural watershed are associated with higher stream nitrate concentration during storms, while DOC concentration in streams across land uses is mainly influenced by storm characteristics/discharge. In both watersheds, DOC concentration quickly increases and decreases with discharge. In the mixed land use watershed, the peak in nitrate is consistently delayed relative to the peak in DOC; however, nitrate peaks vary in relation to discharge in the agricultural watershed. The comparison of the nitrate/DOC concentration patterns to the concentration patterns of major cations during the storms studied suggests that DOC is likely exported via a combination of overland flow and preferential flow through soil macropores in both watersheds. Data suggest nitrate is exported with groundwater, either as tile-drain flow or subsurface flow to the streams. Results also indicate that the connectivity between nitrate/DOC reservoirs and the stream is an important control mechanism on the timing of delivery of nitrate and DOC to the stream in the watersheds studied. Finally, results indicate that even in agricultural watersheds where tile-drain flow is an important contributor to stream flow during storms, there is not necessarily a consistent export mechanism for all solutes. These results underscore the need for further studies investigating the relative importance of matrix flow, overland flow and macropore flow at the watershed scale in order to fully understand event scale processes controlling the transport of solutes to streams in glaciated landscapes of the Midwest under various land use conditions.

광릉 원두부 소유역에서의 우기 중 지하수 함양률 평가

Groundwater recharge rates were estimated and compared in a headwater catchment at the Gwangneung Supersite using three different methods: water-table fluctuation (WTF), mass balance, and hydrograph separation techniques. Data were obtained during the rainy season from June to September 2005. Two different WTF methods estimated the groundwater recharge rate as 25.9% and 23.6%. The mass balance calculation of chloride ions indicated recharge rates of 13.4% on average. Baseflow separation using chloride ion as a tracer from six storm hydrographs produced a 14.0% net baseflow rate on average. Because of the implicit assumption of a long-term steady state without storage change, recharge rates calculated by mass balance and hydrograph separation were smaller than those done with WTF methods, which include the amount of increased storage due to the water-level rise. Subsequently, the WTF method is superior to others in the estimation of groundwater recharge rate to comprehend the dynamic characteristics of the hydrologic cycle.

Assessing catchment-scale spatial and temporal patterns of groundwater and stream salinity

Understanding catchment-scale patterns of groundwater and stream salinity are important in land- and water-salinity management. A large-scale assessment of groundwater and stream data was undertaken in the eastern Mt Lofty Ranges of South Australia using geographical information systems (GIS), regional scale hydrologic data, hydrograph separation and hydrochemical techniques. Results of the study show: (1) salts were mostly of marine origin (75%), while sulfate and bicarbonate from mineral weathering comprised most of the remainder, (2) elevated groundwater salinities and stable water isotopic compositions similar to mean rainfall indicated that plant transpiration was the primary salt accumulation mechanism, (3) key factors explaining groundwater salinity were geology and rainfall, with overall catchment salinity inversely proportional to average annual rainfall, and groundwater salinity ‘hotspots’ (EC >8 mS/cm) associated with geological formations comprising sulfidic marine siltstones and shales, (4) shallow groundwater correlated with elevated stream salinity, implying that baseflow contributed to stream salt loads, with most of the annual salt load (estimated to be 24,500 tonnes) occurring in winter when baseflow volume was highest. Salt-load analysis using stream data could be a practical, low-cost technique to rapidly target the investigation of problem areas within a catchment.

Regional-scale, fully coupled modelling of stream–aquifer interaction in a tropical catchment

The planning and management of water resources in the Pioneer Valley, north-eastern Australia requires a tool for assessing the impact of groundwater and stream abstractions on water supply reliabilities and environmental flows in Sandy Creek (the main surface water system studied). Consequently, a fully coupled stream-aquifer model has been constructed using the code MODHMS, calibrated to near-stream observations of watertable behaviour and multiple components of gauged stream flow. This model has been tested using other methods of estimation, including stream depletion analysis and radon isotope tracer sampling. The coarseness of spatial discretisation, which is required for practical reasons of computational efficiency, limits the model's capacity to simulate small-scale processes (e.g., near-stream groundwater pumping, bank storage effects), and alternative approaches are required to complement the model's range of applicability. Model predictions of groundwater influx to Sandy Creek are compared with baseflow estimates from three different hydrograph separation techniques, which were found to be unable to reflect the dynamics of Sandy Creek stream-aquifer interactions. The model was also used to infer changes in the water balance of the system caused by historical land use change. This led to constraints on the recharge distribution which can be implemented to improve model calibration performance. (c) 2006 Elsevier B.V. All rights reserved.

Quantifying the relative contributions of riparian and hillslope zones to catchment runoff

The spatial and temporal sources of headwater catchment runoff are poorly understood. We quantified the contributions of hillslopes and riparian zones to streamflow for two storm events in a highly responsive, steep, wet watershed located on the west coast of the South Island of New Zealand. We examined the spatial and temporal components of catchment storm flow using a simple continuity‐based approach. We tested this with independent isotopic/solute mass balance hydrograph separation techniques. We monitored catchment runoff, internal hydrological response, isotopic, and solute dynamics at a trenched hillslope, and at hillslope and riparian positions in a 2.6‐ha catchment. The gauged hillslope was used to isolate and quantify (by difference) riparian and hillslope zone contributions to the 2.6‐ha headwater catchment. Utilizing flow‐based approaches and a tracer‐based mass balance mixing model, we found that hillslope runoff comprised 2–16% of total catchment storm runoff during a small 27‐mm event and 47–55% during a larger 70‐mm event. However, less than 4% of the new water collected at the catchment outlet originated from the hillslopes during each event. We found that in the 27‐mm rain event, 84–97% of total storm runoff was generated in the riparian zone. In a larger 70‐mm event, riparian water dominated total flow early in the event, although the hillslope became the main contributor once hillslope runoff was initiated. Despite the large amount of subsurface hillslope runoff in total storm runoff during the second larger event, riparian and channel zones accounted for 96% of the new water measured at the catchment outlet. Riparian water dominated between events, throughout small runoff events, and during early portions of large events. While this sequencing of catchment position contributions to flow has been conceptualized for some time, this is the first study to quantify this timing, constrained by hydrometric, isotopic, and solute approaches.

Estimating ground water discharge by hydrograph separation.

Iron Mountain is located in the West Shasta Mining District in California. An investigation of the generation of acid rock drainage and metals loading to Boulder Creek at Iron Mountain was conducted. As part of that investigation, a hydrograph separation technique was used to determine the contribution of ground water to total flow in Boulder Creek. During high-flow storm events in the winter months, peak flow in Boulder Creek can exceed 22.7 m3/sec, and comprises surface runoff, interflow, and ground water discharge. A hydrograph separation technique was used to estimate ground water discharge into Boulder Creek during high-flow conditions. Total ground water discharge to the creek approaches 0.31 m3/sec during the high-flow season. The hydrograph separation technique combined with an extensive field data set provided reasonable estimates of ground water discharge. These estimates are useful for other investigations, such as determining a corresponding metals load from the metal-rich ground water found at Iron Mountain and thus contributing to remedial alternatives.