Evaluation of infiltration‐based stormwater management to restore hydrological processes in urban headwater streams

Urbanization threatens headwater stream ecosystems globally. Watershed restoration practices, such as infiltration‐based stormwater management, are implemented to mitigate the detrimental effects of urbanization on aquatic ecosystems. However, their effectiveness for restoring hydrologic processes and watershed storage remains poorly understood. Our study used a comparative hydrology approach to quantify the effects of urban watershed restoration on watershed hydrologic function in headwater streams within the Coastal Plain of Maryland, USA. We selected 11 headwater streams that spanned an urbanization–restoration gradient (4 forested, 4 urban‐degraded, and 3 urban‐degraded) to evaluate changes in watershed hydrologic function from both urbanization and watershed restoration. Discrete discharge and continuous, high‐frequency rainfall‐stage monitoring were conducted in each watershed. These datasets were used to develop 6 hydrologic metrics describing changes in watershed storage, flowpath connectivity, or the resultant stream flow regime. The hydrological effects of urbanization were clearly observed in all metrics, but only 1 of the 3 restored watersheds exhibited partially restored hydrologic function. At this site, a larger minimum runoff threshold was observed relative to the urban‐degraded watersheds, suggesting enhanced infiltration of stormwater runoff within the restoration structure. However, baseflow in the stream draining this watershed remained low compared to the forested reference streams, suggesting that enhanced infiltration of stormwater runoff did not recharge subsurface storage zones contributing to stream baseflow. The highly variable responses among the 3 restored watersheds were likely due to the spatial heterogeneity of urban development, including the level of impervious cover and extent of the storm sewer network. This study yielded important knowledge on how restoration strategies, such as infiltration‐based stormwater management, modulated—or failed to modulate—hydrological processes affected by urbanization, which will help improve the design of future urban watershed management strategies. More broadly, we highlighted a multimetric approach that can be used to monitor the restoration of headwater stream ecosystems in disturbed landscapes.     

The effect of urbanization on stream hydrology in hillslope watersheds in central Texas

This study examined the effect of urbanization on stream hydrology in hillslope watersheds. Ten streams (seven in hillslope and three in gentle slope watersheds) around Austin, Texas were selected for analysis. For each stream, we compared parameters of transfer function (TF) models estimated from daily rainfall and streamflow data collected in two study periods (October 1988–September 1992 and October 2004–September 2008) representing different degrees of watershed urbanization. As expected, the streams became more intermittent as the watersheds were more urbanized in all the study streams. However, the effect of urbanization on peakflow differs between hillslope and gentle slope watersheds. After watershed urbanization, peakflow increased in gentle slope watersheds, but decreased in hillslope watersheds. Based on the results of the TF models, we found that urbanization made stream not flashier but drier in hillslope watersheds. Overpumpage of aquifer has been recognized as a problem that leads to the stream dryness in the study area. However, the overpumpage alone cannot explain the differences in hydrological changes between the two types of watersheds. We attributed the reduced peakflow and stream dryness in the hillslope watersheds to land grading for construction forming stair‐stepped or terraced landscape. Compared with natural hillslope, a stair‐stepped landscape could infiltrate more stormwater by slowing down surface runoff on tread portions of the stair. Our findings suggest that a watershed management scheme should take into account local hydrogeologic conditions to mitigate the stream dryness resulting from urbanization in hillslope watersheds. Copyright © 2010 John Wiley & Sons, Ltd.     

The application of electrical conductivity as a tracer for hydrograph separation in urban catchments

Two‐component hydrograph separation was performed on 19 low‐to‐moderate intensity rainfall events in a 4·1‐km² urban watershed to infer the relative and absolute contribution of surface runoff (e.g. new water) to stormflow generation between 2001 and 2003. The electrical conductivity (EC) of water was used as a continuous and inexpensive tracer, with order of magnitude differences in precipitation (12–46 µS/cm) and pre‐event streamwater EC values (520–1297 µS/cm). While new water accounted for most of the increased discharge during storms (61–117%), the contribution of new water to total discharge during events was typically lower (18–78%) and negatively correlated with antecedent stream discharge (r² = 0·55, p < 0·01). The amount of new water was positively correlated with total rainfall (r² = 0·77), but hydrograph separation results suggest that less than half (9–46%) of the total rainfall on impervious surfaces is rapidly routed to the stream channel as new water. Comparison of hydrograph separation results using non‐conservative tracers (EC and Si) and a conservative isotopic tracer (δD) for two events showed similar results and highlighted the potential application of EC as an inexpensive, high frequency tracer for hydrograph separation studies in urban catchments. The use of a simple tracer‐based approach may help hydrologists and watershed managers to better understand impervious surface runoff, stormflow generation and non‐point‐source pollutant loading to urban streams. Copyright © 2007 John Wiley & Sons, Ltd.     

Assessing urban rainfall-runoff response to stormwater management extent

We report an empirical analysis of the hydrologic response of three small, highly impervious urban watersheds to pulse rainfall events, to assess how traditional stormwater management (SWM) alters urban hydrographs. The watersheds vary in SWM coverage from 3% to 61% and in impervious cover from 45% to 67%. By selecting a set of storm events that involved a single rainfall pulse with >96% of total precipitation delivered in 60 min, we reduced the effect of differences between storms on hydrograph response to isolate characteristic responses attributable to watershed properties. Watershed-average radar rainfall data were used to generate local storm hyetographs for each event in each watershed, thus compensating for the extreme spatial and temporal heterogeneity of short-duration, intense rainfall events. By normalizing discharge values to the discharge peak and centring each hydrograph on the time of peak we were able to visualize the envelope of hydrographs for each group and to generate representative composite hydrographs for comparison across the three watersheds. Despite dramatic differences in the fraction of watershed area draining to SWM features across these three headwater tributaries, we did not find strong evidence that SWM causes significant attenuation of the hydrograph peak. Hydrograph response for the three watersheds is remarkably uniform despite contrasts in SWM, impervious cover and spatial patterns of land cover type. The primary difference in hydrograph response is observed on the recession limb of the hydrograph, and that change appears to be associated with higher storm-total runoff in the watersheds with more area draining to SWM. Our findings contribute more evidence to the work of previous authors suggesting that SWM is less effective at attenuating urban hydrographs than is commonly assumed. Our findings also are consistent with previous work concluding that percent impervious cover may have greater influence on runoff volume than percent SWM coverage.     

Hydrogeologic controls on streamflow sensitivity to climate variation

Climate models project warmer temperatures for the north‐west USA, which will result in reduced snowpacks and decreased summer streamflow. This paper examines how groundwater, snowmelt, and regional climate patterns control discharge at multiple time scales, using historical records from two watersheds with contrasting geological properties and drainage efficiencies. In the groundwater‐dominated watershed, aquifer storage and the associated slow summer recession are responsible for sustaining discharge even when the seasonal or annual water balance is negative, while in the runoff‐dominated watershed subsurface storage is exhausted every summer. There is a significant 1 year cross‐correlation between precipitation and discharge in the groundwater‐dominated watershed (r = 0·52), but climatic factors override geology in controlling the inter‐annual variability of streamflow. Warmer winters and earlier snowmelt over the past 60 years have shifted the hydrograph, resulting in summer recessions lasting 17 days longer, August discharges declining 15%, and autumn minimum discharges declining 11%. The slow recession of groundwater‐dominated streams makes them more sensitive than runoff‐dominated streams to changes in snowmelt amount and timing. Copyright © 2008 John Wiley & Sons, Ltd.     

HydRun: A MATLAB toolbox for rainfall–runoff analysis

Understanding the nature of streamflow response to precipitation inputs is at the core of hydrological applications and water resource management. Indices such as the base flow index, recession constant, and response lag of a watershed retain an important place in hydrology as metrics to compare watersheds and understand the impact of human activity, geology, geomorphology, soils, and climate on precipitation–runoff relations. Extracting characteristics of the hyetograph–hydrograph relationship is often done manually, which is time consuming and may result in subjective and potentially inconsistent outcomes. Here, we present a MATLAB‐based toolbox, called HydRun, for rapid and flexible rainfall–runoff analysis. HydRun uses a series of flexible routines to extract base flow from the hydrograph and then computes commonly used time instants of the rainfall–runoff relationship. HydRun provides users the flexibility to decide thresholds and limits of analysis, but objectively computes hydrometric indices. The toolkit includes a graphical user interface and example files. In this paper, we apply HydRun to 4 watersheds, 3 in Scotland and 1 in Canada, to demonstrate the software functions and highlight important decisions the user must make in its application.     

Hydrologic prediction for urban watersheds with the Distributed Hydrology–Soil–Vegetation Model

Some relatively straightforward modifications to the Distributed Hydrology–Soil–Vegetation Model (DHSVM) are described that allow it to represent urban hydrological processes. In the modified model, precipitation that falls on impervious surfaces becomes surface runoff, and a spatially varying (depending on land cover) fraction of surface runoff is connected directly to the stream channel, with the remainder stored and slowly released to represent the effects of stormwater detention. The model was evaluated through application to Springbrook Creek watershed in a partially urbanized area of King County, Washington. With calibration, the modified DHSVM simulates hourly streamflow from these urbanized catchments quite well. It is also shown how the revised model can be used to study the effects of continuing urbanization in the much larger Puget Sound basin. Model simulations confirm many previous studies in showing that urbanization increases peak flows and their frequency, and decreases peak flow lag times. The results show that the urbanization parameterizations for DHSVM facilitate use of the model for prediction and/or reconstruction of a range of historic and future changes in land cover that will accompany urbanization as well as other forms of vegetation change. Copyright © 2008 John Wiley & Sons, Ltd.     

Comparative streamflow characteristics in urbanizing basins in the Portland Metropolitan Area,Oregon, USA

This study investigates changes in streamflow characteristics for urbanizing watersheds in the Portland Metropolitan Area of Oregon for the period from 1951 to 2000. The objective of this study was to assess how mean annual runoff ratio, mean seasonal runoff ratio, annual peak runoff ratio, changes in streamflow in response to storm amount, the fraction of time that the daily mean flow exceeds the annual mean flow, 3‐day recession constants, and dry/wet flow ratio vary among watersheds with different degrees of urban development. There were no statistically significant changes in annual runoff ratio and annual peak runoff ratio for the mixed land‐use watershed (Tualatin River watershed) and the urban watershed (Johnson Creek watershed) during the entire study period. The Tualatin River watershed, where most of the urban development occurred in a lower part of the watershed, showed a statistically significant increase in annual peak runoff ratio during the 1976 and 2000 period. The Upper Tualatin River watershed illustrated a significant decrease in annual peak runoff ratio for the entire study period. With significant differences in seasonal runoff ratio, only Johnson Creek exhibited a significant increase in both wet and dry season runoff ratios. Streamflow during storm events declined rapidly in the urban watershed, with a high 3‐day recession constant. At an event storm scale, streamflow in Fanno Creek, which is the most urbanized watershed, responded quickly to precipitation input. The fraction of time that the daily mean flow exceeded the annual mean flow and dry/wet flow ratio are all lower in Johnson Creek. This suggests a shorter duration of storm runoff and lower baseflow in the urbanized watershed when compared to the mixed land use watershed. The findings of this study demonstrate the importance of spatial and temporal scale, climate variability, and basin physiographic characteristics in detecting the hydrologic effects of urbanization in the Pacific Northwest of the USA. Copyright © 2006 John Wiley & Sons, Ltd.     

Land cover effects on runoff patterns in eastern Piedmont (USA) watersheds

Physiography and land cover determine the hydrologic response of watersheds to climatic events. However, vast differences in climate regimes and variation of landscape attributes among watersheds (including size) have prevented the establishment of general relationships between land cover and runoff patterns across broad scales. This paper addresses these difficulties by using power spectral analysis to characterize area‐normalized runoff patterns and then compare these patterns with landscape features among watersheds within the same physiographic region. We assembled long‐term precipitation and runoff data for 87 watersheds (first to seventh order) within the eastern Piedmont (USA) that contained a wide variety of land cover types, collected environmental data for each watershed, and compared the datasets using a variety of statistical measures. The effect of land cover on runoff patterns was confirmed. Urban‐dominated watersheds were flashier and had less hydrologic memory compared with forest‐dominated watersheds, whereas watersheds with high wetland coverage had greater hydrologic memory. We also detected a 10–15% urban threshold above which urban coverage became the dominant control on runoff patterns. When spectral properties of runoff were compared across stream orders, a threshold after the third order was detected at which watershed processes became dominant over precipitation regime in determining runoff patterns. Finally, we present a matrix that characterizes the hydrologic signatures of rivers based on precipitation versus landscape effects and low‐frequency versus high‐frequency events. The concepts and methods presented can be generally applied to all river systems to characterize multiscale patterns of watershed runoff. Copyright © 2013 John Wiley & Sons, Ltd.     

Quantifying urbanization-associated changes in terrestrial hydrologic system memory

A common feature of watershed urbanization is increased hydrograph ‘flashiness,’ whereby river discharge fluctuations grow more erratic. Such changes might be intuitively interpreted as a decrease in watershed-scale hydrologic system memory. Here, I investigate this hypothesis through a paired-catchment experiment. The serial correlation coefficient, a common metric of short-term time series memory, is applied to daily winter streamflow data from urbanizing and rural watersheds in the Puget Sound lowland of Washington State, USA. Statistical comparisons confirm that this metric shows highly significant decreases over time in the catchment undergoing land use change, but not in the control watershed, which remains rural over the hydrometric record. Moreover, the mean serial correlation coefficients are statistically indistinguishable between the two catchments over the early period of record, when both watersheds are largely rural, whereas the system memory is far weaker in the urbanized stream relative to the rural stream over the late period, following land use change in the former. The results appear readily interpretable in terms of the physical hydrologic changes typically associated with urbanization. The serial correlation coefficient thus appears to be an instructive measure of urbanization impacts for small streams in this region.     

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

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

Estimating effective impervious area in urban watersheds using land cover,soil character and asymptotic curve number

Knowledge of the effective impervious area (EIA) or the degree to which impervious surfaces are hydraulically connected to the drainage system is useful for improving hydrological and environmental models and assessing the effectiveness of green stormwater infrastructure in urban watersheds. The goal of this research is to develop a method to estimate EIA fraction in urban watersheds using readily available data. Since EIA is dependent on rainfall–runoff response and cannot be solely determined based on the physical characteristics of a watershed, the EIA is linked with the asymptotic curve number (CN), a watershed index that represents runoff characteristics. In order for the method to be applicable to ungauged watersheds, the asymptotic CN is predicted using land cover and soil data from 35 urban catchments in Minnesota and Texas, USA. Similar data from 11 other urban catchments in Wisconsin and Texas, USA, are used to validate the results. A set of runoff depth versus EIA fraction curves is also developed to assess the impact of EIA reduction on discharge from an urban watershed in land-use planning studies.     

Effectiveness of landscape‐based green infrastructure for stormwater management in suburban catchments

Land cover changes associated with urbanization have negative effects on downstream ecosystems. Contemporary urban development attempts to mitigate these effects by designing stormwater infrastructure to mimic predevelopment hydrology, but their performance is highly variable. This study used in situ monitoring of recently built neighbourhoods to evaluate the catchment‐scale effectiveness of landscape decentralized stormwater control measures (SCMs) in the form of street connected vegetated swales for reducing runoff volumes and flow rates relative to curb‐and‐gutter infrastructure. Effectiveness of the SCMs was quantified by monitoring runoff for 8 months at the outlets of 4 suburban catchments (0.76–5.25 ha) in Maryland, USA. Three “grey” catchments installed curb‐and‐gutter stormwater conveyances, whereas the fourth “green” catchment built parcel‐level vegetated swales. The catchment with decentralized SCMs reduced runoff, runoff ratio, and peak runoff compared with the grey infrastructure catchments. In addition, the green catchment delayed runoff, resulting in longer precipitation–runoff lag times. Runoff ratios across the monitoring period were 0.13 at the green catchment and 0.37, 0.35, and 0.18 at the 3 grey catchments. Runoff only commenced after 6 mm of precipitation at the decentralized SCM catchment, whereas runoff occurred even during the smallest events at the grey catchments. However, as precipitation magnitudes reached 20 mm, the green catchment runoff characteristics were similar to those at the grey catchments, which made up 37% of the total precipitation in only 10 of 72 events. Therefore, volume‐based reduction goals for stormwater using decentralized SCMs such as vegetated swales require additional redundant SCMs in a treatment train as source control and/or end‐of‐pipe detention to capture a larger fraction of runoff and more effectively mimic predevelopment hydrology for the relatively rare but larger precipitation events.     

Storm‐event rainfall–runoff modelling approach for ungauged sites in Taiwan

This work develops a top‐down modelling approach for storm‐event rainfall–runoff model calibration at unmeasured sites in Taiwan. Twenty‐six storm events occurring in seven sub‐catchments in the Kao‐Ping River provided the analytical data set. Regional formulas for three important features of a streamflow hydrograph, i.e. time to peak, peak flow, and total runoff volume, were developed via the characteristics of storm event and catchment using multivariate regression analysis. Validation of the regional formulas demonstrates that they reasonably predict the three features of a streamflow hydrograph at ungauged sites. All of the sub‐catchments in the study area were then adopted as ungauged areas, and the three streamflow hydrograph features were calculated by the regional formulas and substituted into the fuzzy multi‐objective function for rainfall–runoff model calibration. Calibration results show that the proposed approach can effectively simulate the streamflow hydrographs at the ungauged sites. The simulated hydrographs more closely resemble observed hydrographs than hydrographs synthesized using the Soil Conservation Service (SCS) dimensionless unit hydrograph method, a conventional method for hydrograph estimation at ungauged sites in Taiwan. Copyright © 2008 John Wiley & Sons, Ltd.     

Comparing satellite derived precipitation datasets using the Hillslope River Routing (HRR) model in the Congo River Basin

In this paper, three satellite derived precipitation datasets (TRMM, CMORPH, PERSIANN) are used to drive the Hillslope River Routing (HRR) model in the Congo Basin. The precipitation data are compared spatially and temporally in two forms: (1) precipitation magnitudes, and (2) resulting streamflow and water storages. Simulated streamflow is assessed using historical monthly discharge data from in situ stream gauges and recent stage data based on water surface elevations derived from ENVISAT radar altimetry data. Simulated total water storage is assessed using monthly storage change values derived from GRACE data. The results show that the three precipitation datasets vary significantly in terms of magnitudes but generally produce a reasonable hydrograph throughout much of the basin, with the exception of the equatorial regions of the watershed. The satellite datasets provide unreasonably high values for specific periods (e.g. all three in Oct–Nov; only CMORPH and PERSIANN in Mar–Apr) in the equatorial regions. Overall, TRMM (3B42) provides the best spatial and temporal distributions and magnitudes or rainfall based on the assessment measures used here. Both CMORPH and PERSIANN tend to overestimate magnitudes, especially in the equatorial regions of the Basin. Copyright © 2011 John Wiley & Sons, Ltd.     

Is there a limit to bioretention effectiveness? Evaluation of stormwater bioretention treatment using a lumped urban ecohydrologic model and ecologically based design criteria

In this study, we developed the urban ecohydrology model (UEM) to investigate the role of bioretention on watershed water balance, runoff production, and streamflow variability. UEM partitions the land surface into pervious, impervious, and bioretention cell fractions. Soil moisture and vegetation dynamics are simulated in pervious areas and bioretention cells using a lumped ecohydrological approach. Bioretention cells receive runoff from a fraction of impervious areas. The model is calibrated in an urban headwater catchment near Seattle, WA, USA, using hourly weather data and streamflow observations for 3 years. The calibrated model is first used to investigate the relationship between streamflow variability and bioretention cell size that receives runoff from different values of impervious area in the watershed. Streamflow variability is quantified by 2 indices, high pulse count (HPC), which quantifies the number of flow high pulses in a water year above a threshold, and high pulse range (HPR), which defines the time over which the pulses occurred. Low values of these indices are associated with improved stream health. The effectiveness of the modelled bioretention facilities are measured by their influence on reducing HPC and HPR and on flow duration curves in comparison with modelled fully forested conditions. We used UEM to examine the effectiveness of bioretention cells under rainfall regimes that are wetter and drier than the study area in an effort to understand linkages between the degree of urbanization, climate, and design bioretention cell size to improve inferred stream health conditions. In all model simulations, limits to the reduction of HPC and HPR indicators were reached as the size of bioretention cells grew. Bioretention was more effective as the rainfall regime gets drier. Results may guide bioretention design practices and future studies to explore climate change impacts on bioretention design and management.     

Influences of climate,terrain and land cover on stream salinity in southeastern Australia,and implications for management through reforestation

Reforestation of cleared land has the potential to reduce groundwater recharge, salt mobilization and streamflow. Stream salinity change is the net result of changes in stream salt load and streamflow. The net effect of these changes varies spatially as a function of climate, terrain and land cover. Successful natural resource management requires methods to map the spatial variability of reforestation impacts. We investigated salinity data from 2000 bores and streamflow and salinity measurements from 27 catchments in the Goulburn–Broken region in southeast Australia to assess the main factors determining stream salinity and opportunities for management through reforestation. For groundwater systems of similar geology, relationships were found between average annual rainfall and groundwater salinity and between groundwater salinity and low‐flow salinity. Despite its simplicity, we found that the steady‐state component of a simple conceptual coupled water–salt mass balance model (BC2C) adequately explained the spatial variation in streamflow and salinity. The model results suggest the efficiency of afforestation to reduce stream salinity could be increased by more than an order of magnitude through spatial planning. However, appreciable reductions in stream salinity in large rivers through land cover change alone would still require reforestation on an unprecedented scale. Copyright © 2008 John Wiley & Sons, Ltd.     

Hydrograph characteristics of quick and slow runoffs of a watershed outlet,Taiwan

This study explores the hydrograph characteristics of quick and slow runoffs in watershed outlet hydrographs. The quick and slow runoffs were modelled using a conceptual model of three linear cascade reservoirs that have exponential decay response expressions. Mean rainfall for model inputs was estimated using the block Kriging method. The 107 storms during the 1966–2008 events were classified as large, medium and small events according to the observed streamflow. The optimal hydrograph parameters for 61 rainfall‐runoff events were calibrated using the shuffled complex evolution optimal algorithm. The efficacy of the model was verified using the seven averaged parameters of three types of 46 events and was compared with three evaluation criteria resulting from the Nash model. The 61 calibrations were used to analyse and compare the characteristics of quick and slow flows in early and later periods (1966–2002 and 2003–2008). Finally, the following five conclusions were obtained: (1) The base time of a slow runoff hydrograph is the same as that of a total runoff hydrograph. (2) A quick runoff with a long period occurs when soil antecedent moisture is low and with a short period under a high value. (3) The time to peak of hydrograph components is directly proportional to peak time of a hyetograph; the time to peak of quick and slow flows is about 0·97 and 1·12 times the peak time of a hyetograph, respectively. (4) The peak of hydrograph components is relative to a total runoff hydrograph; the percentages for quick runoff are approximately 71% and 13% for slow flow. Finally, (5) the volume of a quick runoff component is 49% of a total runoff volume and 37% for a slow runoff volume. Copyright © 2010 John Wiley & Sons, Ltd.     

Precipitation induced stream flow: An event based chemical and isotopic study of a small stream in the Great Plains region of the USA

A small stream in the Great Plains of USA was sampled to understand the streamflow components following intense precipitation and the influence of water storage structures in the drainage basin. Precipitation, stream, ponds, ground-water and soil moisture were sampled for determination of isotopic (D, ¹⁸O) and chemical (Cl, SO₄) composition before and after two intense rain events. Following the first storm event, flow at the downstream locations was generated primarily through shallow subsurface flow and runoff whereas in the headwaters region – where a pond is located in the stream channel – shallow ground-water and pond outflow contributed to the flow. The distinct isotopic signatures of precipitation and the evaporated pond water allowed separation of the event water from the other sources that contributed to the flow. Similarly, variations in the Cl and SO₄ concentrations helped identify the relative contributions of ground-water and soil moisture to the streamflow. The relationship between deuterium excess and Cl or SO₄ content reveals that the early contributions from a rain event to streamflow depend upon the antecedent climatic conditions and the position along the stream channel within the watershed. The design of this study, in which data from several locations within a watershed were collected, shows that in small streams changes in relative contributions from ground water and soil moisture complicate hydrograph separation, with surface-water bodies providing additional complexity. It also demonstrates the usefulness of combined chemical and isotopic methods in hydrologic investigations, especially the utility of the deuterium excess parameter in quantifying the relative contributions of various source components to the stream flow.     

Application and evaluation of a snowmelt runoff model in the Tamor River basin,Eastern Himalaya using a Markov Chain Monte Carlo (MCMC) data assimilation approach

Previous studies have drawn attention to substantial hydrological changes taking place in mountainous watersheds where hydrology is dominated by cryospheric processes. Modelling is an important tool for understanding these changes but is particularly challenging in mountainous terrain owing to scarcity of ground observations and uncertainty of model parameters across space and time. This study utilizes a Markov Chain Monte Carlo data assimilation approach to examine and evaluate the performance of a conceptual, degree‐day snowmelt runoff model applied in the Tamor River basin in the eastern Nepalese Himalaya. The snowmelt runoff model is calibrated using daily streamflow from 2002 to 2006 with fairly high accuracy (average Nash–Sutcliffe metric ~0.84, annual volume bias < 3%). The Markov Chain Monte Carlo approach constrains the parameters to which the model is most sensitive (e.g. lapse rate and recession coefficient) and maximizes model fit and performance. Model simulated streamflow using an interpolated precipitation data set decreases the fractional contribution from rainfall compared with simulations using observed station precipitation. The average snowmelt contribution to total runoff in the Tamor River basin for the 2002–2006 period is estimated to be 29.7 ± 2.9% (which includes 4.2 ± 0.9% from snowfall that promptly melts), whereas 70.3 ± 2.6% is attributed to contributions from rainfall. On average, the elevation zone in the 4000–5500 m range contributes the most to basin runoff, averaging 56.9 ± 3.6% of all snowmelt input and 28.9 ± 1.1% of all rainfall input to runoff. Model simulated streamflow using an interpolated precipitation data set decreases the fractional contribution from rainfall versus snowmelt compared with simulations using observed station precipitation. Model experiments indicate that the hydrograph itself does not constrain estimates of snowmelt versus rainfall contributions to total outflow but that this derives from the degree‐day melting model. Lastly, we demonstrate that the data assimilation approach is useful for quantifying and reducing uncertainty related to model parameters and thus provides uncertainty bounds on snowmelt and rainfall contributions in such mountainous watersheds. Copyright © 2013 John Wiley & Sons, Ltd.