Continuous Hydrologic Simulation Research Articles

Global resilience analysis of combined sewer systems under continuous hydrologic simulation

Managing and reducing combined sewer overflow (CSO) discharges is crucial for enhancing the resilience of combined sewer systems (CSS). However, the absence of a standardised resilience analysis approach poses challenges in developing effective discharge reduction strategies. To address this, our study presents a top-down method that expands the existing Global Resilience Analysis to quantify resilience performance in CSS. This approach establishes a link between threats (e.g., rainfall) and impacts (e.g., CSOs) through continuous and long-term simulation, accommodating various rainfall patterns, including extreme events. We assess CSO discharge impacts from a resilience perspective by introducing eight new metrics. We conducted a case study in Fehraltorf, Switzerland, analysing the performance of three green infrastructure (GI) types (bioretention cells, green roofs, and permeable pavements) over 38 years. The results demonstrated that GI enhanced all resilience indices, with variations observed in individual CSO performance metrics and their system locations. Notably, in Fehraltorf, green roofs emerged as the most effective GI type for improving resilience, while the downstream outfall displayed the highest resilience enhancement. Overall, our proposed method enables a shift from event-based to continuous simulation analysis, providing a standardised approach for resilience assessment. This approach informs the development of strategies for CSO discharge reduction and the enhancement of CSS resilience.

Nonstationary Annual Maximum Flood Frequency Analysis Using a Conceptual Hydrologic Model with Time-Varying Parameters

Recent evidence of the impact of watershed underlying conditions on hydrological processes have made the assumption of stationarity widely questioned. In this study, the temporal variations of frequency distributions of the annual maximum flood were investigated by continuous hydrological simulation considering nonstationarity for Weihe River Basin (WRB) in northwestern China. To this end, two nonstationary versions of the GR4J model were introduced, where the production storage capacity parameter was regarded as a function of time and watershed conditions (e.g., reservoir storage and soil-water conservation land area), respectively. Then the models were used to generate long-term runoff series to derive flood frequency distributions, with synthetic rainfall series generated by a stochastic rainfall model as input. The results show a better performance of the nonstationary GR4J model in runoff simulation than the stationary version, especially for the annual maximum flow series, with the corresponding NSE metric increasing from 0.721 to 0.808. The application of the nonstationary flood frequency analysis indicates the presence of significant nonstationarity in the flood quantiles and magnitudes, where the flood quantiles for an annual exceedance probability of 0.01 range from 4187 m3/s to 8335 m3/s for the past decades. This study can serve as a reference for flood risk management in WRB and possibly for other basins undergoing drastic changes caused by intense human activities.

Calibration of a continuous hydrologic simulation model in the urban Gowrie Creek catchment in Toowoomba, Australia

Study regionToowoomba, Queensland, Australia Study focusIn this study we derive loss model parameters suitable for use in the dynamic loss Australian Representative Basin Model (ARBM) through the calibration of a continuous simulation hydrologic model. We compare the derived parameters to those published in the literature, and our results highlight the need to develop a database of calibrated loss parameters for urban catchments. New hydrological insightsThe development of design storms for flood modelling commonly uses the initial loss/continuous loss model to estimate the conversion of rainfall to runoff. This loss model, when applied to pervious areas, uses parameters that have been calibrated for gauged rural catchments. These same parameters are often applied to the pervious component of ungauged urban catchments with minimal understanding of the resulting impact on runoff. This research uses a continuous simulation modelling approach to calibrate parameters suitable for use in the ARBM loss model built into the hydrological modelling software XPRAFTS. Through a two-stage calibration approach, the model offered a satisfactory fit (Nash Sutcliffe Efficiency > 0.5) for 9 of the 11 selected storm events, with seven events exceeding a Nash Sutcliffe Efficiency of 0.75. Events used in the calibration/validation included peak flows as low as 9 m3/s and as high as 600 m3/s. Developing these loss model parameters offers new insights into the suitability of a dynamic loss model approach in an urban catchment in regional Australia and provides an alternative to the parameters already available in the literature which were found to overestimate the peak flow in frequent events.

Quantification of Expected Changes in Peak Flow Quantiles in Climate Change by Combining Continuous Hydrological Modelling with the Modified Curve Number Method

Climate projections point to modifications in the magnitude, frequency and timing of floods in the future. However, robust methodologies to quantify how climate change will modify the catchment response in flood events are required. Continuous hydrological modelling usually smooth magnitudes of extreme events. This paper proposes a methodology to improve the assessment of flood changes in the future driven by climate change. Climate change projections of the EURO-CORDEX programme obtained under the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) supplied are used. Four catchments located on the Douro River Basin have been considered as case studies. Precipitation and temperature projections have been bias corrected to reduce errors with observations in the control period (1971–2004). The HBV continuous hydrological simulation model has been used to simulate the soil moisture content on the day of occurrence of the maximum annual rainfalls in the four catchments. The modified curve number method has been utilized to obtain the changes expected in the future in flood magnitudes, considering the initial soil moisture contents estimated with the HBV model and the expected changes in annual maximum rainfalls. The methodology has been applied to the control period (1971–2004) to check the validity of the process. Then, the methodology has been applied to the future period (2011–2095), to quantify the changes expected in the future in flood magnitudes under climate change conditions. The combined use of the HBV continuous hydrological simulation with the modified curve number method improves the results provided by the HBV model. The proposed methodology allows a better characterization of the response of catchments in flood events. It also considers the expected variation in the antecedent moisture content in catchments in the future, driven by increasing temperatures and decreasing mean annual precipitations in the future. The results show that flood quantiles will increase in three of the four catchments considered.

Integrating stormwater management and stream restoration strategies for greater water quality benefits.

Urbanization alters the delivery of water and sediment to receiving streams, often leading to channel erosion and enlargement, which increases loading of sediment and nutrients, degrades habitat, and harms sensitive biota. Stormwater control measures (SCMs) are constructed in an attempt to mitigate some of these effects. In addition, stream restoration practices such as bank stabilization are increasingly promoted as a means of improving water quality by reducing downstream sediment and pollutant loading. Each unique combination of SCMs and stream restoration practices results in a novel hydrologic regime and set of geomorphic characteristics that interact to determine stream condition, but in practice, implementation is rarely coordinated due to funding and other constraints. In this study, we examine links between watershed-scale implementation of SCMs and stream restoration in Big Dry Creek, a suburban watershed in the Front Range of northern Colorado. We combine continuous hydrologic model simulations of watershed-scale response to SCM design scenarios with channel evolution modeling to examine interactions between stormwater management and stream restoration strategies for reducing loading of sediment and adsorbed phosphorus from channel erosion. Modeling results indicate that integrated design of SCMs and stream restoration interventions can result in synergistic reductions in pollutant loading. Not only do piecemeal and disunited approaches to stormwater management and stream restoration miss these synergistic benefits, they make restoration projects more prone to failure, wasting valuable resources for pollutant reduction. We conclude with a set of recommendations for integrated planning of SCMs and stream restoration to simultaneously achieve water quality and channel protection goals.

Process-based flood frequency analysis in an agricultural watershed exhibiting nonstationary flood seasonality

Abstract. Floods are the product of complex interactions among processes including precipitation, soil moisture, and watershed morphology. Conventional flood frequency analysis (FFA) methods such as design storms and discharge-based statistical methods offer few insights into these process interactions and how they “shape” the probability distributions of floods. Understanding and projecting flood frequency in conditions of nonstationary hydroclimate and land use require deeper understanding of these processes, some or all of which may be changing in ways that will be undersampled in observational records. This study presents an alternative “process-based” FFA approach that uses stochastic storm transposition to generate large numbers of realistic rainstorm “scenarios” based on relatively short rainfall remote sensing records. Long-term continuous hydrologic model simulations are used to derive seasonally varying distributions of watershed antecedent conditions. We couple rainstorm scenarios with seasonally appropriate antecedent conditions to simulate flood frequency. The methodology is applied to the 4002 km2 Turkey River watershed in the Midwestern United States, which is undergoing significant climatic and hydrologic change. We show that, using only 15 years of rainfall records, our methodology can produce accurate estimates of “present-day” flood frequency. We found that shifts in the seasonality of soil moisture, snow, and extreme rainfall in the Turkey River exert important controls on flood frequency. We also demonstrate that process-based techniques may be prone to errors due to inadequate representation of specific seasonal processes within hydrologic models. If such mistakes are avoided, however, process-based approaches can provide a useful pathway toward understanding current and future flood frequency in nonstationary conditions and thus be valuable for supplementing existing FFA practices.

Estimating design hydrographs at the basin scale: from event-based to continuous hydrological simulation

ABSTRACT Design hydrographs are widely used in practical hydrologic engineering problems. Typical applications adopt event-based (EBM) methods, using rainfall-runoff models to convert design hyetographs into design hydrographs. Uncertainties include the definition of antecedent conditions and the assumption of equivalence between hyetograph and hydrograph return periods. An alternative is to use continuous simulation (CSM) methods, by forcing a rainfall-runoff model with long precipitation series, and directly analyzing the output discharges. To better understand uncertainties in the EBM method and differences between CSM and EBM ones, we applied a hydrological model in the Itajaí-Açu river basin to compare a CSM method with 730 different simulations of an EBM one, considering different basin antecedent conditions and design hyetographs (10- and 50-years). Results indicated that the EBM method leads to a large range of design discharges depending on the antecedent condition. CS-based 10- and 50-years maximum discharges corresponded to percentiles between 30% and 50% of the EBM estimates. Higher discharge variation occurred in sub-basins with larger maximum soil water storage. Our conclusions agree with the literature, which points towards CSM-based methods to estimate design discharges.

A new method for establishing hydrologic fidelity of snow depth measurements based on snowmelt–runoff hydrographs

ABSTRACTFor snowmelt-driven flood studies, snow water equivalent (SWE) is frequently estimated using snow depth data. Accurate measurements of snow depth are important in providing data for continuous hydrologic simulations of such watersheds. A new hydrologic fidelity metric is proposed in this study to evaluate the potential contribution of particular snow depth datasets to flow characteristics using observed data and hydrologic modeling using the Variable Infiltration Capacity (VIC) model. Data-based hydrologic fidelity of snow depth measurements is defined as a categorical skill score between the snow depth in the watershed and the hydrograph peak or volume at the watershed outlet. Similarly, model-based hydrologic fidelity is defined as a categorical skill score between the model-simulated snow depth and the model-simulated hydrograph peak or volume. The proposed framework is illustrated using the Pecatonica River watershed in the USA, indicating which sites have a higher hydrologic fidelity, which is preferred in hydrologic studies.

Estimate of Continuous Hydrographs and Sediment Graphs in Nestos River Basin

The objective of this study is to present continuous hydrologic simulations, as well as continuous simulations of soil and streambed erosion processes in the mountainous part of Nestos River basin (Macedonia-Thrace border, northeastern Greece). The characteristic of the basin is the presence of a dam at its upper boundary and two irrigation canals very close to the outlet of the basin. For attaining the above, a composite mathematical model, consisting of three submodels, is applied: a rainfall-runoff submodel, a soil erosion submodel and a sediment transport submodel for streams. The application of the latter mathematical model results in continuous hydrographs and sediment graphs at the basin outlet. The computed stream discharge and sediment discharge values are compared with field measurements. Statistic efficiency criteria provide satisfactory results. Therefore, it is concluded that the continuous hydromorphologic modeling can be successfully applied to Nestos River basin.

Performance and Cost-Based Comparison of Green and Gray Infrastructure to Control Combined Sewer Overflows

Rainwater harvesting (RWH) is being used more often today as a water supply and stormwater management green infrastructure (GI). In recent years, GIs in urban water engineering have gained attention due to their lower lifecycle costs—in both implementation and operation phases—rather than traditional gray approaches. The research described in the present paper compared implementation of RWH systems to gray approaches previously designed as a part of the long-term control plan (LTCP) for combined sewer overflow (CSO) control in Toledo, Ohio. RWH scenarios in this study were defined based on different system capacities and release policies, and then combined gray and green scenarios were analyzed according to their hydrologic performance and cost. This study employed long-term continuous hydrologic and hydraulic (H&H) simulations as well as lifecycle cost (LCC) analysis techniques. The results showed that greening the LTCP via RWH could improve the lifecycle cost-effectiveness by 48%. The captured rainwater was considered to supply toilet flushing water demand in buildings.

A Modular Approach to Optimizing Green Infrastructure Design through Unit Processes and Continuous Hydrologic Simulation

A Modular Approach to Optimizing Green Infrastructure Design through Unit Processes and Continuous Hydrologic Simulation

Improvement of the Variable Storage Coefficient Method with Water Surface Gradient as a Variable

<abstract> <bold><sc>Abstract.</sc></bold> The variable storage coefficient (VSC) method has been used for streamflow routing in continuous hydrological simulation models such as the Agricultural Policy/Environmental eXtender (APEX) and the Soil Water Assessment Tool (SWAT) for more than 30 years. APEX operates on a daily time step and offer options for simulating processes on shorter time steps (e.g., hourly). However, APEX is not adequate for applications such as designing flood control structures or estimating flood damages because of a fundamental assumption in the VSC method: the normal flow condition. The storage with variable slope (SVS) method and an enhanced variable storage coefficient (VSCe) method are proposed as new routing methods for continuous simulation models that will improve flow routing and water quality simulation at subdaily time scales. This study describes the principle of the SVS method and the VSCe method and their performances against HEC-RAS unsteady flow results for various hydraulic and geometric conditions. Results show that the peak flow and the time to peak flow improved by up to 20% with SVS and VSCe on mild slopes (less than 0.0005 m m^-1) and small time steps of less than 1 h when compared to the conventional VSC method, although the difference narrowed as the channel slope and time interval increased. A case study on a small agricultural watershed in Texas indicates that both VSCe and VSC are reliable in watershed applications, but the improvement in streamflow prediction can be marginal in watersheds with steep slopes.

Cost Efficiency of Low Impact Development (LID) Stormwater Management Practices

Stormwater management has focused increasingly on Low Impact Development (LID) techniques in recent years. Although their effectiveness has been demonstrated in a number of cases, methodologies for the selection of most appropriate solutions for individual sites are still evolving. The cost efficiency of implementing a wide range of LID techniques in a proposed land development in the City of London, Ontario, Canada was investigated using continuous hydrologic simulation and a recently developed LID costing tool. The results indicate that infiltration trench and infiltration trench in combination with green roof were the most cost efficient solutions for runoff reduction.

Hydrologic modeling analysis of a passive, residential rainwater harvesting program in an urbanized, semi-arid watershed

Summary This paper presents the results of a long-term, continuous hydrologic simulation analysis of a watershed-scale residential rainwater harvesting (RWH) program in the Chollas Creek watershed, San Diego, California, USA. The U.S. Environmental Protection Agency’s Storm Water Management Model (SWMM) simulated rainfall-runoff responses for variations in a RWH network, including the RWH unit storage size, the number of implementing households, the amount of time before a unit is allowed to release captured runoff (i.e. drain delay), and the time it takes for the unit to drain (i.e. coefficient of discharge). Comparison of results found reductions to increase linearly with capacity and implementation. Maximum long-term watershed volumetric reductions between 10.1% and 12.4% were observed for the period of analysis (1948–2011) with a range of RWH storage sizes (227 L barrels to 7571 L cisterns). The ratio of overflow to underdrain flow, ranging from 5.17 to 0.014 (227–7571 L), exhibits the ability of cisterns to fully capture the majority of annual and long-term events. Sensitivity analysis found regional precipitation characteristics and disconnection of rooftop runoff to impact long-term watershed reduction potential more so than available RWH capacity. Drain delay control and dry duration time increased reduction variability with cisterns, though long-term reductions were not significantly impacted. Normalization of net present value (NPV) to volumetric reductions yielded a RWH unit cost of $0.20–$1.71 per 1000 L of watershed runoff reduced on average per year. Minor variations in cost based on the extent of watershed implementation highlights the potential to incrementally institute RWH programs. For the case study location, the 227-L rain barrel provided the greatest cost-effectiveness, reducing an average 6500 L of runoff per dollar invested for the analysis period.

The SCHADEX method: A semi-continuous rainfall–runoff simulation for extreme flood estimation

Summary This paper presents the SCHADEX probabilistic method for extreme flood estimation, developed and applied since 2006 at Electricite de France (EDF) for dam spillway design. SCHADEX is based on a semi-continuous rainfall–runoff simulation process: a continuous hydrological simulation provides an exhaustive description of the possible hydrological states of the catchment, while floods are generated on an event basis. The method has been built around two models: the Multi-Exponential Weather Pattern (MEWP) distribution for rainfall probability estimation, and the MORDOR hydrological model. The simulation process allows the production of an estimated distribution of flood volumes at the study’s time step, as well as a distribution of flood peaks based on a peak-to-volume ratio computed from observed hydrographs of significant floods. A wide range of rainy events are simulated on each hydrological state, generating an exhaustive set of crossings between precipitation and soil saturation hazards. Consequently, SCHADEX stands clearly apart from the “ N -years flood is generated by a N -years rainfall” paradigm, as a great diversity of hydrological scenarios can generate a flood of a given return period, with many variables taken into account. Since its development, SCHADEX has been widely applied in France for industrial studies, to catchments from several to several thousand square kilometres. This has allowed the testing and improvement of the method. In this paper, the basic concepts and hypothesis of the method are given in detail and illustrated throughout with the example of the River Tarn at Millau (2170 km 2 , South of France). The different steps of the method are successively exposed, up to the estimated distribution of flood daily discharges and flood peaks and their comparison with observed values. Lastly, some methodological perspectives to enhance the method are presented.

Copula-based interpretation of continuous rainfall–runoff simulations of a watershed in northern Iran

In the present work, the joint response of key hydrologic variables, including total precipitation depths and the corresponding simulated peak discharges, are investigated for different antecedent soil moisture conditions using the copula method. The procedure started with the calibration and validation of the soil moisture accounting (SMA) loss rate algorithm incorporated in the Hydrologic Engineering Center – hydrologic modeling system (HEC–HMS) model for the study watershed. A 1000 year long time series of hourly rainfall was then generated by the Neyman–Scott rectangular pulses (NSRP) rainfall generator, which was then transformed into the runoff rate by the HEC–HMS model. This long-term continuous hydrological simulation resulted in characterizing the response of the watershed for various input conditions such as initial soil moisture content (AMC), total rainfall depth, and rainfall duration. For each initial soil moisture class, the copula method was employed to determine the joint probability distribution of rainfall depth and peak discharge. For instance, for dry AMC condition and 1 h rainfall duration, the Joe family fitted best to the data, compared with six other one-parameter families of copulas. Results showed that the bivariate analysis of rainfall–runoff using the copula method can well characterize the watershed hydrological behavior. The derived offline curves could provide a probabilistic real-time peak discharge forecast.

Evolution of the SCS Runoff Curve Number Method and Its Application to Continuous Runoff Simulation

AbstractThe Natural Resources Conservation Service (NRCS) [previously Soil Conservation Service (SCS)] developed the SCS runoff curve-number (CN) method for estimating direct runoff from storm rainfall. The NRCS uses the CN method for designing structures and for evaluating their effectiveness. Structural design is usually based on a single event of a certain probability of occurrence. During the years when many floodwater-retarding watershed projects were planned and constructed (1950–1980), the CN equation was used in a continuous mode to evaluate the projects. To operate CN in a continuous mode, runoff was estimated from a daily rainfall record of approximately 30 years. For each day of recorded rainfall, the five-day antecedent rainfall was used to assign a CN1 (dry condition), CN2 (average condition), or CN3 (wet condition), and runoff was estimated with the appropriate CN. With the development of continuous hydrologic simulation models, CN was related directly to soil water content or estimated usin...

Linking Hydrologic Alteration to Biological Impairment in Urbanizing Streams of the Puget Lowland, Washington, USA1

We used a retrospective approach to identify hydrologic metrics with the greatest potential for ecological relevance for use as resource management tools (i.e., hydrologic indicators) in rapidly urbanizing basins of the Puget Lowland. We proposed four criteria for identifying useful hydrologic indicators: (1) sensitive to urbanization consistent with expected hydrologic response, (2) demonstrate statistically significant trends in urbanizing basins (and not in undeveloped basins), (3) be correlated with measures of biological response to urbanization, and (4) be relatively insensitive to potentially confounding variables like basin area. Data utilized in the analysis included gauged flow and benthic macroinvertebrate data collected at 16 locations in 11 King County stream basins. Fifteen hydrologic metrics were calculated from daily average flow data and the Pacific Northwest Benthic Index of Biological Integrity (B-IBI) was used to represent the gradient of response of stream macroinvertebrates to urbanization. Urbanization was represented by percent Total Impervious Area (%TIA) and percent urban land cover (%Urban). We found eight hydrologic metrics that were significantly correlated with B-IBI scores (Low Pulse Count and Duration; High Pulse Count, Duration, and Range; Flow Reversals, TQmean, and R-B Index). Although there appeared to be a great deal of redundancy among these metrics with respect to their response to urbanization, only two of the metrics tested – High Pulse Count and High Pulse Range – best met all four criteria we established for selecting hydrologic indicators. The increase in these high pulse metrics with respect to urbanization is the result of an increase in winter high pulses and the occurrence of high pulse events during summer (increasing the frequency and range of high pulses), when practically none would have occurred prior to development. We performed an initial evaluation of the usefulness of our hydrologic indicators by calculating and comparing hydrologic metrics derived from continuous hydrologic simulations of selected basin management alternatives for Miller Creek, one of the most highly urbanized basins used in our study. We found that the preferred basin management alternative appeared to be effective in restoring some flow metrics close to simulated fully forested conditions (e.g., TQmean), but less effective in restoring other metrics such as High Pulse Count and Range. If future research continues to support our hypothesis that the flow regime, particularly High Pulse Count and Range, is an important control of biotic integrity in Puget Lowland streams, it would have significant implications for stormwater management.

Evaluation of Statistical Rainfall Disaggregation Methods Using Rain-Gauge Information for West-Central Florida

Rainfall disaggregation in time can be useful for the simulation of hydrologic systems and the prediction of floods and flash floods. Disaggregation of rainfall to timescales less than 1h can be especially useful for small urbanized watershed study, and for continuous hydrologic simulations and when Hortonian or saturation-excess runoff dominates. However, the majority of rain gauges in any region record rainfall in daily time steps or, very often, hourly records have extensive missing data. Also, the convective nature of the rainfall can result in significant differences in the measured rainfall at nearby gauges. This study evaluates several statistical approaches for rainfall disaggregation which may be applicable using data from West-Central Florida, specifically from 1h observations to 15min records, and proposes new methodologies that have the potential to outperform existing approaches. Four approaches are examined. The first approach is an existing direct scaling method that utilizes observed 15min rainfall at secondary rain gauges, to disaggregate observed 1h rainfall at more numerous primary rain gauges. The second approach is an extension of an existing method for continuous rainfall disaggregation through statistical distributional assumptions. The third approach relies on artificial neural networks for the disaggregation process without sorting and the fourth approach extends the neural network methods through statistical preprocessing via new sorting and desorting schemes. The applicability and performance of these methods were evaluated using information from a fairly dense rain gauge network in West-Central Florida. Of the four methods compared, the sorted neural networks and the direct scaling method predicted peak rainfall magnitudes significantly better than the remaining techniques. The study also suggests that desorting algorithms would also be useful to randomly replace the artificial hyetograph within a rainfall period.

Development of a continuous soil moisture accounting procedure for curve number methodology and its behaviour with different evapotranspiration methods

AbstractThe curve number (CN) method is widely used for rainfall–runoff modelling in continuous hydrologic simulation models. A sound continuous soil moisture accounting procedure is necessary for models using the CN method. For shallow soils and soils with low storage, the existing methods have limitations in their ability to reproduce the observed runoff. Therefore, a simple one‐parameter model based on the Soil Conservation Society CN procedure is developed for use in continuous hydrologic simulation. The sensitivity of the model parameter to runoff predictions was also analysed. In addition, the behaviour of the procedure developed and the existing continuous soil moisture accounting procedure used in hydrologic models, in combination with Penman–Monteith and Hargreaves evapotranspiration (ET) methods was also analysed. The new CN methodology, its behaviour and the sensitivity of the depletion coefficient (model parameter) were tested in four United States Geological Survey defined eight‐digit watersheds in different water resources regions of the USA using the SWAT model. In addition to easy parameterization for calibration, the one‐parameter model developed performed adequately in predicting runoff. When tested for shallow soils, the parameter is found to be very sensitive to surface runoff and subsurface flow and less sensitive to ET. Copyright © 2007 John Wiley &amp; Sons, Ltd.