😅Hydrological Modeling Unit 8 – Streamflow Routing and Hydrographs

Key Concepts and Definitions

• Streamflow routing predicts the timing and magnitude of flow at a downstream point based on upstream flow data
• Hydrographs graphically represent the rate of flow (discharge) versus time past a specific point in a river
• Baseflow represents the sustained flow in a stream channel derived from natural storage sources (groundwater, lakes, wetlands)
• Direct runoff is the portion of streamflow directly attributed to a specific rainfall or snowmelt event
• Time of concentration ($T_c$) time required for water to flow from the most hydraulically distant point to the watershed outlet
  • Influenced by factors such as watershed shape, size, slope, and surface roughness
• Unit hydrograph (UH) direct runoff hydrograph resulting from one unit (e.g., 1 cm or 1 inch) of effective rainfall uniformly distributed over a watershed for a specified duration
• Effective precipitation portion of rainfall that contributes to direct runoff after accounting for losses (infiltration, evaporation, interception)

Streamflow Basics

• Streamflow, or discharge ($Q$), volume of water flowing through a channel cross-section per unit time (usually expressed in cubic meters per second or cubic feet per second)
• Factors influencing streamflow include precipitation, watershed characteristics, and antecedent moisture conditions
• Streamflow measurement techniques involve direct methods (current meters, acoustic Doppler velocimeters) and indirect methods (weirs, flumes, rating curves)
• Stage-discharge relationship (rating curve) empirical relationship between water level (stage) and discharge at a specific stream cross-section
  • Developed by measuring discharge at various stages and fitting a curve to the data points
• Hydrologic processes contributing to streamflow generation include surface runoff, interflow, and baseflow
• Streamflow variability influenced by factors such as climate, land use, and watershed management practices
• Flow duration curve (FDC) graphical representation of the percentage of time a specific streamflow is equaled or exceeded

Types of Hydrographs

• Storm hydrograph represents the streamflow response to a single rainfall event, typically showing a rapid rise and gradual recession
• Seasonal hydrograph depicts the variation in streamflow over a year, influenced by factors such as snowmelt, monsoons, and evapotranspiration
• Annual hydrograph shows the streamflow patterns over multiple years, useful for identifying long-term trends and variability
• Flood hydrograph focuses on high-flow events, essential for flood risk assessment and management
• Synthetic hydrograph generated using mathematical models and statistical techniques when observed data is unavailable
  • Commonly used synthetic hydrograph methods include the Snyder's method, SCS (Soil Conservation Service) method, and Clark's method
• Dimensionless hydrograph obtained by dividing the discharge ordinates by the peak discharge and the time ordinates by the time to peak, allowing for comparison between different watersheds

Hydrograph Components and Analysis

• Rising limb steep increase in discharge from the onset of direct runoff to the peak flow
• Peak flow ($Q_p$) maximum discharge during a hydrograph, influenced by factors such as rainfall intensity, duration, and watershed characteristics
• Recession limb gradual decrease in discharge after the peak flow, representing the depletion of water stored in the watershed
• Time to peak ($T_p$) time from the beginning of direct runoff to the peak flow
• Lag time ($T_{lag}$) time difference between the center of mass of effective rainfall and the peak flow
• Baseflow separation techniques (fixed interval, sliding interval, local minimum) used to estimate the contribution of baseflow to total streamflow
• Hydrograph separation methods (straight-line, concave, chemical mass balance) used to separate direct runoff from baseflow
  • Straight-line method assumes a constant baseflow rate during the storm event
  • Concave method considers the gradual depletion of baseflow during the recession limb

Streamflow Routing Methods

• Lumped routing methods (Muskingum, level pool routing) consider the watershed as a single unit and use simplified equations to route flow
  • Muskingum method uses a linear relationship between storage, inflow, and outflow in a river reach
• Distributed routing methods (kinematic wave, dynamic wave) divide the watershed into smaller elements and solve complex equations to route flow
  • Kinematic wave method assumes that the flow is dominated by gravity and neglects pressure and inertial forces
• Hydraulic routing methods (Saint-Venant equations, Manning's equation) based on the principles of conservation of mass and momentum in open-channel flow
• Hydrologic routing methods (unit hydrograph, time-area method) based on the concept of a linear system response to an input (effective precipitation)
  • Unit hydrograph theory assumes that the direct runoff response is linearly proportional to the effective precipitation input
• Muskingum-Cunge method combines the Muskingum routing approach with the kinematic wave approximation, allowing for variable parameters along the river reach
• Reservoir routing methods (modified Puls, level pool) used to predict the outflow hydrograph from a reservoir given the inflow hydrograph and storage-discharge relationship

Modeling Tools and Software

• HEC-HMS (Hydrologic Engineering Center - Hydrologic Modeling System) widely used software for simulating precipitation-runoff processes and streamflow routing
  • Offers various methods for loss estimation, transform, baseflow separation, and routing
• SWMM (Storm Water Management Model) dynamic rainfall-runoff simulation model used for single event or long-term simulations of urban hydrology and hydraulics
• MIKE HYDRO River comprehensive river modeling package for simulating flow, sediment transport, and water quality in rivers and floodplains
• HEC-RAS (Hydrologic Engineering Center - River Analysis System) software for performing one-dimensional and two-dimensional hydraulic calculations for natural and constructed channels
• Python libraries (NumPy, SciPy, Pandas) offer powerful tools for data manipulation, analysis, and visualization in hydrological modeling
• R packages (hydromad, topmodel, EcoHydRology) provide functions and tools for hydrological modeling, statistical analysis, and graphical representation
• GIS (Geographic Information Systems) software (ArcGIS, QGIS) used for spatial data processing, watershed delineation, and parameter estimation in hydrological modeling

Practical Applications

• Flood forecasting and warning systems rely on streamflow routing models to predict the timing and magnitude of flood peaks downstream
• Water resource management decisions (reservoir operations, irrigation scheduling, hydropower generation) informed by streamflow routing simulations
• Environmental impact assessments (EIAs) use streamflow routing models to evaluate the potential effects of land use changes, dam construction, or climate change on river flow regimes
• Sediment transport modeling coupled with streamflow routing to predict the movement and deposition of sediment in rivers and reservoirs
• Water quality modeling applications use streamflow routing to simulate the transport and fate of pollutants in river systems
• Ecological flow assessments employ streamflow routing models to determine the flow requirements for maintaining aquatic ecosystem health and biodiversity
• Watershed restoration projects use streamflow routing to design and evaluate the effectiveness of interventions such as riparian buffers, wetland construction, or channel modifications

Challenges and Limitations

• Data availability and quality issues (missing data, measurement errors, spatial and temporal resolution) can affect the accuracy and reliability of streamflow routing models
• Parameter estimation uncertainty due to the complexity of hydrological processes and the variability of watershed characteristics
  • Calibration and validation techniques (manual, automatic) used to estimate model parameters and assess model performance
• Equifinality multiple parameter sets may produce similar model outputs, leading to uncertainty in model predictions
• Scaling issues arise when applying models developed at one spatial or temporal scale to another (e.g., from small watersheds to large river basins)
• Model structure uncertainty related to the simplification of complex hydrological processes and the choice of appropriate model components
• Computational constraints can limit the spatial and temporal resolution of streamflow routing models, especially for large-scale applications
• Climate change and land use change impacts on hydrological processes may not be adequately captured by models calibrated using historical data
  • Scenario analysis and uncertainty assessment techniques used to explore the range of possible future conditions and their effects on streamflow