HEC-HMS Runoff Computation

Modeling Direct Runoff with HEC-HMS Empirical models - traditional UH models - a causal linkage between runoff and excess precipitation without detailed consideration of the internal processes A conceptual model - kinematic-wave model of overland flow - represent possible physical mechanism

User-specified Unit Hydrograph Basic Concepts and Equations Qn=storm hydrograph ordinate Pm=rainfall excess depth Un-m+1=UH ordinate

User-specified Unit Hydrograph Estimating the Model Parameters 1. Collect data for an appropriate observed storm runoff hydrograph and the causal precipitation 2. Estimate losses and subtract these from precipitation. Estimate baseflow and separate this from the runoff

User-specified Unit Hydrograph Estimating the Model Parameters 3. Calculate the total volume of direct runoff and convert this to equivalent uniform depth over the watershed 4. Divide the direct runoff ordinates by the equivalent uniform depth

User-specified Unit Hydrograph Application of User-specified UH - In practice, direct runoff computation with a specified-UH is uncommon. - The data are seldom available. - It is difficult to apply.

Snyder’s UH Model Basic Concepts and Equations

Snyder’s UH Model Basic Concepts and Equations - standard UH - If the duration of the desired UH for the watershed of interest is significantly different from the above equation, tR=duration of desired UH, tpR=lag of desired UH

Snyder’s UH Model Basic Concepts and Equations - standard UH - for other duration Up=peak of standard UH, A=watershed drainage area Cp=UH peaking coefficient,C=conversion constant(2.75 for SI)

Snyder’s UH Model Estimating Snyder’s UH Parameters - Ct typically ranges from 1.8 to 2.0 - Cp ranges from 0.4 to 0.8 - Larger values of Cp are associated with smaller values of Ct

SCS UH Model Basic Concepts and Equations

SCS UH Model Basic Concepts and Equations - SCS suggests the relationship A=watershed area; C=conversion constant(2.08 in SI) t=the excess precipitation duration;tlag=the basin lag

SCS UH Model Estimating the SCS UH Model Parameters

Clark Unit Hydrograph Models translation and attenuation of excess precipitation Translation: movement of excess from origin to outlet based on synthetic time area curve and time of concentration Attenuation: reduction of discharge as excess is stored in watershed modeled with linear reservoir

Clark Unit Hydrograph Required Parameters: TC Not Time of Concentration!!! Storage coefficient

Clark Unit Hydrograph Estimating parameters: Time of Concentration: Tc Estimated via calibration SCS equation Storage coefficient Flow at inflection point of hydrograph divided by the time derivative of flow

ModClark Method Models translation and attenuation like the Clark model Attenuation as linear reservoir Translation as grid-based travel-time model Accounts for variations in travel time to watershed outlet from all regions of a watershed

ModClark Method Excess precipitation for each cell is lagged in time and then routed through a linear reservoir S = K * So Lag time computed by: tcell = tc * dcell / dmax All cells have the same reservoir coefficient K

ModClark Method Required parameters: Gridded representation of watershed Gridded cell file Time of concentration Storage coefficient

ModClark Method Gridded Cell File Contains the following for each cell in the subbasin: Coordinate information Area Travel time index Can be created by: GIS System HEC’s standard hydrologic grid GridParm (USACE) Geo HEC-HMS

Kinematic Wave Model Conceptual model Models watershed as a very wide open channel Inflow to channel is excess precipitation Open book:

Kinematic Wave Model HMS solves kinematic wave equation for overland runoff hydrograph Can also be used for channel flow (later) Kinematic wave equation is derived from the continuity, momentum, and Manning’s equations

Kinematic Wave Model Required parameters for overland flow: Plane parameters Typical length Representative slope Overland flow roughness coefficient Table in HMS technical manual (Ch. 5) % of subbasin area Loss model parameters Minimum no. of distance steps Optional

Baseflow Three alternative models for baseflow Constant, monthly-varying flow Exponential recession model Linear-reservoir volume accounting model

Baseflow Constant, monthly-varying flow User-specified Empirically estimated Often negligible Represents baseflow as a constant flow Flow may vary from month to month Baseflow added to direct runoff for each time step of simulation

Baseflow Exponential recession model Defines relationship of Qt (baseflow at time t) to an initial value of baseflow (Q0) as: Qt = Q0Kt K is an exponential decay constant Defined as ratio of baseflow at time t to baseflow one day earlier Q0 is the average flow before a storm begins

Baseflow Exponential recession model

Baseflow Exponential recession model Typical values of K 0.95 for Groundwater 0.8 – 0.9 for Interflow 0.3 – 0.8 for Surface Runoff Can also be estimated from gaged flow data

Baseflow: Exponential recession model: Applied at beginning and after peak of direct runoff User-specified threshold flow defines when recession model governs total flow

Baseflow Linear Reservoir Model: Used with Soil Moisture Accounting loss model (last time) Outflow linearly related to average storage of each time interval Similar to Clark’s watershed runoff

Applicability and Limitations Choice of model depends on: Availability of information Able to calibrate? Appropriateness of assumptions inherent in the model Don’t use SCS UH for multiple peak watersheds Use preference and experience