55 https://www.publications.usace.army.mil/Portals/76/Publications/EngineerManuals/EM_1110-2-1417.pdf?ver=VFC- A5m2Q18fxZsnv19U8g%3d%3d
56 https://www.hec.usace.army.mil/confluence/hmsdocs/hmsguides/applying-loss-methods-within-hec-hms/introduction-to-the-loss- rate-tutorials
removed from the upper-soil profile storage. The model then continues as described above for the precipitation periods.
Required Parameters
Parameters that are required to utilize this method within HEC-HMS include the amounts of storage within the soil, groundwater 1, and groundwater 2 layers that are initially filled [percent], the maximum infiltration rate [in/hr or mm/hr], directly connected impervious area [percent], the maximum soil storage [in or mm], tension storage [in or mm], the maximum soil percolation rate [in/hr or mm/hr], the maximum groundwater layer 1 storage [in], the maximum groundwater layer 1 percolation rate [in/hr or mm/hr], the groundwater layer 1 coefficient [hr], the maximum groundwater layer 2 storage [in], the maximum groundwater layer 2 percolation rate [in/hr or mm/hr], and the groundwater 2 layer coefficient [hr].
The amount of initial storage refers to the initial saturation of each layer at the beginning of a simulation and should be determined through model calibration. The maximum infiltration rate sets the upper bound on infiltration from the surface storage into the soil. This is the upper bound on infiltration; the actual infiltration in a particular time interval is a linear function of the surface and soil storage, if a surface method is
selected. Without a selected surface method, water will always infiltrate at the maximum rate. Soil storage represents the total storage available in the soil layer. Tension storage specifies the amount of water storage in the soil that does not drain under the effects of gravity. Percolation from the soil layer to the upper groundwater layer will occur whenever the current soil storage exceeds the tension storage. Water in tension storage is only removed by ET. By definition, tension storage must be less than soil storage. The soil percolation sets the upper bound on percolation from the soil storage into the upper groundwater layer.
The actual percolation rate is a linear function of the current storage in the soil and the current storage in the upper groundwater layer. The maximum groundwater layer 1 storage represents the total storage in the upper groundwater layer. The groundwater layer 1 percolation rate sets the upper bound on percolation from the upper groundwater into the lower groundwater layer. The groundwater layer 2 layer percolation rate sets the upper bound on deep percolation out of the system. The aforementioned parameters are typically estimated using the predominant soil texture and literature values. The groundwater layer 1 and groundwater layer 2 coefficients are used as the time lag on a linear reservoir for transforming water in storage to lateral outflow.
A Note on Parameter Estimation
The values presented here are meant as initial estimates. This is the same for all sources of similar data including Engineer Manual 1110-2-1417 Flood-Runoff Analysis55 and the Introduction to Loss Rate Tutorials56. Regardless of the source, these initial estimates must be calibrated and validated.
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The following table contains a list of various advantages and disadvantages regarding the aforementioned loss methods available for use within HEC-HMS. However, these are only guidelines and should be
supplemented by knowledge of, and experience with, the methods and the watershed in question.
Method Advantages Disadvantages
Initial and Constant "Mature" method that has been used successfully in thousands of studies throughout the U.S.
Easy to set up and use.
Parameters can be related to predominant soil textures and estimated using multiple literature sources.
Method is parsimonious; it includes only a few parameters necessary to explain the variation of runoff volume.
Difficult to apply to ungaged areas due to lack of direct physical relationship of parameters and watershed properties.
Method may be too simple to predict losses within event, even if it does predict total losses well.
Does not allow for continuous simulation.
Does not allow for surface storage to occur prior to soil saturation.
Deficit and Constant Similar to advantages of the Initial and Constant method.
Method is scalable in that it allows for continuous simulation (but is not required for use).
Similar to disadvantages of the Initial and Constant method.
Green and Ampt Parameters can be related to predominant soil textures and estimated using multiple literature sources.
Predicted values are in accordance with classical
unsaturated flow theory (good for ungaged watersheds).
Allows for surface storage to occur prior to soil saturation.
Not widely used, so less mature.
Not as much experience in professional community as simpler methods.
Less parsimonious than simpler methods.
Does not allow for continuous simulation.
Layered Green and
Ampt Similar to advantages of the
Green and Ampt method.
Allows for continuous simulation.
Similar to disadvantages of the Green and Ampt method.
Requires more parameters than the Green and Ampt method.
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Method Advantages Disadvantages
SCS Curve Number Simple, predictable, and stable method.
Relies on only one parameter, which varies as a function of soil group, land use and treatment, surface condition, and antecedent moisture condition.
Features readily understood and well-documented.
Well established method widely accepted for use in U.S. and abroad.
Parameters can be related to predominant soil group/land use and estimated using multiple literature sources.
Predicted values not in accordance with classical unsaturated flow theory (infiltration rate will approach zero during a storm of long duration rather than a constant rate).
Developed with data from small agricultural watersheds in midwestern U.S., so applicability elsewhere is uncertain.
Default initial abstraction (0.2*S) does not depend upon storm characteristics or timing.
Rainfall intensity is not considered when computing losses (i.e., the same loss volume will be calculated for 1 in rainfall distribution over 1 hour or 1 day).
Does not allow for continuous simulation.
Does not allow for surface storage to occur prior to soil saturation.
Exponential Predicted values are in accordance with classical unsaturated flow theory.
Similar to disadvantages of the Green and Ampt method.
Requires more parameters than Green and Ampt.
Parameters cannot be related to predominant soil textures and estimated using (not good for ungaged watersheds).
Does not allow for continuous simulation.
Does not allow for surface storage to occur prior to soil saturation.
Smith Parlange Similar to advantages of the Green and Ampt method.
Similar to disadvantages of the Green and Ampt method.
Requires more parameters than the Green and Ampt method.
Does not allow for continuous simulation.
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Method Advantages Disadvantages
Soil Moisture
Accounting Parameters can be estimated for ungaged watersheds from information about soils.
Predicted values are in accordance with classical
unsaturated flow theory (good for ungaged watersheds).
Allows for continuous simulation.
Allows for surface storage to occur prior to soil saturation.
Not widely used, so less mature, not as much experience in professional community.
Features not widely understood.
Less parsimonious than simple empirical methods.