Simple Canopy

Basic Concepts and Equations

This method is a simple representation of a plant canopy. All precipitation is intercepted until the canopy storage capacity is filled. Once the storage is filled, all further precipitation falls to the surface, or directly to the soil if no representation of the surface is included. If a non zero directly connected impervious percentage is defined in the loss method, a portion will go to direct runoff prior to falling to the surface or soil defined as:

All potential evapotranspiration will be used to empty the canopy storage until the water in storage has been eliminated. The potential evapotranspiration is multiplied by the crop coefficient to determine the amount of evapotranspiration from canopy storage and later the surface and soil components. Only after the canopy storage has been emptied will unused potential evapotranspiration be used by the surface and soil components.

A gridded Simple Canopy method is also included within the program. This method presumes a subbasin is composed of regularly spaced cells with uniform length and width. This method permits the user to specify initial conditions and parameters for each grid cell separate from the neighboring cells.

Required Parameters

Parameters that are required to utilize this method within HEC-HMS include the initial storage percentage [%] or depth [in or mm], crop coefficient [unitless], maximum storage [in or mm], evapotranspiration Coincidence method, and uptake method.

The initial storage or depth defines the starting saturation of the canopy.  This parameter is a function of the antecedent canopy moisture content at the beginning of the simulation.  It may be estimated in the same manner as the initial abstraction for loss models.

The crop coefficient is a ratio applied to the potential evapotranspiration (computed in the Meteorologic Model) when computing the amount of water to actually extract from the soil.  This canopy is typically initially estimated using land use estimates but it should be calibrated using observed data.

The maximum storage defines the maximum volume of water that can be held within the canopy.  This value is typically initially estimated using land use estimates but it should be calibrated using observed data. 

The initial storage must be less than or equal to the maximum storage.  Both parameters are specified as effective depths (e.g., inches or millimeters). 

The Evapotranspiration Coincidence method defines when infiltrated water will be extracted from the surface and/or soil.  The Only Dry Periods method will result in evapotranspiration only occurring during time steps with no precipitation and/or snowmelt.  The Wet and Dry Periods method will allow for evapotranspiration during periods of both precipitation/snowmelt and no precipitation/snowmelt.

The Wet and Dry Periods method can improve simulated results when using a long computational time interval (e.g., 1-day) or during a snowmelt simulation.

The Uptake method defines if and how water will be extracted from the surface and/or soil. The Simple method extracts water at a rate equivalent to the potential evapotranspiration and can be used with the Deficit Constant or Soil Moisture Accounting loss methods. The Tension Reduction method can be used with the Soil Moisture Accounting Method and extracts water at the potential evapotranspiration rate from the gravity zone but reduces the rate when extracting from the tension zone.

No water is extracted from the soil unless the Simple or Tension Reduction method is selected.