Subbasin Data – Selecting a Canopy Method

The canopy component represents the presence of plants in the landscape. Plants intercept precipitation, reducing the amount of precipitation that arrives at the ground surface. Intercepted water evaporates between storm events. Plants also extract water from the soil in a process called transpiration. Evaporation and transpiration are often combined as evapotranspiration.

In GeoHECHMS, the Simple Canopy method is used to compute the precipitation loss due to evapotranspiration. Selecting a canopy method is optional and generally used for continuous simulation applications. If no canopy method is selected, the subbasin will not compute any losses due to evapotranspiration and all precipitation will be treated as direct precipitation.

The Simple Canopy method provides a simple representation of a plant canopy. In this method, all the precipitation is intercepted until the canopy storage capacity is full. Once the canopy storage is filled, all further precipitation falls to the surface or directly to the soil if no surface representation is included. 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 from surface and soil components. Only after the canopy storage has been emptied will the surface, and soil components, use the unused potential evapotranspiration.

Note that the Dynamic Canopy and Gridded Simple Canopy methods are not supported in GeoHECHMS since they are mostly used by researchers or the Army Corps of Engineers.

Follow the steps given below to select a canopy method:

1. From the Input ribbon menu, select the Subbasin Data command.
   
2. The Subbasin Data dialog box will be displayed.
   
3. From the Subbasin ID dropdown combo box, select the subbasin to assign a canopy method.
   
4. From the Canopy method dropdown combo box, select the canopy method.
   
5. To enter the parameters for the selected canopy method, select the Canopy Data option from the Subbasin Specifications dropdown combo box.
   Note that the Canopy Data option will be grayed out (i.e., unavailable) when None is selected for the canopy method.
6. The Canopy Data panel will be displayed.
   
7. Specify the initial condition of the canopy storage using the Initial storage spin control button. The initial condition is specified as the percentage of the canopy storage that is full of water at the beginning of the simulation.
8. In the Maximum storage field, specify the maximum amount of water that can be held on leaves before through-fall to the surface begins. The amount of storage is specified as an effective depth of water.
9. Specify the Crop coefficient, which is the ratio applied to potential evapotranspiration. It is used for calculating the quantity of water extracted from the soil during crop development.
10. From the Evapotranspiration dropdown combo box entry, select one of the following options:
    • Dry Periods Only
      Use this option to set the canopy to evaporate water from storage and extract water from the soil only during dry periods with no precipitation.
    • Wet & Dry Periods
      Use this option to set the canopy to evaporate water from storage and extract water from the soil during both dry and wet periods.
      Note that the choice for simultaneous precipitation and evapotranspiration can improve results when using a long interval.
11. From the Uptake method dropdown combo box entry, select the method for extraction of water from the soil.
    The Simple method extracts water at the potential evapotranspiration rate and can be used with the deficit constant or soil moisture accounting infiltration loss methods. In contrast, 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. When None is selected, there will be no soil water extraction. To learn how to select an infiltration method for a subbasin, refer to this article in our knowledge base.
12. When the data has been defined, click the [Close] button to close the Subbasin Data dialog box.

Canopy Methods along with the Kinematic Wave Runoff Method

If Kinematic Wave is selected as the runoff method, then the Canopy Data panel will be divided into two surface area sections as shown below.

By default, Surface area #1 is active with the Subbasin area field set as 100 %. However, changing the Subbasin area in Surface area #1 activates the Surface area #2 fields.

In addition, changing the Subbasin area in either Surface area #1 or Surface area #2 adjusts the Subbasin area in the other counterpart such that the total Subbasin area adds to 100%.

To learn how to select a runoff method for a subbasin, refer to this article in our knowledge base.

Pros and Cons of Simple Canopy Method

Pros	Cons
It is simple to use and parameterize.	Multiple factors such as the type of storm, time of year, vegetation type, and vegetation density can affect canopy storage. Hence calibration and validation of parameters can be challenging.