7. Once the overlay process is complete, the user may proceed with determination of hydrologic response units (HRUs).
SECTION 6.2: HRU D
EFINITIONPurpose
Once the land use, soil, and slope data layers have been imported overlaid, the distribution of hydrologic response units (HRUs) within the watershed must be determined. The HRUs Definition command in the HRU Analysis menu allows the user to specify criteria used in determining the HRU distribution. One or more unique land use/soil/slope combinations (hydrologic response units or HRUs) can be created for each subbasin.
Application
Subdividing the watershed into areas having unique land use and soil combinations enables the model to reflect differences in evapotranspiration and other hydrologic conditions for different land covers/crops and soils. Runoff is predicted separately for each HRU and routed to obtain the total runoff for the watershed. This increases the accuracy of load predictions and provides a much better physical description of the water balance.
The user has two options in determining the HRU distribution: assign a single HRU to each subwatershed or assign multiple HRUs to each subwatershed. If a single HRU per subbasin is selected, the HRU is determined by the dominant land use category, soil type, and slope class within each watershed. If multiple HRUs are selected, the user may specify sensitivities for the land use, soil, and slope data that will be used to determine the number and kind of HRUs in each watershed.
Key Procedures
• Select single or multiple HRUs per subwatershed
• For multiple HRUs, define land use and soil threshold levels
• Optional: Set land use refinement parameters to split HRU land use classes and define special land uses exempt from threshold levels
• Click the OK button to determine the HRU distribution
1. Select HRU Definition from the HRU Analysis menu. The HRU Definition dialog box will be displayed with the HRU Thresholds tab activated (Figure 6.57).
Figure 6.57
2. The dialog box has three radio buttons: Dominant Land Use, Soils, Slope, Dominant HRU, and Multiple HRUs. The user must select the button for the method used to create HRUs.
a. The Dominant Land Use, Soil, and Slope option will create one HRU for each subbasin. The dominant land use, soil, and slope class in the subbasin are simulated in the HRU.
b. The Dominant HRU option will create one HRU for each subbasin. The dominant unique combination of land use, soil, and slope class in the subbasin are used to simulate the HRU.
c. The Multiple HRUs option will create multiple HRUs within each subbasin. This option is selected by default. To activate this option, select the radio button.
Three slide bars and the Threshold options are now enabled (Figure 6.58). The thresholds can be based on either a percentage of area or an absolute area. These options are controlled by the Percentage and Area radio button options in the Threshold section.
Figure 6.58
The land use slider bar controls the threshold level used to eliminate minor land uses in each subbasin. Land uses that cover a percentage (or area) of the subbasin area less than the threshold level are eliminated. After the elimination process, the area of the remaining land uses is reapportioned so that 100% of the land area in the subbasin is modeled.
For example, assume there is a subbasin that contains
•
35% agricultural land in corn•
30% pasture•
21% forest•
10% agricultural land in orchard•
4% urbanIf the threshold level for land use is set to 20%, HRUs would be created for pasture, forest, and corn. The areas of modeled land uses would be modified as follows:
•
corn: (35% ÷ 86%) x 100% = 41%•
pasture: (30% ÷ 86%) x 100% = 35%•
forest: (21% ÷ 86%) x 100% = 24%where 86% was the percentage of the subbasin originally covered by pasture, forest, and corn.
The soil slider bar controls the creation of additional HRUs based on the distribution of the selected land uses over different soil types. This scale is used to eliminate minor soils within a land use area. As with the land use areas, once minor soil types are eliminated, the area of remaining soils is reapportioned so that 100% of the land use area is modeled.
For example, assume that the overlay performed by the interface during the land use, soil, slope overlay identified the following soil distribution for pastureland in the subbasin:
•
20% Houston Black•
25% Branyon•
15% Heiden•
10% Austin•
7% Stephen•
6% Denton•
5% Frio•
4% Purves•
3% Bastrop•
2% Altoga•
1% Eddy•
1% San Saba•
1% FerrisIf the threshold level for soils within a land use area is set to 10%, the following HRUs will be created for this example:
•
pasture/Houston Black•
pasture/Branyon•
pasture/Heiden•
pasture/AustinThis process is performed for every land use modeled in the subbasin.
The slope slider bar controls the creation of additional HRUs based on the distribution of the selected soil types over different slope classes.
This scale is used to eliminate minor slope classes within a soil on a specific land use area. As with the land use areas and soil areas, once minor slope classes are eliminated, the area of remaining slope classes is reapportioned so that 100% of the soil area is modeled.
For example, assume that the overlay performed by the interface during the land use, soil, slope overlay identified the following slope distribution for Branyon soil on pastureland in the subbasin:
•
50% 0 -1% slopes•
35% 1%-2% slopes•
15% > 2% slopesIf the threshold level for slope within a soil on a land use area is set to 20%, the following HRUs will be created for this example:
•
pasture/Branyon/0-1% slopes•
pasture/Branyon/1%-2% slopesThis process is performed for every soil on every land use modeled in the subbasin.
The threshold levels set for multiple HRUs is a function of the project goal and the amount of detail desired by the modeler. For most
applications, the default settings for land use threshold (20%) and soil threshold (10%) and slope threshold (20%) are adequate.
i. Specify the Landuse threshold level by moving the pointer on the first slide bar (Figure 6.59).
Figure 6.59
ii. Specify the Soil threshold level by moving the pointer on the second slide bar (Figure 6.60).
Figure 6.60
iii. Specify the Slope threshold level by moving the pointer on the third slide bar (Figure 6.61).
Figure 6.61
iv. The slider bar minimum and maximum values will be set based on valid values within the datasets. However, if you type a value in to one of the text boxes that is beyond the valid range for the slider, you will receive the following error message:
Figure 6.62
3. At this point, the user has the option of performing some additional land use refinements before applying the thresholds and creating the HRUs.
On the Land Use Refinement (Optional) tab, the user has the option to define certain land use classes to split into multiple “sub-land uses” and the option to set special land uses “exempt” from the land use threshold setting defined.
Figure 6.63