To download the zipped code for the Runoff to Rainfall Ratio module,
right click on one of the following links and choose "Save Link As..."

(GeneralRatio is for broader application. SEPA_Ratio should only be used
if the study site is one of the watersheds listed for download
or if it is similarly located in southeastern Pennsylvania.)

GeneralRatio.zip
SEPA_Ratio.zip

Either file will need to be unzipped in WinZip before it can be compiled.

If you would rather download the code as is
and edit out the characters that get added during file transfer (^M), use one of the following links instead

GeneralRatio.cpp
SEPA_Ratio.cpp

1. Save the GeneralRatio.cpp or SEPA_Ratio.cpp code in a directory named "Hydrology" (or whatever you prefer). This directory can contain both this module and the SCS Direct Runoff module. Compile the code. (On a Sun Solaris 7, an example compile command is "CC GeneralRatio.cpp -o ratio". This will generate the output "ratio," which is a compiled program ready to run.)
   
   
2. Make sure your ascii landuse files are also in this same directory. These files are the echoes of the original resolution land use maps - NOT the ones resampled to 1-km². You should have created these files following the instructions at the "Data Preparation" link. They should have names such as land87.asc, land96.asc, etc. Your landuse maps must have been based on the following classification scheme: 1=urban, 2=agriculture (bare soil and short vegetation), 3=forest and 4=water.
   
   
3. This module requires that your study site be a defined watershed with a mapped channel. It should preferably be small and definitely not much larger than 350 square miles. The techniques used to develop the module were based on an ideal 24-hour storm discharge response, and thus the results are not suitable for basins with large storage areas such as lakes. You will need to know elevation values throughout the basin; you can get these from the DEM that was used to prepare the SLEUTH model data layers.
   
   
4. Be sure you know the following characteristics of your basin:

   
   

   size

   This is the size of the basin in square miles.

   head-outlet

   This parameter represents the decrease in elevation that occurs from a point near the headwater source area to a point at the basin outlet. The actual curving channel path should be used to determine the point 80% up the main channel; this is defined as the headwater reference, while the site where the stream gauge for the basin would be located represents the outlet point. Both elevation values should be in meters, so the final units for this parameter are meters. (**This parameter is used with GeneralRatio.)

   channel/range

   This parameter indicates how well the decrease in elevation within the main channel represents the actual potential for terrain effects within the entire basin. "channel" is the value determined for "head-outlet" above, while "range" is the maximum elevation observed within the entire basin minus the minimum observed. Both the numerator and denominator should be in meters. (**This parameter is used with GeneralRatio.)

   channel potential

   This parameter is the "head-outlet" value (see above) divided by the straight line distance between the headwater reference and the outlet point. Both the numerator and denominator should be in meters. (**This parameter is used with SEPA_Ratio.)

   
   
   
5. **If you will be using the SEPA_Ratio module, decide if you want to separate the urban land class based on its proximity to the water channel. If so, you will need to have prepared a 250-meter buffer layer for the watershed. This must have been converted to ascii format (see the "Data Preparation" link) and be at the same spatial resolution as your original land use maps. You should construct this just as if you were preparing another gif image for input to the SLEUTH model. The value in each cell should be 1 (one) if it is located within the 250-meter buffer and 0 (zero) if it is not.
   
   
6. Run the GeneralRatio or SEPA_Ratio program (just type the name of the program that came out of compiling). All required inputs will be requested through prompts within the module.
   
   
7. The output file that is generated will either be called ratios.txt or sepa_ratios.txt. An example of this output for the GeneralRatio program is given below. **The output from the SEPA_Ratio program separates the developed land proportions into that in and out of the buffer (if this option was chosen). It also calculates the ratio under both typical and dry antecedent moisture conditions. Note the general warning in the sample output for 1996 regarding the relative proportion of the land class "water" within the basin. Warnings may be written throughout the output depending on your basin's particular situation. The equations within this module can only reasonably be applied to scenarios that fall within the range of land use patterns and physical characteristics observed in the study basins. Otherwise, unphysical runoff to rainfall ratios (greater than 1 or less than 0) may result. The module monitors your input and notes when the runoff to rainfall ratio cannot be reliably estimated. Warnings may occur if, for example, the basin is too steep or too large, if one particular land use dominates the basin, or if a negative ratio is calculated.
   
   
8. Use these output text files to monitor the runoff to rainfall ratio for each year of predicted land use. For ratios.txt, interpret this ratio as an index that represents the basin's runoff response to a typical storm under normal antecedent moisture conditions during the non-summer months. For sepa_ratios.txt, there is the additional ability to see how the ratio may change when the antecedent moisture conditions during this same time period are dry. Keep in mind that these results are not an attempt at flood prediction, but merely a strategy to allow communities to determine when their water resources might become noticeably impacted by urban growth.