Prácticas educativas a desarrollar con la población de estudio

After the NEXRAD Level II Conversion tool or the NWS Precipitation tool has been run,

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Runoff can be calculated using the Soil Conservation Service (SCS) Curve Number (CN)

method. This employs the CN grid derived from land cover and soil data, as described in

Chapter 4. An estimation of water losses and infiltration within the soil can also be

derived from this method. It should be noted that the estimated runoff from this method

approximates the maximum potential runoff possible, and the estimated water infiltration

represents the maximum possible infiltration. This processing workflow is shown in

Runoff-Infiltration Process Expanded Start Imports, inputs and environments Convert Precipitation units Set up Raster Variables Potential Runoff? Runoff Calculation Convert to Numpy Arrays Water Infiltration Calculation Output Infiltration and Mosaic Dataset Option Output Runoff and Mosaic Dataset Option End Delete Intermediate Files CN Grid Resample Runoff Create Mask Flow Direction Extract by Mask Flow Accumulation Convert to Volume Output Accumulated Runoff and Mosaic

Dataset Option Implement ARC Choice Yes No

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The runoff and infiltration script imports the Arcpy and numpy modules, and

specifically, the Spatial Analyst module from Arcpy. The script then sets up variables for

the user-defined parameters, which specify the location and name of the precipitation

image to be used, the CN grid to be used, the flow direction grid to be used, and the

output runoff , water infiltration, and accumulated runoff locations and names. The first

operation is to activate the Spatial Analyst extension. Even if the license is installed in

the local environment, the Spatial Analyst functions are not available for use within a

script unless this command is run. This extension is also released at the end of the script.

The use of the Spatial Analyst module allows calculations to be performed on rasters.

The precipitation data are converted into units of inches, which is required for the SCS

runoff calculation, and the CN grid choice is implemented to calculate maximum

retention. If runoff is a possibility, the runoff equation is applied. The amount of water

retained is calculated each raster. If the evaluation for runoff produces a value less than

or equal to zero, the runoff calculation is skipped and the total precipitation for that cell is

translated to water infiltrated into the soil.

Since it is quite possible that a storm event may not generate any runoff, an internal

check looks at the overall results from the runoff calculation. If sum of the runoff is zero,

a message is sent to the user stating that no runoff was produced for that event and the

final runoff image is not produced. Figure 5.7 shows an example runoff map from the

Figure 5.7: Potential runoff generated from NEXRAD Level II data for the September 13, 2011 storm event

Figure 5.8 shows the potential water infiltration from the same event. It is

important to note that the values represented here indicate the maximum possible runoff

and the maximum possible infiltration. Evaporation and groundwater recharge are not

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Figure 5.8: Infiltrated water from NEXRAD Level II precipitation data for the September 13, 2011 storm event

If a valid runoff product is generated, an accompanying accumulated runoff map is

also generated. This initiates a set of raster calculations that generate a raster of runoff

drainages and their accumulated volume in cubic feet. The accumulated runoff process

uses the flow direction raster that was created previously, as described in Chapter 4. This

grid is used to resample the runoff raster to the spatial resolution of the flow direction

grid, which is 5 by 5 meters. The purpose of this is to generate a more accurate

representation of the runoff drainages. The model does not currently consider how far the

runoff will travel beyond the impacted areas, as this would require additional information

about the timeline and intensity of the storm, as well as travelling time for the surface

water. Therefore a mask using the resampled runoff was used to generate a subset of

impacted areas. This mask extracts the flow direction grid for the impacted areas. The

flow accumulation tool is then used with two inputs: the resampled runoff as a weighted

number of cells that would flow into a specific cell. By adding the runoff grid, the runoff

for each cell is added, generating an accumulated runoff map that delineates the

drainages. The process limits the volume of accumulated runoff shown to 0.1 cubic feet

or more. This allows the drainages generated from runoff to be clearly defined. An

example output from this process is shown in Figure 5.9. The rasters are saved to TIFF

images in the user defined location. The mosaic dataset condition is checked and new

layers in the corresponding mosaic datasets are made. At this point, the intermediate files

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Figure 5.9: Top: runoff map, as a locator map. Bottom: accumulated runoff

5.5 Summary

This chapter reviewed the implementation of the tools built for this project, both as how

they were used during testing and how they should be used in the future. While this

chapter has focused on how the tools were built and their proper use, Chapter 6 will focus

on the output products from these processes and how they can be used for analysis, as

well as discuss the analysis that went into certain parameters of each tool and how the

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