Erosion hidrica del suelo - USLE - Parte 4 de 5 - FACTOR LS - ArcGIS ONLINE

Erosion Control Training: Part 4 - Topography Factor Calculation

Introduction to Topography in Erosion Analysis

• Greetings and introduction to the fourth part of the training on soil water erosion, focusing on the topographic factor (ls) calculation. This segment is particularly interesting as it builds upon previous lessons.

Required Inputs for Topographic Factor Calculation

• The current tutorial requires only two inputs: a digital elevation model (DEM) of the watershed and the shapefile of the study area. These are essential for further analysis.

Overview of Previous Tutorials

• A recap of prior tutorials is provided, emphasizing that this session will focus on determining aspects related to topography, leading to the creation of a topographic raster (ls). This raster is crucial for calculating potential erosion in subsequent tutorials.

Understanding the Topographic Factor (ls)

• The topographic factor is derived from two components:

• Factor L: Represents slope length.

• Factor S: Represents slope steepness.

• Reference to Foster's 1977 proposal for calculating parameter L using intermediate values such as lambda and m, where m depends on beta (the slope).

Calculating Parameters Using Geographic Information Systems (GIS)

• Lambda represents the length analyzed in a given section; Velázquez's formula from 2008 will be utilized based on earlier studies.

• The parameters involved include:

• A: Flow accumulation at pixel level.

• D: Pixel size which needs determination.

• X: Shape coefficient, typically set to one when working with square pixels in GIS applications.

Steps for Calculating Factors L and S

• To calculate factor S:

• Two formulas are proposed based on whether the tangent of slope is less than or greater than specific thresholds.

• Historical context indicates that earlier calculations were labor-intensive without modern raster concepts; GIS has significantly reduced time requirements for these analyses.

Procedure Using ArcGIS Software

1. Establish a slope map using ArcGIS tools, where "Slope" refers to beta in English terminology.

2. Calculate parameter F based on slope before moving onto parameter M.

3. Apply a comprehensive formula involving flow accumulation and pixel size once all parameters are established.

4. Generate both rasters needed for further analysis after completing these calculations step-by-step through ArcGIS software functionalities.

This structured approach ensures clarity while navigating through complex topics related to soil erosion analysis using advanced geographic information systems techniques.

Spatial Analysis and Hydrology Techniques

Filling Imperfections in DEM

• The process begins with filling imperfections in the Digital Elevation Model (DEM) using the "spatial analy Tools" from the toolbox, specifically under hydrology.

• The filled DEM is referred to as "fill," which will be used for subsequent analyses, allowing users to focus on this processed input.

Slope Calculation

• Users are instructed to select "Slow slope" under surface analysis, ensuring that slope values are expressed in degrees rather than percentages for clarity.

• A slope map is generated, highlighting areas prone to water erosion based on soil quality; regions with slopes between 29 and 51 degrees are particularly noted.

Raster Calculations

• It’s important to convert degree values into radians by multiplying by 0.01745 for compatibility with ArcGIS processing requirements.

• The next step involves calculating a parameter 'F' using raster algebra through the "map algebra" tool within spatial analy tools.

Parameter F Formula

• The formula for parameter 'F' includes sine calculations of Beta (the slope), requiring conversion into radians before applying it in a mathematical expression involving both numerator and denominator components.

• Careful attention is needed when entering parentheses during calculations to avoid errors that could prevent successful execution of the raster generation.

Generating Additional Rasters

• After successfully generating parameter 'F', another raster representing parameter 'm' is created by dividing 'F' by (1 + F), resulting in a new set of values ranging from 0 to 742.

Determining Pixel Size and Flow Accumulation

• To determine pixel size ('D'), users check properties of the fill raster, noting it as approximately 76.44 after rounding.

• The flow accumulation calculation requires first establishing flow direction using the fill raster before proceeding with further hydrological analysis steps.

Flow Accumulation and Calculation Process

Understanding Flow Accumulation

• The process of flow accumulation is discussed, emphasizing that it typically takes longer due to the formation of river channels through slope analysis.

• The initial step involves calculating the numerator using map algebra, where a specific parameter 'l' is defined for this calculation.

Calculating the Numerator

• The numerator consists of two parts: one raised to M + 1 and another shorter term also raised to M + 1.

• It includes the flow accumulation plus a squared dimension (76.44), both elevated to M + 2, indicating careful attention to mathematical powers in calculations.

Denominator Calculation

• Transitioning to the denominator, it is calculated similarly with parameters defined as 'd' raised to M + 2.

• The formula incorporates another value (22.13), which is also raised to its respective power, ensuring consistency in calculations.

Error Checking and Adjustments

• An error in previous calculations prompts a reevaluation; adjustments are made by rewriting formulas completely for clarity.

• A detailed breakdown of the numerator shows how each component contributes, ensuring all parentheses are correctly counted and closed.

Finalizing Factor Calculations

• After confirming accuracy in both numerator and denominator, focus shifts towards calculating factor 'S', which depends on specific conditions related to angle tangents.

• A conditional formula is introduced based on whether the tangent of an angle exceeds a certain threshold (0.09), guiding how factor 'S' should be computed.

Calculating Raster Factors in GIS

Understanding the Calculation Process

• The speaker discusses the importance of correctly placing parentheses in mathematical operations, emphasizing that improper placement can lead to incorrect results.

• Acknowledgment of a successful calculation process for the Beta factor, with plans to rename it for clarity and further calculations.

• Introduction of raster factors, specifically focusing on multiplying two factors (L and S), which is essential for generating accurate raster data.

• The speaker notes that a similar parameter already exists, suggesting a need for consistency in naming conventions by using "double r" as part of the identifier.

• Confirmation that the calculated parameters are now ready to be applied, including changing colors and classifications within the raster data.

Finalizing Raster Data

• The tutorial concludes with adjustments made to decimal formatting and color classification for better visualization of the raster factor LS. The speaker reflects on the length of the tutorial but emphasizes its necessity for thorough understanding.