Create Curve Number CN Raster Using ArcHydro Tools in ArcGIS Pro
Fundamentals of Curve number hydrology
In the context of the Soil Conservation Service Curve Number (SCS CN) method, the curve number (CN) represents a parameter used to estimate direct runoff from rainfall events in a particular watershed or catchment area. The SCS CN method is widely used in hydrology and water resources engineering to predict runoff volumes for various land uses and soil types.
The curve number is a dimensionless value that ranges between 30 and 100. It is based on a variety of factors such as soil type, land use, hydrologic condition, and antecedent moisture conditions. The higher the curve number, the less water can infiltrate into the soil, resulting in higher runoff potential.
The general concept behind the SCS CN method is that it classifies the land cover and soil characteristics into different hydrological groups, each corresponding to a specific curve number. By knowing the curve number for a particular area, hydrologists and engineers can estimate the runoff volume produced by a given rainfall event.
The SCS CN method has been widely used in the design of stormwater management systems, flood control studies, and the assessment of watershed response to various land use changes and development activities. However, it's essential to note that it's a simplified approach and may have limitations, especially for complex hydrological systems. for example, it's important to use the method within its intended scope and consider other factors that may affect the runoff estimation.
Data and software requirments
To determine the curve number (CN) using a Geographic Information System (GIS) package, you will need several types of data related to the watershed or catchment area of interest. Here are the essential types of data you will typically need:
Digital Elevation Model (DEM): A high-resolution DEM of the study area is crucial for deriving the flow direction and flow accumulation. It helps identify the drainage patterns and delineate the watershed boundaries.
Land Use/Land Cover (LULC) Data: LULC data provides information about the different land use and land cover types within the study area. This information is essential for classifying the hydrological groups and assigning the corresponding curve numbers.
Soil Data: Soil data provides information about the soil types present in the study area. Different soil types have varying infiltration rates, which affect the curve number assignment.
GIS Software: A GIS software package (e.g., ArcGIS, QGIS) is essential to process and analyze the spatial data, perform calculations, and create maps.
Once you have the necessary data, you can use GIS tools to delineate the watershed, overlay the land use and soil data to assign curve numbers, calculate hydrological parameters, and perform runoff simulations using the SCS CN method. Many GIS packages have built-in hydrological modeling tools or plugins that can facilitate this process.
general steps to calculate the curve number raster
Download the required data:
If the study area is in the United States, you can download the land-use and soil datasets from the following websites:
Download and install Arc Hydro for ArcGIS Pro here: https://www.esri.com/en-us/industries/water-resources/arc-hydro/downloads#arc-hydro-for-arcgis-pro
Landuse Data: https://www.mrlc.gov/viewer/
If you want to follow along with the video, download the following ArcGIS Pro Package: CN_ArcGISPro Package
Create a histogram of hydrologic soil groups (HSGs) and replace the missing HSGs with the most frequent HSG
Convert A/D, B/D, and C/D to D
Convert the landuse raster into a feature class (aka a shapefile)
Add 4 columns to the soil table: PctA, PctB, PctC, and PctD
Use the "Union" tool to create a connected soil and land-use shapefile
Create a new column: LandUse = Gridcode
Create a "CN Lookup Table"
Generate the CN raster by using ArcHydro Tools
Analyze the results
The following video walks you through these steps:
Other useful resources
Download and Process Land Use and Land Cover Data for Yor Watershed
Understanding the land use and cover characteristics is essential for accurate modeling, and we'll walk you through the step-by-step procedure to obtain and prepare the necessary data. Whether you're a beginner or looking to enhance your modeling skills, join us as we unlock the potential of NLCD data in your hydrological and watershed analyses.
Curve Number Determination Process by Land Use and Soil Data Using ArcGIS Pro Built-in Tools
The Curve Number (CN) method is a widely used method in hydrology to estimate the amount of stormwater runoff from an area. The CN method is based on the characteristics of the land use, soil type, and antecedent moisture conditions of a particular area. The CN method is commonly used in the design of stormwater management systems, flood control projects, and watershed modeling.
Note that the CN values used in ArcGIS Pro are typically taken from tables or equations that are based on empirical data. The specific CN values used will depend on the land use, soil type, and antecedent moisture conditions of the area being analyzed.
Natural Resources Conservation Service (NRCS)-Curve Number (CN) Method Theory
The Curve Number Method (or simply CN Method) was developed by Natural Resources Conservation Service (NRCS), formerly known as Soil Conservation Service (SCS). It is an empirical method used in hydrology for predicting rainfall excess and infiltration. Below are 4 videos that help you understand the theory behind this popular method (watch in order):
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Unit Hydrograph Theory and Application
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