Creating an Average Annual Rainfall Map Using CHRS Data: A Step-by-Step Guide

In this blog post, we will explore how to prepare an average annual rainfall map using CHRS data, specifically focusing on rainfall data from the years 2016 to 2020. This guide will walk you through the entire process, from downloading the necessary data to visualizing the results in ArcMap.

The CHRS (Climate Hazards Group InfraRed Precipitation with Station data) data portal provides access to various rainfall datasets. For our purpose, we will be using this portal to download daily rainfall data for a specific region in India, which we will then use to calculate the average annual rainfall.

Step 1: Downloading the Rainfall Data

To begin, you need to access the CHRS data portal. You can do this by searching for "CHRS data portal" on Google. Once on the portal, follow these steps:

1. Select the Data Type: Choose the type of rainfall data you need. Options include hourly, daily, monthly, and yearly rainfall data. For our analysis, we will select daily rainfall data.
2. Choose the Resolution: You can select different resolutions, such as 0.25° x 0.25° or 0.04° x 0.04°. For this example, we will use the higher resolution data.
3. Select the Region: You can download data for specific political divisions or entire countries. In this case, we will select Maharashtra.
4. Specify the Time Frame: Choose the years for which you want to download data. We will download data from 2017 to 2021.
5. Download the Data: After selecting the desired options, provide your email address and click on the download option. The data will be sent to your email.

Once you receive the data, extract the files and organize them in a folder for easy access.

Step 2: Setting Up in ArcMap

After downloading and extracting the rainfall data, the next step is to set it up in ArcMap:

1. Open ArcMap: Launch ArcMap and navigate to the Catalog window.
2. Add the Rainfall Data: Copy the path of the extracted rainfall data and paste it into the Catalog. Add the files to your map.
3. Define Spatial Reference: Right-click on each layer, go to properties, and set the spatial reference to WGS 1984 (EPSG: 4326).

Step 3: Calculating Average Annual Rainfall

To calculate the average annual rainfall from the five years of data, follow these steps:

1. Open Raster Calculator: In the ArcMap toolbox, navigate to Spatial Analyst Tools > Map Algebra > Raster Calculator.

2. Input the Formula: Use the formula to calculate the average:

   (Rainfall_2017 + Rainfall_2018 + Rainfall_2019 + Rainfall_2020 + Rainfall_2021) / 5

   Make sure to replace the layer names with the actual names of your rainfall layers.

3. Specify Output Location: Choose a location to save the output file, naming it "average_rainfall.tif".

4. Run the Calculation: Click OK to execute the calculation.

Step 4: Extracting Data for Specific Regions

If you want to focus on a specific district, such as Aurangabad, you can extract that area from the average rainfall map:

1. Add District Boundary: Import the shapefile containing the district boundaries.
2. Use Extract by Mask Tool: Navigate to Spatial Analyst Tools > Extraction > Extract by Mask. Select the average rainfall layer and the district boundary shapefile.
3. Save the Extracted Data: Specify a name for the output file and run the extraction.

Step 5: Increasing Resolution with IDW

To enhance the resolution of the rainfall data, you can convert the raster data to point data and apply the Inverse Distance Weighting (IDW) method:

1. Convert Raster to Point: Use the conversion tool to change the average rainfall raster to point data.
2. Project the Point Data: Ensure the point data is projected correctly, selecting UTM Zone 43 for Aurangabad.
3. Apply IDW Interpolation: In the Spatial Analyst Tools, select Interpolation > IDW. Choose the grid code as the input and specify the output file name.

Step 6: Visualizing the Rainfall Map

Once you have generated the high-resolution rainfall map, you can visualize it:

1. Change Symbology: Adjust the color scheme to represent different rainfall amounts effectively.
2. Create Map Layout: Set up the map layout by adding elements such as a title, north arrow, scale bar, and legend.
3. Export the Map: Finally, export the map in your desired format (JPG, PNG, or PDF).

Conclusion

Creating an average annual rainfall map using CHRS data is a straightforward process that involves downloading data, processing it in ArcMap, and visualizing the results. By following the steps outlined in this guide, you can generate accurate rainfall maps for any region of interest. If you have any questions or need further assistance, feel free to leave a comment below.