USGS TerraWeb: USGS: Rainfall Mapping

Rainfall Mapping

Because of the importance of rainfall to eolian erosion, and global change in general, digital rainfall image generation and analysis is being investigated. Digital rainfall images of Arizona have been generated for all 12 months of 1980, 1981, 1982, and 1983 which correspond to normal, moderately dry, normal, and extremely wet years, respectively. The digital rainfall images were generated using monthly records available from the National Weather Service for approximately 150 gauge stations throughout the state of Arizona. The information was converted from tabular to raster/image format and surface interpolation was used to 'fill' in the gaps. Obviously, the larger the gaps the more 'guessing' had to be done to fill in the holes. The first pair of images shows the gauge locations in rasterized format and the surface interpolation results, which represents the final digital rainfall image. There were months that had missing readings at various gauges, so these images have holes that were not filled (the amount of surface interpolation was limited to not fill/guess beyond a certain spatial resolution). The digital rainfall image maps are being used to analyze spatial and temporal rainfall variability for normal, dry, and wet years. They will also be correlated with image products generated using remotely sensed multispectral images (e.g., images of vegetation change and rainfall during the period of vegetation change). The correlation will help with the analysis of the various parameters important for eolian mapping, vegetation change detection, and ecosystem monitoring. The images below show preliminary digital rainfall image examples and temporal differences/ comparisons. Also shown is a short temporal rainfall movie made using the 48 months of data, however, remember that the data used for this movie have been spatially compressed to keep the data volumes at an acceptable level within the World Wide Web.

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The two images above show the surface interpolation results. The image on the left shows the location and color-coded rainfall totals for the various rain gauges in Arizona for a six month period in 1983. The image on the right shows the interpolated results.

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These four images were generated by taking the individual monthly image results and computing a sum using the three winter rain months (January, February, and March) and the three summer/monsoon rain months (July, August, and September). The resulting images represent six month rainfall totals for 1983 (upper left/wet year), 1982 (upper right/ normal year), 1981 (lower left/moderately dry year), and 1980 (lower right/normal year). The colors represent a total rainfall range from 3 to 28 inches for the six month period shown.

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The digital image data can be used to compare differences in both the temporal and spatial distribution of rainfall. One method of doing this is by generating difference/change images between the various temporal image products. The two images shown above are color-coded difference images generated using the 1982/normal year differenced with the 1983/wet year (left) and then differenced with the 1981/moderately dry year (right). These image products show both where, and how much, the wet and moderately dry years differed from the normal year rainfall. The scale at the bottom gives the difference amounts in inches and the background color, which represents a difference of zero, can be used as a guide as to where the rainfall was more or less than the normal year. Notice that even during a wet year there are areas that are dryer than the normal year, as well as in a dry year there are areas that are wetter than the normal year. This shows that care must be taken when working with ground based instrument readings because regional wet-/dry-year ratings may not necessarily mean that those conditions exist at the instrumented ground site, or vice versa.

Choose a video format below to view a movie showing rainfall from 1981 to 1983.

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Once the monthly rainfall images are generated they can be analyzed and compared in any desired combination (e.g., in six month sums shown above). However, they can also be used on an individual monthly basis over a selected period of time to visually and statistically study and compare temporal and spatial relationships and changes in rainfall. As an example, the 48 months of rainfall data were all color coded to the same range (0 to 6 inches) and displayed in the short temporal movie shown above. This type of visualization can help in the analysis and comparison of temporal and spatial patterns and variations.

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Last Modified: Thu Oct 31 15:12:07 MST 2002