Spatial Characteristics of Drought

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To characterize the spatial distribution of drought throughout the North Central Region, the HPR was associated with the geographic center of each county. ArcView GIS software (ESRI 1999) was used to interpolate a surface of HPR based on the spatial association between points using the inverse distance weighting (IDW) algorithm and a spatial moving average by associating 12 nearest neighbors to interpolate values of HPR between the 1055 points. Surface grid maps of HPR were developed based on monthly values of HPR at each location, and maps of cumulative HPR from May to July were produced. Two maps based on HPR were developed using this method: one shows the 20-year mean (figure 4.7b) and one shows only 1988 values (figure 4.7a). The higher HPR values (i.e., HPR > 15), based on the 20-year mean, occurred primarily in the western third of the region. However, in 1988, high values of HPR (HPR >30) occurred in central Michigan, Illinois, and Iowa, and in parts of Minnesota and North Dakota.

The magnitude of the difference between the 20-year mean HPR for each month (May-August) and the corresponding1988 HPR values is expressed spatially in figure 4.8 by computing the percent difference between the two spatial data set grids. The following equation was used to compute the difference:

Average HPR

Figure 4.7 Spatial distribution of (a) the 1988 May-July HPR for the time interval (19721991) and (b) the average May-July HPR.

Average HPR

Figure 4.7 Spatial distribution of (a) the 1988 May-July HPR for the time interval (19721991) and (b) the average May-July HPR.

This computation was applied to grid maps for May, June, July, and August. Differences, expressed as a percentage, illustrate the patterns of the 1988 HPR deviation from the 20-year mean, with the darker portions of the maps representing the maximum percent differences for May (600%), June (1600%), July (700%), and August (600%). Note the corresponding areas for each of the classes. The month of June 1988 shows the greatest potential plant stress, particularly in the eastern portions of the NCR.

July August

Figure 4.8 Percent change in HPR relative to 1988.

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