Spatial distribution of Iowa vineyards

From 2004 to 2011 the number of Iowa vineyards grew from 165 to 301. In 2011 the wine and vineyard industry had a 234.3 million dollar impact on Iowa’s economy. A spatial evaluation of current vineyard sites may assist with management of vineyards; lead to new generalizations that can be used to create prescriptive models for selecting sites for future vineyards. To achieve these goals we developed 3 spatial questions: 1) How are vineyards distributed in Iowa? 2) Are there statistically significant clusters of vineyards? 3) What are site characteristics of vineyards?

We created maps of existing Iowa vineyards beginning by geocoding addresses of vineyards from extension specialists client lists, and government agencies. Political boundaries, land cover, and infrastructure data were obtained from the Iowa Department of Natural Resources via the Iowa Geospatial Data Clearinghouse. We used ArcGIS, GeoDa, and Microsoft Excel to analyze the data and run statistical analyses.

The number of vineyards per county was calculated and a box plot map showed higher concentrations of vineyards across the center of the state, particularly around larger populations, and revealed three upper outliers. While regression analysis of county population to the number of vineyards in the county showed r²=0.20. A test for spatial autocorrelation using univariate Moran’s I showed I = 0.32, two counties that are clusters of low-low (counties with low numbers of vineyards surrounded by counties with low numbers of vineyards), and four counties that are clusters of high-high (counties with high numbers of vineyards surrounded by counties with high numbers of vineyards) at pseudo p = 0.001.

Data was extracted from raster layers depicting topography and land cover to compare current vineyard, recommended, and typical Iowan site characteristics. Vineyards are more likely to be located near a river. Sixty-six percent are within 2.41 kilometers compared to 49% of Iowa land, Chi square = 47.65 at p=0.0001. We found that while 57% of vineyards are located on recommended eastern or southern aspects and 33% on moderate slopes of 5-15%, there are no statistical differences between the percent of vineyards located on these slopes and the percent of total Iowa land matching these conditions. Meaning these results could be an artifact of Iowa’s landscape, not deliberate site planning. These results are likely due to a number of reasons including suitable conditions for growing grapes, criteria for other uses such as facility rentals, and competition from other land uses.