T he moisture regime of the natural soil is thought to be the most important attribute affecting vineyard establishment and maintenance (Roach and Arnold 1971). Drainage is influenced by many factors including texture, depth to impervious layer, slope and landscape position. Additional inferences can be made about soil permeability, runoff and seasonably high water tables. The general characteristics of the subsoil should be considered when a site is selected (Jordan et al. 1980). Tiling may improve soil drainage; however, the better the soil drainage the better the site is for grape production.

To find out more about importace of soil moisture read this article about soil moisture.

To generate the map of suitable soil drainage areas I downloaded the soil survey of Napa from the U.S Geological Survey website. The map is in 1:20,000 scale. Then I linked the table containing the soil drainage information to the soil survey parcels layer using the key "classcode" common on both the layer and the table. In the soil survey database the classifications for the soil drainage are as following.

The codes identify the natural drainage condition of the soil and refer to the frequency and duration of periods when the soil is free of saturation.

To classify soil drainage suitability I used the following ranking.

To classify the soil parcels to the classification above, first for each class I selected all the parcels with the matching drainage codes. Then I made a layer from selected features (parcels) and that left me with 4 layers, one for each class. Then for each layer I merged the the adjacent parcels using he ArcGIS GeoProcessing wizard. I then unioned all the parcels using an exta field with a unique value in each layer to get the final result.

Click here to see the final result.

Degree Days Suitability, Solar Radiation Suitability, Final Result