G rowing of the grape requires careful consideration of the microclimate of the selected location. Growing conditions are determined largely by the solar radiation received at the site. The sun is the primary energy source for practically all biological processes on the earth. Solar radiation has a direct affect on the air and soil temperature, transpiration, soil moisture and atmospheric humidity, etc.

The amount of solar radiation received at a site varies widely with slope and aspect of the area. Particularly in hilly and mountainous areas. The total growing season solar radiation can be measured in Mega Joules per square meter received through the period of April first to October 31th. The accumulated seasonal solar radiation tends to be lower on slopes facing north, north east and north west and higher for the slopes facing south.

Figure (3). Table of solar radiation variation with respect to Slope and Aspect of the Okanagan valley (1987).

The weather data archive for the state of California is available online from the university of California's web site. Unfortunately the number of stations with sensors for detecting solar radiation in this region is limited to 3 stations and the website does not provide any information about the slope and aspect of these stations. So, to generate a table like the table in Figure 3, since the state of California and the province of British Columbia are approximately at the same geographical longitude I used the the variability of the solar radiation with changes in the slope in Okanagan valley (Figure 3) as reference and interpolated a solar radiation table for Napa County based on that.

To interpolate I found the average seasonal solar radiation for the slopes of less than 10 degrees in Summerland - Okanagan. This average is the same as the average seasonal solar radiation for flat areas in Okanagan. From that I computed the percentage difference in the value observed in each field of the table in Figure 3 and the Average Seasonal solar radiation of the flat areas in Okanagan. The calculated values are summerized in Figure 4.

Figure (4). The percentage of variation between the average seasonal solar radiation of flat areas and the values in table of Figure 3.

Average of Seasonal Solar Radiation of Summerland in flat areas = 3856.75

Finally I calculated the expected seasonal solar radiation for Napa County using the table in Figure 4 and the average seasonal solar radiation for years between 1995 to 2002 for Napa-C station (1) which is located at the heart of the Napa valley and is known to be at a flat area (no aspect).

Figure (5). The expected values of the solar radiation in the Napa county with changes to Slope and aspect of the area.

To map the table in Figure 5, I first generated the slopes of the Napa using the 3m DEM of the county and the 3D Analyst extension of the ArcGIS. Then I used the reclassify command of the spatial analyst of the ArcGIS to classify the slopes (0-10,10,20,20-30,30-40,40-50,50-90). I applied the same procedure again but this time to compute the aspect map.

Using the the raster calculator of the spatial analysis extension of the ArcGIS I generated a map from slopes and aspect layer that satisfied the conditions in Figure 6.

Figure (6). Solar radiation suitability classes

This classification is based on the classification used to generate the Atlas of Suitable Grape Growing Regions of Okanagan Valley (1987).

Click here to see the final map

Growing Degree Suitability, Soil Drainage Suitability, Final Results