High concentrations of ambient ozone significantly reduce yields of many important crops grown in California as demonstrated by numerous controlled studies over the past 40 years. Past efforts to model crop losses used this information to estimate the magnitude of yield loss using aggregated county wide statistics. The work reported herein expanded the methodological basis of the Crop Loss Assessment Program by using GIS technology to disaggregate the statistics used to estimate yield loss based on the geographic distribution of production areas for major commodities and interpolated (1/d2) ozone exposure indices based on 1991 and 1992 ARB ozone data. The analytical procedures used were the same in 1991 and 1992, except that in 1992, SUM06 models were used in addition to 7-hour and 12-hour seasonal mean models for estimating the yield losses for several crops. Additionally, the graphic representations of estimated yield losses were enhanced by color coded altitudinal ramping for better topographic definition.
Ozone concentrations on a monthly basis were interpolated within state air basins using ARB air quality statistics and an imposed 2,000 foot altitudinal barrier to transport. Monthly 7-hour means, a widely used exposure index for plant response functions, were used for the statewide interpolations. Ozone concentrations were highest during summer months when seven-hour means in the southern San Joaquin Valley were comparable to those observed in parts of the South Coast Air Basin.
The intensity of potential yield loss was determined using ARB air quality data and published yield response functions. Areas where potential yield losses occurred were delimited by the location and extent of irrigated farmlands within an agricultural region. Results were graphically displayed to illustrate geographic variability. Using interpolated ozone exposure indices, for example, potential yield losses for cotton grown in the San Joaquin Valley ranged from less than 10% to almost 30%.
Statewide crop by county estimated yield losses in 1991 and 1992 were comparable. For most crops, loss estimates varied less than 20% between years. The variability was not consistent, in that losses were higher for come crops while lower for others when the two years were compared. No discernible regional trends between years were apparent.
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