Principal Investigator: Randall Mutters
University of California, Davis
March 1998

ARB Contract No. 94-345 (Full Report)

ABSTRACT

Numerous controlled studies over the past four decades demonstrated that high concentrations of ambient ozone significantly reduced yields of many important crops grown in California. Past efforts to model crop losses on a regional scale used plant response models from these studies and aggregated ozone exposure indices to estimate yield losses at the county level. The work reported herein not only employs this traditional approach (important for long-term trend analysis), but also expands the methodological basis of the Crop Loss Assessment Program by usig GIS technology to estimate the yield loss based on interpolated (1 /d2) ozone exposure indices. Analytical procedures used 7-hour and 12-hour seasonal mean models, and SUM06 Wiebull functions for estimating the yield losses in several crops. Interpolated yield loss contours are graphically represented and enhanced by color-coded altitudinal ramping.

Ozone concentrations on a monthly basis were interpolated within the state air basins using ARB air quality statistics and an imposed 2000-ft 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 the summer months when 7-hour means in the southern San Joaquin Valley were comparable to those observed in parts of the South Coast Air Basin.

The severity of potential yield loss was determined using ARB 1993 air quality data and published yield response functions. Statewide crop by county estimated yield reductions ranged from less than 1% in fresh market tomato to a high of over 30% for cantaloupe in 1993. Countywide estimated yield losses for 15 out of 20 crops in 1993 were higher than in the previous two years (1991 and 1992). The increase in estimated yield reductions between 1991 and 1993 ranged from a 2% increased loss in lemon to 56% in processing tomato.

For selected major crops, results were graphically displayed to illustrate the variability that occured within individual counties and production zones. Yield loss projections were confined to areas delimited by the location and extent of irrigated farmlands within an agricultural region. Using interpolated ozone exposure indices, for example, potential yield losses for cotton grown in Kern County ranged from less than 15% in the Buttonwillow area to more than 30% near Arvin. In contrast, the aggregated county wide yield loss for cotton was estimated at 25%. The latter technique, although useful, lacks the resolution to describe locally relevant variations in ozone effects.

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