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Publication Abstract

Response of Corn and Soybean Yields to Precipitation Augmentation, and Implications for Weather Modification in Illinois Hollinger, S.E., and Stanley A. Changnon, Jr., 1993  Illinois State Water Survey, Champaign, IL,  ISWS B-73    Full Text Available

Illinois State Water Survey weather modification studies since 1970 have focused on developing a technology to enhance rainfall in Illinois and studying how that increased rainfall might impact Illinois agriculture and water resources. The effects of added rainfall on crop production were initially defined using weather-crop yield models. The availability of agricultural test plots where rainfall could be controlled offered an opportunity to better define the effects of added rain on major crops. The objective of this study was to determine how additional rainfall in a series of summers would impact corn (Lea mays) and soybean (Glycine max) yields in Illinois. The ultimate goal was to determine the feasibility of different weather modification programs in actual Illinois summers.

Two types of experiments were conducted each year from 1987 through 1991. In one experiment, corn and soybeans were grown in plots that could be covered to keep natural rain off the crops. The crops were exposed to the natural environment whenever it was not raining. During the months of June, July, and August, all natural rain was kept off the crops, and water treatments were applied at rates and on days that simulated typical dry, average, and wet summers, plus increases of 25 percent to the typical dry, average, and wet summer rain regimes.

In the other field experiment, corn and soybeans were grown in an open area under natural conditions where the plots received natural rainfall. At the end of each rainfall event, additional water was applied to the crop to simulate the effects of different weather modification capabilities. Ten different treatments, selected to reflect various potential cloud seeding capabilities, were applied: natural rainfall; 10, 25, and 40 percent increases to all rain events; 10, 25, and 40 percent increases to moderate rain events [rains greater than 2.54 millimeters (mm) up to 25.4 mm]; 10 and 40 percent increases to heavy rain events (rains greater than 25.4 mm); and 40 percent increases to light rain events (rains less than 2.54 mm).

Cultural practices used to grow the corn and soybean crops were typical of those used in eastcentral Illinois. At maturity the crops were harvested and final grain yield was determined. The yield components that made up the final yield were also measured and studied to determine their response to rainfall during different growth stages.

Additional water applications in the typical dry, average, and wet summers resulted in increased corn and soybean yields. Generally, as the total summer rainfall increased, the final yield increased. An unexpected finding in the study was that corn yields varied more between years than within years under the same water treatments. Soybean yields did not vary as much between and within years as did corn.

Analysis of the year-to-year corn yield variations showed that temperature during the period from planting to tassel initiation (the first 20 to 30 days after planting) was related to final yield. Warmer temperatures during this early growth period resulted in lower yields.

During the summers of 1989 to 1991, corn was planted on two different planting dates and at two plant densities. While population did not have an effect on final yield, corn yields from later planting dates were lower than those from earlier planting dates. Temperature during the early growth stage was correlated to the yields resulting from the two planting dates in the three years. Early growing-season temperature had a greater effect on final yield than did the amount of water received by the plant during the growing season.

The best rainfall enhancement treatment for corn in the open-area experiment, as determined by consistently high yields across all years of the experiment, was 40 percent additional water applied to light or heavy rains. The best treatment for soybeans was natural rainfall and 40 percent increases to all moderate rains.

Integration of these results with climatic, economic, and legal factors affecting rainfall enhancement showed that expected yield increases would be less than those calculated from the five-year open-area experiment. A 25 percent rain increase on days with moderate rains would amount to only 20 percent of the total yield increase observed in the open-area study. With a 25 percent rainfall increase on all rain days, the expected yield increase would be only 43 percent of the experimental increase. The reduction in yields from the experimental increases are due to limitations in weather modification techniques, i.e., the inability to take advantage of seedable events occurring at night and during severe weather conditions.

  1. Additional studies need to be conducted to determine the cause of the response of corn to early-season temperature. If the observed response has a physical or physiological explanation, an early-season estimate of final yield potential could be obtained and used in defining the appropriate weather modification program for each summer.
  2. The design of the rain shelter experiments did not allow for the identification of the stage of corn growth that results in the greatest yield benefit to the crop. Therefore, additional experiments need to be conducted to identify the optimal time to apply additional water to the corn crop.
  3. The 1987-1991 results of the open-area study do not encourage the use of cloud seeding to achieve major yield increases in the deep-soil areas of Illinois and the Corn Belt. The results of this research indicate that with current constraints relating to night seeding and severe weather conditions, only marginal benefits could be produced, except in the occasional drier summers.
  4. Existing climatological- or statistical-based techniques to predict regional summer-season rainfall (above normal, near normal, or below normal) should be used on a continuing yearto- year basis to decide on the need for cloud seeding. The capability of predicting the level of summer precipitation by early June is needed in deciding which rain treatment to employ. Knowledge of future hot dry conditions would call for seeding of all possible rain events. Knowledge of near-normal rain conditions would call for seeding only moderate to heavy rains during critical crop periods from late June to early August, depending on the crop's stages. Knowledge of an upcoming wet summer would preclude the use of any rain enhancement.
  5. Seeding techniques should be developed that allow for the delivery of seeding material into nighttime convective clouds.
  6. A cloud seeding technology needs to be developed that is capable of increasing summer rainfall by 25 percent or more.

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