ESTABLISHING DIFFERENTIAL IRRIGATION LEVELS USING TEMPERATURE-TIME THRESHOLDS

The requirements of precision crop management or limited water supply requires that either plant water status be varied during the growing season to optimize crop performance or a continuous level of limited irrigation must be practiced to match irrigation water availability. Canopy temperature (TC) has been successfully used to time irrigation applications for well-watered crop growing conditions. The cumulative daily time that TC exceeds a crop-specific temperature threshold, designated as stress time (ST), is used to indicate the need for irrigation. Manipulation of the ST value required to indicate the need for irrigation changes irrigation frequency and seasonal irrigation. The ST value that generates the irrigation signal is the time threshold (TT). This study developed a procedure for estimating the relationship between TT for cotton and seasonal irrigation rates. Data were analyzed from irrigation studies that measured the water application and cotton yield response to a range of average daily ST. All irrigation studies included a common control TT of 330 min/d above a canopy temperature threshold of 28°C as the criteria for the irrigation signal that maintained cotton in a well-watered status. In order to develop relationships between daily ST and final yield, or water input during the season, daily ST values were calculated and averaged over the irrigation season. The same procedure was used to calculate the average TC. The procedure for identifying control TT to establish different crop water status levels included the following steps. First, a linear relationship was defined between average daily ST and lint yield. Second, separate linear relationships were established between average daily ST and total water or irrigation input. Third, based on the slope of the trend line between average daily ST and yield, 1-h differences in average daily ST were selected to produce sufficiently large differences in total water and irrigation input to affect yield. These average ST values were 408, 468, and 528 min/d, and corresponded with control TT values of 330, 390, and 450 min/d. These control TT should result in different amounts of water application during the growing season and produce differences in cotton yield.