ARS | Publication request: Establishing Differential Irrigation Levels Using Temperature-Time Thresholds

Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 1, 2003
Publication Date: October 20, 2003
Citation: Wanjura, D.F., Upchurch, D.R., Mahan, J.R. 2003. Establishing differential irrigation levels using temperature-time thresholds [abstract]. Transactions of the ASAE. 20(2):201-206

Interpretive Summary: Center pivot irrigation systems using low pressure discharge of water at the soil surface and subsurface drip irrigation systems are gaining wide acceptance because of their capability to achieve improved control of frequency and amount of irrigation. To achieve the potential benefits of these systems, a dependable and useable method of timing irrigation is needed. A common symptom of plants which need water is high leaf temperature. Infrared thermometers were used to continuously measure canopy temperature of cotton and the cumulative time that temperature remains above specific temperature thresholds (stress time) can be used to indicate the need for irrigation. A method for selecting specific values of stress time (time thresholds) that produces different water input levels with irrigation timing was delineated. First, a linear relationship was identified between stress time and lint yield, then a linear relationship was established between stress time and total water or irrigation input, and finally one-hour differences in stress time were judged to produce sufficiently large differences in total water and irrigation input to produce differences yield. For a semi-arid environment time thresholds of 330, 390, and 450 min/day above 28 °C for cotton were selected to produce significant differences in irrigation and cotton yield. The choice of a time threshold depends on the combination of available irrigation water supply and the desired level of lint production.

Technical Abstract: The requirements of precision crop management or limited water supply requires that either plant water status be varied to optimize crop performance or a continuous level of limited irrigation must be practiced to match irrigation water availability. Canopy temperature (TC) has been 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 signal 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 from irrigation studies that collectively measured the water application and yield response of cotton to a range of average daily ST were analyzed. All irrigation studies included a common control TT of 330 min/day above a temperature threshold of 28 °C as criteria for the irrigation signal that maintained cotton in a well-watered status. In order to develop relationships between daily stress ST and final yield, or water input during the season, ST was calculated for each day 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, one-hour differences in average daily ST were selected to produce sufficiently large differences in total water and irrigation input to affect yield. These average TT values were 408, 468, and 528 min./day, and corresponded with control TT values of 330, 390, and 450 min/day. These control TT should result in different amounts of water application during the growing season and produce differences in cotton yield.