Submitted to: Irrigation Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/18/2005
Publication Date: 5/1/2006
Citation: Wanjura, D.F., Upchurch, D.R., Mahan, J.R. 2006. Behavior of temperature-based water stress indicators in biotic controlled irrigation. Irrigation Science. 24:223-232. Interpretive Summary: The BIOTIC method for timing irrigation has demonstrated the capability of producing different soil water regimes for crop production using measured canopy temperature. A critical component in the method is a temperature-based index identified as stress time (ST) that monitors the water stress level of the crop and produces the signal to irrigate. Stress time is the accumulation of time when the crop's canopy temperature is warmer than the optimum temperature of the crop. A 3-year study was conducted using subsurface drip irrigation and ST to determine when to apply irrigation. Stress times between 5.5 and 8.5 hours/day were evaluated. Other temperature indices (daily average canopy temperature, the difference between canopy and air temperature, and a relative crop water stress index) were computed and compared with ST to evaluate their sensitivity to crop water status and their correlation with cotton yield. The ST index was more highly correlated with amount of irrigation over the 3-years than the other temperature indices. In addition only ST demonstrated a common relationship for estimating cotton yield across the 3-years. These results indicate consistency of ST for controlling multiple irrigation regimes under the different growing environments present among years.
Technical Abstract: A 3-year cotton subsurface drip irrigation study used continuous measurement of canopy temperature to determine signals for irrigation control. Timing of irrigation applications was controlled by the BIOTIC protocol which used a combination of stress time (ST) and a crop specific optimum temperature threshold to indicate water stress and the associated need for irrigation. Stress time (ST) is the cumulative daily quantity of time while cotton canopy temperature exceeds 28 °C which is the optimum temperature for cotton. Different values of ST generated varying numbers of irrigation signals that produced different irrigation regimes. Time thresholds (TT) varied from 5.5 to 8.5 hours in 1-hour increments. The objective of this investigation was to examine the degree of association among ST, daily average canopy temperature, the difference between canopy temperature and air temperature (Tc ' Ta), and the relative crop water stress index (RCWSI) including their relationship with cotton lint yield. The data analyzed was collected as 15-minute averages and analyzed for the irrigation period from DOY 187 to DOY 243 each year. The number of irrigation signals was linearly related with TT with an estimated change of '10.2 and -8.7 irrigation events per 1-hour increase of TT in 2003 and 2004. There was a significant curvilinear relationship between TT and the average daily stress on days with irrigation signals and those without irrigation signals across years. Daily average canopy temperature increased linearly with ST within years for days when irrigation signals occurred; however, the level of correlation was non-significant in most instances. The percentage of positive daily (Tc-Ta) values during the irrigation season, an indication of water stress, increased with TT level in all years. Mean daily (Tc-Ta) values also increased with daily canopy temperature. The rate of change for RCWSI, referenced to the 5.5 hr TT treatment, per 1-hour increase in TT value was 0.24 and 0.42 in 2003 and 2004. Daily ST was the only temperature index having a common relationship with lint yield across all years which estimated an average decline of 343 kg lint/ha for each 1-hour increase of ST for days with irrigation signals during the irrigation period. ST, mathematically the most simple of the canopy temperature-based parameters, provided the most consistent estimate of crop water stress and correlation with lint yield. The power of ST to characterize water stress effects on crop productivity evolves from being an integrated value of time while canopy temperature exceeds a physiologically based threshold value.