Submitted to: World Cotton Research Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: September 9, 1998
Publication Date: N/A
Interpretive Summary: Cotton is grown under both rainfed and irrigated production systems. The lack of water under either system results in water stress which reduces yield. By quantifying the degree of water stress corrective procedures such as irrigation can be applied or effect of the water stress on cotton yield can be estimated. An irrigated study of cotton used multiple irrigation levels where canopy temperatures and weather were measured at the field site. The canopy temperatures and weather factors were used to make daily estimates of water stress indicators that reflect the degree of water stress experienced by the crop. A stress index computed as a summation of time when the plant temperature was above its optimum temperature and a crop water stress index that used a well-watered reference plot were both sensitive to the water stresses resulting from the different levels of irrigation. These same stress indicators were also correlated with crop yield. Water stress indices that accurately indicate when a crop is experiencing a deficiency of soil water can improve crop production management to either maximize yield or increase the efficiency of water use where water resources are limited. Good production management will increase profitability and sustainability of cotton production.
Technical Abstract: Crop water stress evaluation is essential for accurate irrigation schedul- ing. Cotton was grown in 1997 using 4 irrigation levels where canopy tem- peratures (TC) were continuously measured with infrared thermometers. The water level treatments were WL1-rainfall, WL2-1/3 ET, WL3-2/3 ET, and WL4- 1.0 ET. This report compares the accuracy of different stress indices for describing crop water stress. The first method for estimating water stress was the daily summation of stress time (ST) when TC exceeded 28 deg C. The second method for estimating plant water stress was the Crop Water Stress Index (CWSI). The CWSI methods included the theoretical procedure, CWSI-T, and the empirical method, CWSI-E, that used computed TCmax value and a measured value to TCmin, as the TC in the highest water level treatment WL4. ST was compared for 3 daily periods - Midday, Daytime, and Entire Day, that were defined by threshold levels of net irradiance and air tempera- ture. Cotton ST for the Daytime and Entire Day periods increased as the amount of water applied declined and were significantly different among all water levels. The CWSI-T and CWSI-E values also declined as the quantity of water applied increased. CWSI-E was more accurate than CWSI-T because its stress values for the well-watered treatment WL4 were restricted to zero and values for all water levels were within the theoretical range from 0 to 1.0. CWSI and ST were linearly related with yield. The CWSI approach to characterizing water stress is a potentially accurate method for comparing across environments, but its use is more difficult than ST because of additional environmental factors that must be measured or computed. ST was a sensitive indicator of water stress but its consistency among years and geographical locations requires further evaluation.