Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/29/2008
Publication Date: 11/1/2008
Citation: Li, L., Nielsen, D.C., Yu, Q., Ma, L., Ahuja, L.R. 2008. Evaluating the Crop Water Stress Index and its Correlation with Latent Heat and CO2 Fluxes Over Winter Wheat and Maize in the North China Plain. Agricultural Water Management. 97:1146-1155. doi:10.1016/j.agwat.2008.09.015. Interpretive Summary: The Crop Water Stress Index (CWSI) is easily calculated from measurements of infrared canopy temperature, air temperature, vapor pressure deficit, net radiation, and wind speed. This study found that CWSI was correlated with latent heat flux (crop evapotranspiration) and CO2 flux (photosynthesis) over wheat and corn canopies, and therefore should be valuable to schedule irrigations in the North China Plain when net radiation levels are greater than 300 W/m2.
Technical Abstract: Plant water status is a key factor impacting crop growth and agricultural water management. Crop water stress may alter canopy temperature, the energy balance, transpiration, photosynthesis, canopy water use efficiency, and crop yield. The objective of this study was to calculate the Crop Water Stress Index (CWSI) from canopy temperature and energy balance measurements and evaluate the utility of CWSI to quantify water stress by comparing CWSI to latent heat and CO2 flux measurements over canopies of winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.). The experiment was conducted at the Yucheng Integrated Agricultural Experimental Station of the Chinese Academy of Sciences from 2003 to 2005. Latent heat and carbon dioxide fluxes (by eddy covariance), canopy and air temperature, relative humidity, net radiation, wind speed, and soil heat flux were averaged at half-hour intervals. Leaf area index and crop height were measured every seven days. CWSI was calculated from measured canopy-air temperature differences using the Jackson method. Calculated values of minimum canopy-air temperature differences over a range of vapor pressure deficits were similar to values used to construct previously published empirically-determined non-water-stressed baselines, but only under high net radiation conditions (greater than 500 W m-2). Valid measures of CWSI were only obtained when canopy closure minimized the influence of viewed soil on infrared canopy temperature measurements (leaf area index was greater than 2.5 m2 m-2). Wheat and maize latent heat flux and canopy CO2 flux generally decreased linearly with increases in CWSI when net radiation levels were greater than 300 W m-2. The response of latent heat flux and CO2 flux to CWSI did not demonstrate a consistent relationship in wheat that would recommend it as a reliable water stress quantification tool. The responses of latent heat flux and CO2 flux to CWSI were more consistent in maize, indicating its usefulness in identifying and quantifying water stress conditions when net radiation was greater than 300 W m-2. The results suggest that CWSI calculated by the Jackson method under varying solar radiation and wind speed conditions may be used for irrigation scheduling and agricultural water management of maize in irrigated agricultural regions, including the North China Plain.