Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 27, 2012
Publication Date: March 7, 2012
Citation: O'Shaughnessy, S.A., Evett, S.R., Colaizzi, P.D., Howell, T.A. 2012. A crop water stress index and time threshold for automatic irrigation scheduling of grain sorghum. Agricultural Water Management. 107:122-132. Interpretive Summary: The improvement of irrigation scheduling using a plant feedback method has been successful with continuous crop canopy measurements. In this study, a theoretical Crop Water Stress Index (CWSI) was summed over daylight hours and investigated for its effectiveness as a trigger for automatic irrigation scheduling of two varieties of grain sorghum. Crop biomass and dry grain yield responses from the automatic treatment plots compared well to those from manual scientific irrigation scheduling based on soil water content, in the highest irrigation treatment level (I80%) in 2009 and in all irrigation treatment levels in 2010. Soil water variability early in growing season 2009 affected yield production, and may have affected the results at the two lower irrigation amounts (I50% and I30%). The constant irrigation applications in the amount of twice peak daily crop water usage on a frequent basis with LEPA drag socks was a favorable method for irrigation delivery to both early and late maturing grain sorghum. Although this automated irrigation scheme performed well, there are some disadvantages associated with it, which are common to all thermal-based indices using radiometric sensors, are that early in the season, false positive irrigation triggers may be generated. Such frequent events could lead to over irrigation. Another disadvantage is that cloud cover can reduce irrigation triggering at any time during the growing season, and IRT temperature readings can be influenced by changing crop aspect (e.g., leaf orientation and erectness, head formation) throughout the growing season. On the whole, this study indicates that using a CWSI calculated over daylight hours can be an effective method for scheduling irrigations since it produced similar results to scientific irrigation scheduling using a neutron probe. While farmers are not likely to use a neutron probe to take soil water measurements, they may invest in moving sprinkler systems that are outfitted with sensor networks for automated control and continuous plant water status feedback as a means to manage irrigation scheduling. Further research is necessary to investigate whether the CWSI-TT is effective for irrigation scheduling of other crops in this region and to determine which decision support methods would best augment plant feedback irrigation scheduling.
Technical Abstract: Variations of the Crop Water Stress Index (CWSI) have been used to characterize crop water stress and schedule irrigations. For the most part, this thermal-based stress index has been calculated from measurements taken once daily or over a short period of time, in both cases near solar noon. A method of integrating the CWSI over a day was developed to avoid the noise that may occur if weather prevents a clear CWSI signal near solar noon. This CWSI-TT index was the accumulated time that CWSI was greater than a threshold value (0.45); and it was compared with a time threshold (CWSI-TT) based on a well-watered crop. We investigated the effectiveness of the CWSI-TTindex to automatically control irrigation of short and long season grain sorghum hybrids (Sorghum bicolor (L.) Moench, NC+5C35 and Pioneer 84G62); and to examine crop response to deficit irrigation treatments (i.e. 80%, 55%, 30% and 0% of full replenishment of soil water depletion to 1.5-m depth). Biomass, grain yield, water use efficiency (WUE), and irrigation water use efficiency (IWUE). Results from automated irrigation scheduling were compared to those from manual irrigation based on weekly neutron probe readings. In 2009, results from the Automatic irrigation were mixed; biomass yields in the 55% and 0% treatments, dry grain yields in the 80% and 0% treatments, and WUE in the 80%, 55%, and 0% treatments were not significantly different from those in the corresponding Manual treatments. However, dry grain yields in the 55% and 30% treatments were significantly less than those in the manual control plots. These differences were due mainly to soil water variability in the beginning of the growing season. This conclusion is reinforced by the fact that IWUE was not significantly different for 30% and 55% treatments, and was significantly greater for Automatic control at 80%. In 2010, there were no significant differences in biomass, dry grain yield, WUE, or IWUE for irrigation control methods when compared across the same amount treatments. Similar results between irrigation methods for at least the highest irrigation rate indicates that the CWSI-TT method can be an effective trigger for automatically scheduling irrigations for grain sorghum in a semi-arid region.