Submitted to: Irrigation Association Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 12/6/2016
Publication Date: 12/6/2016
Citation: Evett, S.R., O'Shaughnessy, S.A., Colaizzi, P.D., Schwartz, R.C. 2016. Relating soil available water fraction to water stress indices. In: Technical Program Proceedings - Agriculture Sessions. Irrigation Association Show and Technical Conference, December 5-9, 2016, Las Vegas, Nevada. p. 280-289.
Interpretive Summary: Fresh water resources for agriculture and irrigation are diminishing because of decreases in supply and competition for other uses; therefore irrigation application needs to be as efficient as possible. Variable rate center pivot irrigation systems are being offered by all major manufacturers to improve irrigation efficiency, but there are no practical decision support tools to support management of these systems. Soil water sensors are too costly to install in sufficient numbers. Crop water stress maps based on crop leaf temperature can be made with relative ease; but maps of crop stress do not help decide how much to irrigate, only where and when. Scientists at the USDA ARS laboratory at Bushland, Texas, investigated the relationship between easily sensed and mapped crop leaf temperature and soil water content. The relationships were sometimes not strong, and the nature of the relationships changed during the growing season. The scientists did learn that simple soil water content is likely not useful for building a relationship with crop temperature. The next step is to convert soil water content to soil water potential that relates better to crop water stress.
Technical Abstract: Thermometric infrared sensing of production agricultural fields is becoming more commonplace using satellite, aerial and proximal remote sensing platforms, including arrays of infrared thermometers mounted on moving irrigation systems. Using plant canopy temperature data for irrigation management has been thoroughly explored and working solutions have been implemented to trigger irrigations when plant water stress reaches a given threshold value. Using spatial canopy temperature data, maps of plant stress indices, such as the crop water stress index (CWSI), can be produced and used to trigger irrigation in those parts of a field in which the stress index exceeds the threshold, thus enabling variable rate irrigation (VRI). However, knowing the degree of plant water stress does not clearly translate into knowing the amount of irrigation to apply. Past research has shown a strong correlation between crop water stress index (CWSI) and stem water potential. The stem water potential is strongly correlated to the soil water potential in the root zone, although an exact analytical expression for the relationship is lacking, largely due to the dynamic nature of root growth and soil water redistribution in response to irrigation and precipitation. A strong relationship between CWSI and root zone soil water storage has been demonstrated during the latter part of the growing season in crops that were irrigated at different levels throughout the season, but the relationship earlier in the season was not strong in that earlier research, likely because the crop had not yet been severely stressed. At any rate, determining the soil water status before the onset of severe stress is the real objective and one not met by focusing only on soil water storage. Idso and Jackson demonstrated a relationship between the CWSI and the fraction of plant available soil water storage, not just the entire soil water storage. The present work involves a preliminary analysis of the relationship between fractional plant available soil water and CWSI measured at three levels of deficit irrigation of corn in 2016. The strength of correlations depended on the deficit level and the period of the season for which data were analyzed, and correlations were sometimes linear and sometimes nonlinear, particularly for stronger deficits later in the season. It appears that the effective soil water potential over the root zone will have to be computed in order to develop a stronger relationship with CWSI.