Development of a Soil-Moisture Based Plant-Stress Index for Site Drought Characterization.

Although the large number of dryland cotton (Gossypium hirsutum, L.) variety trials conducted each year allow producers to examine the yield response of varieties in similar growing conditions to their own, local trials may not fully express all varietal characteristics, specifically those of drought stress. A method to quantify experienced drought in these trials could serve as the framework on which to compile trail results across locations and thereby more accurately define varietal yield response to drought stress.  Therefore, the main objective of this research was to develop a soil moisture-based index to quantify drought stress in dryland cotton variety trials.  Five field trials consisting of a range of replicated varieties under water-deficit stressed conditions and a well-watered strip were conducted across the Cotton Belt during the 2012 and 2013 growing seasons.  Soil moisture in each plot profile was inferred by (4) Decagon 5TE sensors (Decagon Devices, Inc., Pullman, WA).  This data was then used to calculate an adjusted soil moisture stress index (ASMSI) and a relative reduction in evapotranspiration (1-(ET_{c adj}/ET_c)).  Results indicate both approaches appear to have potential in characterizing the amount of stress experienced within dryland variety trials.  The utility of these approaches will depend on the quality of the soil moisture and meteorological measurements collected to calculate these indices and the soundness of the assumption that yield is restricted by water-deficit stress.  More research should be conducted on reducing these location responses before maximum separation between varietal responses will be recognized.