Soil Water Extraction of Bollgard® II Cotton in a Heavy-Clay Soil

ABSTRACT: Water scarcity is the main limiting factor in cotton (Gossypium hirsutum L.) production in Australia, and sustaining productivity and profitability with limited water is one of the biggest challenges facing the industry.  A good understanding of the magnitude, timing and spatial distribution of cotton soil water extraction is important for proper irrigation management, and for development of accurate crop models and decision support systems. The overall objective of this study was to evaluate the water extraction distribution of Bollgard^® II cotton under different irrigation regimes. Specific objectives were to quantify: (1) the depth of soil water extraction as a function of time, (2) the percent of seasonal water extraction from each soil depth, and (3) the relationship between depth of soil water extraction and canopy height. To meet these specific objectives, daily and seasonal cotton soil water extraction was determined from continuous records of water content in the soil profile measured from four irrigation treatments at a field experiment conducted at Kingsthorpe, Queensland. We found that cotton extracted soil water from as deep as 150 cm, but the percent of seasonal extraction sharply decreased with soil depth. The top 50 cm soil layer accounted for 75% of the seasonal extraction and the top 80 cm, for 90%.  We also found that from 32 days after sowing (DAS) to 100 DAS, the depth of soil water extraction increased linearly at a rate of 1.89 cm day^-1 or 2.36 times the increase in crop canopy height. These findings suggest that cotton producers should manage irrigations to maintain adequate moisture in the top 80 cm of the soil profile rather than relying on moisture stored deeper in the profile.