Screening Production Strategies for Declining Irrigation Capacity and Predictable Climate Conditions.

The declining saturated thickness of the Ogallala Aquifer beneath the Southern High Plains decreases irrigation well capacity and necessitates deficit irrigation and better precipitation use. Precipitation varies seasonally but also in response to the El Niño–Southern Oscillation (ENSO) of sea surface temperatures, resulting in predictable precipitation patterns in much of North America. This may provide insight into growing season conditions for refining irrigation management strategies. Our objectives were to i) simulate cotton lint yields in response to initial soil water, emergence date, and irrigation rate and duration combinations for La Niña, neutral, and El Niño ENSO phases, and ii) optimize variable application deficit irrigation strategies for those phases. The simulation model GOSSYM was used to calculate growth and yield of cotton emerging on day 145, 152 and 159 from soil with 50 or 75% initial available soil water for all combinations of irrigation duration (4, 6, 8, and 10 weeks) and rate (dryland plus 2.5, 3.75, and 5.0 mm d^-1 applied weekly). Using ANOVA methods, where years were random effects, we determined that lower initial soil water and delayed emergence consistently decreased yield by 15 to 20 % per week for all ENSO conditions. Growing season precipitation and thermal energy varied with ENSO phase, with the drier and warmer La Niña fall periods extending the growing season slightly. Limited rain during the La Niña phase produced the poorest lint yields compared with wetter neutral and El Niño phases. Yield optimizing irrigation strategies that focused water resources were best suited to drier La Niña phase conditions and limited initial soil water and irrigation applications. Net lint yield was optimized during El Niño phase years by using uniform deficit irrigation (water spreading); whereas, neutral phase years favored focused application strategies as the initial soil water or irrigation amount decreased.