Simulated irrigation water productivity and related profit effects in U.S. Southern High Plains cotton production

To explore management practices that increase irrigation water productivity (IWP) in U.S. Southern High Plains (SHP) cotton production, the CROPGRO-Cotton crop simulation model was used to evaluate the yield, IWP, and profit effects of irrigation amount and timing. Using 2005–2019 weather input data from 21 SHP weather stations, lint yields were simulated for each of the 315 station-years under unirrigated ‘dryland’ conditions and 18 increasing total irrigation (TIRR) levels. As TIRR was increased to 55.9 cm median lint yields asymptotically approached a maximum. However, irrigation above 35.6 cm increased the incidence of total irrigation plus growing season rainfall exceeding 100% of potential crop ET, leading to decreasing marginal yield effects and decreasing IWP. The highest median IWP (0.321 kg m−3) was found with both 33.0 and 35.6 cm of total irrigation, with 30.5 cm providing slightly lower IWP (0.320 kg m−3). In analyses of irrigated profitability under varying lint price and pumping cost conditions, 30.5 cm (12.0 in) of irrigation increases profits relative to dryland conditions under all but low lint price and high pumping cost conditions. But as TIRR is reduced the probability of these positive profit effects become similar to an evenly weighted coin flip at about 17.8 cm (7.0 in). Simulations that varied the timing of 30.5 cm of irrigation increased median IWP up to 0.434 kg m−3 by limiting irrigation to cotton’s reproductive and maturation periods, with no irrigation during the initial vegetative period. As a result, these simulations show that applying 30.5–35.6 cm (12.0–14.0 in) of irrigation during cotton’s reproductive and maturation phases, with little or no vegetative irrigation, maximizes IWP in SHP cotton production under current climate conditions.