EVALUATING DECISION CRITERIA FOR IRRIGATION SCHEDULING OF COTTON

ABSTRACT Irrigation scheduling methods using three threshold canopy temperatures (28 C, 30 C, and 32 C) were compared with soil water balance methods that replaced depleted soil moisture weekly (SWR) or at approximately two-week intervals (AUS) and a treatment that received only preplant irrigation (DRY). The largest quantity of water applied (rainfall plus irrigation) was to SWR (138 cm). The quantities for AUS (75 cm) and 28C (70 cm) treatments were similar, followed by 30C (46 cm) and 28 C (70 cm) treatments were similar, followed by 30C (46 cm), 32C (36 cm), and DRY (18 cm). Water used by these treatments was 133, 75, 78, 52, 46, and 29 cm, respectively. Volumetric soil profile moisture content of 0.28 in the SWR and AUS treatments at the end of the season showed little depletion while the 28 C, 30 C, and 32C treatments were lower at 0.24. Average canopy temperatures during the period DOY 187-274 when net radiation exceeded 200 w/m^ were 26.0, 26.8, 27.8, 24.1, 25.3, and 28.9 C, respectively, for 28 C, 30 C, 32 C, SWR, AUS, and the DRY treatments. Above 600 w/m^, the difference between canopy and air temperature increased in the 30 C, 32 C, and DRY treatments indicating water stress. Lint yield was positively related to water applied and water used, except for the SWR treatment. Rate of boll maturity was highest in the three threshold canopy temperature and DRY treatments. Cotton fiber length was decreased by water stress, micronaire value was reduced by low temperature, and fiber strength was reduced by either high water stress or low canopy temperature. Irrigation scheduled by threshold canopy temperature permits automated decision-making of both application time and amount.