DRAINAGE AND IRRIGATION EFFECTS ON COTTON PRODUCTION

Excessively wet and dry soil conditions can occur during the same year in the southeastern Coastal Plain of the United States. Water managem ent systems that provide both subsurface drainage during wet soil cond itions and irrigation during dry soil conditions are desired. Several water table management alternatives, possibly with surface irrigation such as microirrigation, could satisfy these needs. Three water table management (WTM) systems and microirrigation were evaluated for three cotton cultivars on a southeastern Coastal Plain soil during 1987-1989 . The WTM systems included controlled drainage-subirrigation (CDSI), c ontrolled drainage (CD), and subsurface drainage (SSD). All WTM system s had both surface microirrigation and rainfed treatments. Cotton cult ivars were Coker 315, DPL 50, and DPL 90. Seasonal rainfall, subirriga tion, and microirrigation amounts varied considerably during the three -year period. Water requirements for subirrigation in the CDSI system were high (1477 to 2841 mm), but neither microirrigation nor subirriga tion water requirements were closely related to seasonal rainfall amou nts. Cotton lint yields among WTM systems were significantly different in two of three years; yields for the CDSI system were lowest (836 an d 766 kg/ha) and yields for CD and SSD were highest (1022 and 942 kg/h a, respectively). Wetter-than-optimum soil conditions in all irrigated treatments, especially in combination with the CDSI system, probably caused the reduced yield. Microirrigation produced significantly great er lint yields than the rainfed treatments in the first two years of t he study (1127 and 1116 kg/ha versus 492 and 801 kg/ha), but not in th e last year (872 versus 874 kg/ha) when seasonal rainfall was kast of the three years but was better distributed. There were significant yie ld differences among cotton cultivars in two years, but no cultivar co nsistently produced the greatest or least yield. Cotton yield increase s obtained with these WTM system-microirrigation combinations suggest the need to control the water table closer to the soil surface in sout heastern Coastal Plain soils when surface irrigation is not used. The CDSI could provide a profitable management alternative if a water tabl e fluctuates near the soil surface much of the time, especially during the growing season. Where subsurface drainage is needed part of the y ear, it may be more profitable to use CD or SSD systems with surface i rrigation, especially when maintaining the water table near the soil s urface in CDSI systems requires a large water volume. However, the com bined cost of the subsurface drainage and microirrigation systems woul d be very high and might not be profitable for crops such as cotton.