Cotton Irrigation Timing with Variable Seasonal Irrigation Capacities in the Texas South Plains

<abstract> <b><sc>Abstract. </sc></b>Within the Ogallala Aquifer Region of Texas, the irrigation capacity (IC) for a given field often changes within a growing season due to seasonal depletion of the aquifer, in-season changes in crop irrigation needs in dry years, or consequences of irrigation volume limits imposed by irrigation district rules. Irrigation planning is further complicated by erratic seasonal rainfall. A field experiment was conducted from 2010 to 2013 to determine cotton ( L.) irrigation productivity using a range of ICs common to the Texas South Plains during three irrigation periods. The treatments included in-season ICs (maximums of 0, 3.2, and 6.4 mm d^-1) in combination with irrigation periods determined by accumulated growing degree days (GDD_15.6) and were designated as period 1 (P1= emergence to 525 GDD_15.6), period 2 (P2= 525 to 750 GDD_15.6), and period 3 (P3>750 GDD_15.6). Combinations of these factor levels resulted in 27 irrigation treatments with applications made by the low energy precision application (LEPA) method. Annual rainfall totals ranged from 137 to 557 mm over the four years. In all years, results indicated that attempting to store water in the soil profile, or irrigating in excess of the cotton evapotranspiration rate, early in the growing season reduced seasonal irrigation water use efficiency (SIWUE), and sometimes yield, compared to treatments with limited or no early irrigations. Treatments with 0 and 3.2 mm d^-1 ICs during P1 used up to 20% less seasonal irrigation with minor yield loss (<2%) compared to treatments with ICs of 3.2 and 6.4 mm d^-1, respectively. This was attributed to water losses caused by evaporation in the region (high wind, low humidity, and high temperatures) from May through June as well as excessive early season vegetative growth resulting in elevated crop transpiration later in the season that could not be met with available IC. Cotton irrigation during the period beyond accumulative cotton GDD_15.6 = 750 was critical for acceptable irrigated yield and high SIWUE. These results provide information to optimize cotton water use in an area with declining irrigation capacity, an advective climate, and increasing pumping restrictions.