Optimizing sowing date to mitigate loss of growing degree days and enhance crop water productivity of groundwater-irrigated spring maize

Groundwater irrigation (GWI) decreases soil temperature and increases crop growth duration and water consumption. Optimizing sowing dates offers a cost-effective solution to mitigate these effects. This study evaluated five sowing date treatments for spring maize: GWI on April 20 (GW420), April 25 (GW425), April 30 (GW430), May 5 (GW505), and May 10 (GW510), with surface water irrigation (SWI) on April 20 (SW420) as the control. The evaluated parameters included soil temperature at 5 cm depth (T-5), soil-temperature-calculated growing degree days (GDD(s)), actual crop evapotranspiration (ETc-act), leaf area index (LAI), grain filling, grain yield, and crop water productivity (WPc). GW420 decreased daily maximum T-5 by 1.8 degrees C (P<0.05) and daily average GDD(s) accumulation by 5.9 % and increased the growth duration by 7.8 d and ETc-act by 33.2 mm compared to SW420. GW420 also delayed LAI growth and decreased the weight of maximum grain filling rate (W-max) and maximum grain filling rate (G(max)), reducing mean LAI (LAI(ave)) by 8.7 %, grain yield by 6.7 %, and WPc by 10.2 % (P<0.05). Late sowing compensated for GDD(s) loss in the GWI treatments, with the highest daily average GDD(s) accumulation observed in GW505 and GW510 (21.3 degrees C d(-1)), followed by SW420 and GW430 (20.2-20.3 degrees C d(-1)), and the lowest in GW420 and GW425 (19.1-19.4 degrees C d(-1)). Late sowing also shortened growth duration and decreased ETc-act, with GW510 showing a 13.9 d shorter growth duration and GW425, GW430, GW505, and GW510 exhibiting 30.6, 36.0, 57.6, and 70.2 mm lower ETc-act, respectively, than GW420. Moderately late sowing (GW430) enhanced G(max) and maintained the active grain filling period (T-agp). Late sowing increased WPc by 7.9 %, 16.8 %, 17.4 %, and 17.2 % in GW425, GW430, GW505, and GW510 (P<0.05), respectively, compared to GW420. While the grain yields of GW430 and SW420 did not significantly differ, GW430 had a higher WPc than SW420, indicating that moderately late sowing fully compensated for the decline in grain yield and WPc of groundwater-irrigated maize. Entropy-TOPSIS analysis revealed that GW430 is the optimal sowing date for groundwater-irrigated maize in arid regions of northwest China, offering a cost-effective method to mitigate GWI-induced GDD(s) loss and enhance WPc.