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 (T₅), soil-temperature-calculated growing degree days (GDD_s), actual crop evapotranspiration (ET_c-act), leaf area index (LAI), grain filling, grain yield, and crop water productivity (WP_c). GW420 decreased daily maximum T₅ by 1.8°C (P<0.05) and daily average GDD_s accumulation by 5.9 % and increased the growth duration by 7.8 d and ET_c-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 WP_c 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°C d^–1), followed by SW420 and GW430 (20.2–20.3°C d^–1), and the lowest in GW420 and GW425 (19.1–19.4°C d^–1). Late sowing also shortened growth duration and decreased ET_c-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 ET_c-act, respectively, than GW420. Moderately late sowing (GW430) enhanced G_max and maintained the active grain filling period (T_agp). Late sowing increased WP_c 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 WP_c than SW420, indicating that moderately late sowing fully compensated for the decline in grain yield and WP_c 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 WP_c.