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Nitrogen (N) accounts for more than 80% of the total mineral nutrients absorbed by plants and it is the most widely limiting element for crop production, particularly under water deficit conditions. For a comprehensive understanding of sunflower Helianthus annuus N uptake under deficit irrigation conditions, experimental and numerical simulation studies were conducted for full (100% ETC) and deficit (65% ETC) irrigation practices under the semi-arid conditions of the Imperial Valley, California, USA. Plants were established with overhead sprinkler irrigation before transitioning to subsurface drip irrigation (SDI). Based on pre-plant soil N testing, 39 kg ha−1 of N and 78 kg ha−1 of P were applied as a pre-plant dry fertilizer in the form of monoammonium phosphate (MAP) and an additional application of 33 kg ha−1 of N from urea ammonium nitrate (UAN-32) liquid fertilizer was made during the growing season. Soil samples at 15-cm depth increments to 1.2 m (8 layers, 15 cm each) were collected prior to planting and at three additional time points from two locations each in the full and deficit irrigation treatments. We used HYDRUS/2D for the simulation in this study and the model was calibrated for the soil moisture parameters (θs and θr), the rate constant factors of nitrification (the sensitive parameter) in the liquid and solid states (μw,3, and μs,3). The HYDRUS model predicted cumulative root water uptake fluxes of 533 mm and 337 mm for the 100% ETC and 65% ETC, respectively. The simulated cumulative drainage depths were 23.7 mm and 20.4 mm for the 100% ETC and 65% ETC which represented only 4% and 5% of the applied irrigation water, respectively. The soil wetting profile after SDI irrigation was mostly around emitters for the last four SDI irrigation events, while the maximum values of soil moisture in the top 30 cm of the soil profile were 0.262 cm3 cm−3 and 0.129 cm3 cm−3 for 100% ETC and 65% ETC, respectively. The 16.5 kg ha−1 (NH2)2CO (50% of the total N) that was applied during the growing season was completely hydrolyzed to NH4+ within 7 days of application, while 4.36 mg cm−1 cumulative decay was achieved by the end of the 98-day growing season. We found that 86% of NH4+ (74.25 mg cm−1) was nitrified to NO3− while 14% remained in the top 50 cm of the soil profile. The denitrification and free drainage of NO3− were similar for 100% ETC and 65% ETC, and the maximum nitrate was drained during the sprinkler irrigation period. By the end of the growing season, 30.8 mg cm−1 of nitrate was denitrified to N2 and the reduction of nitrate plant uptake was 17.1% for the deficit irrigation section as compared to the fully irrigated side (19.44 mg cm−1 vs. 16.12 mg cm−1). This reduction in N uptake due to deficit irrigation on sunflower could help farmers conserve resources by reducing the amount of fertilizer required if deficit irrigation practices are implemented due to the limited availability of irrigation water. [ABSTRACT FROM AUTHOR]