Long-term trend of rice cultivated area, yield, and yield gap relative to the national target in Khuzestan province, Iran, with emphasis on soil salinity in downstream areas (2010-2025)

Introduction: Khuzestan province ranks fourth in Iran in terms of rice cultivated area, owing to its fertile lands and abundant rivers. However, the province faces severe water limitations, increasing soil salinity, and growing competition over water resources. The yield gap is considered an effective tool for identifying opportunities to increase production without expanding cultivated area. This study was conducted with the objectives of: (1) analyzing the 15-year trend (2010–2025) of rice cultivated area and yield across different counties of Khuzestan; (2) quantifying the yield gap; (3) determining the contribution of each county to the total provincial yield gap; and (4) investigating the relationship between soil salinity and rice yield.

Materials and Methods: Annual data on rice cultivated area and paddy yield for 27 counties of Khuzestan province over 15 cropping years were extracted from agricultural statistics. The target yield was determined based on the National Food Security Document of Iran. The trend of cultivated area was assessed using simple linear regression. The yield gap relative to the national target per unit area was calculated as the difference between the target yield and actual yield. The total production yield gap was obtained by multiplying the yield gap per unit area by the average cultivated area. In calculating the averages, only years with active cultivation were included. The Pearson correlation coefficient was calculated between mean soil salinity and rice yield for seven downstream counties of the Karun, Karkheh and Zohreh-Jarrahi basins.

Results and Discussion: The 15-year average rice cultivated area in Khuzestan was 94166 ha, showing a significant increasing trend of 6350 ha yr⁻¹ (p < 0.01). The highest annual increasing slopes were observed in Shushtar (+1730 ha yr⁻¹), Karkheh (+1549 ha yr⁻¹), Ahvaz (+836 ha yr⁻¹), and Bavi (+692 ha yr⁻¹). The 15-year average paddy yield was 4225 kg ha⁻¹, with the highest yield of 4996 kg ha⁻¹ recorded in 2019–2020 and the lowest of 3330 and 3552 kg ha⁻¹ in 2010–2011 and 2022–2023, respectively. Yield trends were stable in 16 counties, decreasing in three counties, and increasing in four counties. The provincial yield gap relative to the national target was estimated at 809 kg ha⁻¹ (equivalent to 16% of the target yield), reaching 2047 kg ha⁻¹ (42%) in Andika county. However, due to the negligible cultivated area of Andika (59 ha), its contribution to the total production yield gap was only 0.3%. Approximately 64% of the total yield gap (68230 tons) was concentrated in five counties: Shushtar (22.7%, 15554 tons), Ahvaz (16%, 11261 tons), Shush (11.6%, 7915 tons), Hamidiyeh (7%, 4785 tons), and Bavi (5.9%, 4053 tons). A strong negative correlation (r = -0.79, p < 0.01) was observed between soil salinity and rice yield in the downstream counties of the Karun and Karkheh and Zohreh–Jarrahi basins. In Hendijan county, with salinity exceeding 12 dS m⁻¹, the yield dropped to 3800 kg ha⁻¹. These findings indicate that rice cultivation in Khuzestan is highly sensitive to short-term water management policies and climatic events, and that soil salinity is one of the most important yield-limiting factors in downstream areas.

Conclusion: Despite severe water constraints, the rice cultivated area in Khuzestan increased by 6350 ha yr⁻¹ over the 15-year period, primarily driven by dam water releases for electricity generation and short-term economic incentives. The greatest potential for increasing production through reducing the yield gap relative to the national target is concentrated in Shushtar, Ahvaz, Shush (Karkheh), Hamidiyeh, and Bavi counties. A 50% reduction in the yield gap in these five counties alone could produce an additional 22000 tons of paddy rice annually without expanding the cultivated area.