Assessment of Vegetative Phases and Optimization of Rainfed Rice Cycle Timing Under Climatic Risk in the Tonkpi Agroecological Zone, Western Côte d’Ivoire

Abstract

Aims: This study aimed to determine the vegetative periods and identify optimal sowing windows for rainfed rice in the Tonkpi agroecological zone of western Côte d’Ivoire, in order to reduce exposure to climatic risks and improve crop-cycle planning. Study Design: Agroclimatic assessment based on long-term climatic analysis using the Eldin climatic risk approach. Place and Duration of Study: The study was conducted in five localities of the Tonkpi Region (Man, Biankouma, Danané, Zouan-Hounien, and Sipilou) using a 40-year climatic dataset spanning the period from 1984 to 2024. Methodology: Water satisfaction probability curves were generated by comparing rainfall (P), potential evapotranspiration (ET₀), and ET₀/2. A climatic risk threshold of 25% was used to identify periods with sufficient moisture for vegetative development. The same threshold was applied to the heading–flowering phase, considered the most sensitive to water deficit. Sowing windows for 100-day and 120-day varieties were established based on the alignment between crop requirements and favorable periods of water availability. Results: Vegetative growth can be sustained from April to late October in Man, Biankouma, and Zouan-Hounien, and from April to mid-October in Danané. In Sipilou, the onset of favorable moisture conditions is delayed until late May due to a higher early-season water deficit. The flowering phase benefits from adequate water availability between late May and mid-October across most sites, with an earlier end in Danané and a later start in Sipilou. For 100-day varieties, optimal sowing occurs from mid-April to early August (late May in Sipilou). For 120-day varieties, sowing should occur earlier, between mid-April and mid-July, to avoid moisture decline during maturation. Conclusion: Sowing windows and vegetative phase durations vary across the Tonkpi Region due to localized climatic disparities. The findings provide evidence-based guidance for climate-smart planning and support the development of locally adapted cropping calendars to enhance the resilience of rainfed rice production systems.