This study was conducted in semi-arid area of Muvumba valley, Nyagatare District, Eastern Province of Rwanda. This work was formulated with the aim of assessing the effect of supplementary sprinkler irrigation on growth and yield of French beans (Phaseolus vulgaris l.) in semi-arid Muvumba valley, Rwanda. The irrigation treatments were EM (early in the morning), EL (late in the evening) and ELE (both early in the morning and late in the evening) and each treatment was replicated three times in a randomized complete block design (RCBD) for two seasons (Dry-Wet and Wet-Dry seasons). Soil water content was monitored with gravimetric method at 0.30 m depth every time irrigation was undertaken and CROPWAT 8.0 was used to estimate the crop evapotranspiration. The results obtained show that the treatments had significantly different effects on plant height, leaf width, leaf length, leaf area and leaf area index. Reduced water application due to evapotranspiration (ET) from 252.7 mm/dec during Dry-Wet season to 223.8 mm/dec during Wet-Dry season increased water use efficiency (WUE) during Wet-Dry season. The highest water use efficiency of French bean was 1.637 Kg/mm, irrigated early in the morning and late in the evening (ELE) during Wet-Dry season, and the lowest water use efficiency of French bean was 0.969 Kg/mm irrigated once only in the morning during Dry-Wet season. The results obtained also show that French bean growth parameters and yield were significantly different (p < 0.01) and were highest in both the seasons when the crop was irrigated twice a day (ELE) than when irrigated either early in the morning (EM) or late in the evening (LE). In both seasons, the highest yield of French bean was 366.8 Kg/ha with a net income of 98,414 Rwandan Franc equivalent to 103.5 USD. This yield was obtained when the crop was irrigated twice a day (ELE) during the Wet-Dry season. The results also indicated that the Wet-Dry season (yields of total irrigation regimes) had the highest yield (1,338.7 Kg/ha) with the net income of 320,443 RWF (337 USD) than. Irrigating early in the morning and late in the evening (ELE) using supplemental sprinkler irrigation system increased WUE and yield than either irrigation in the morning only (LE) or in the evening only (EM). The study recommends that it is financially viable to irrigate twice a day because it is possible to obtain higher return as compared to irrigating in the morning or in the evening only

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