This report presents a study of crop coefficient (Kc) and yield response factors (Ky) for onion crop cultivated under deficit irrigation and different mulch covers in Samaru, Northern Nigeria. The field experiments were conducted at the irrigation fields of the Institute for Agricultural Research (IAR) irrigation field in 2008/2009 and 2009/2010 irrigation seasons using surface irrigation method. The experiments consisted of 16 treatments in each season. They comprised of four levels of water application depths (25, 50, 75, and 100% of weekly reference evapotranspiration (WRET)) and four levels of mulching (no-mulch, using rice straws, black and transparent polyethylene materials). Water applied per irrigation and soil moisture contents before and after irrigation was monitored throughout the seasons, while onion bulbs were harvested at the end of season and weighed. Average daily crop water use (crop consumptive use) were estimated from the soil moisture content using the soil moisture depletion method, while daily reference evapotranspiration (ETo) were computed from weather data using the FAO-Penman Montieth method. Crop coefficient values (Kc) were computed as the ratio of crop water use to ETo. The water stress coefficients (Ks) were computed by relating crop coefficient of the fully irrigated treatments to the deficit irrigated treatments. The yield response factor (Ky) were obtained by relating relative yield decreases to relative crop water use deficits. The study showed that crop water use of the onion crop decreased with increase in irrigation deficit. Applying water at 50% WRET reduced peak consumptive use of the onion crop by about 20%, while applying water at 25% WRET reduced crop water use by about 40%. Kc values of fully irrigated treatments ranged from 0.39 to 1.15, while Kc values of the deficit irrigated treatments varied from 0.24 to 1.13. Mulch materials did not significantly influence crop coefficient, but deficit irrigation did. Kc decreased with increase in water deficit with resultant water stress coefficients (Ks) ranging from 0.59 to 0.96. The relative yield decreases of the onion crop were proportionally greater with increase in evapotranspiration deficit for both mulched and no-mulch conditions. However, the proportional decrease in yield under the no-mulch condition was much higher than the mulched condition. Among the mulch materials, the proportional decrease in yield in the polyethylene materials were over 10% lower than the rice straw mulch. The Ky of the onion crop under no-mulch condition was 1.15, while Ky values for the mulched treatments were 1.13, 1.00 and 1.05 for rice straw-, white polyethylene-, and black polyethylene- mulch, respectively. The crop coefficients and yield response factors developed in this study are reliable as they had similar trends in two seasons, and could be used in irrigation design and scheduling for onion in the study area.
Key words: Onion, deficit irrigation, mulching, crop coefficient (Kc), yield response factor (Ky), crop water use.
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