EFFECTS OF DEFICIT IRRIGATION ON YIELD AND QUALITY OF ONION CROP

Abstract

There is shortage of onions in Uasin Gishu and Nandi counties during dry season from October to March when the demand is high. Rainfall during the period is inadequate for crop development. This study aimed at testing Deficit Irrigation technology as an appropriate irrigation management strategy that could improve crop water productivity and give optimum Onion crop yield. A field trial was conducted in Nandi County with drip irrigation system and six irrigation treatments replicated three times in a randomized complete block design. Full supply of crop water requirement to meet 100% ETc (T100) acted as a control. The crop was subjected to five stress levels T90, T80, T70, T60 and T50 at vegetative and late season growth stages. Establishment and yield formation stages were given adequate water to meet normal crop water demand (ETc). The treatments were protected from receiving extra water from the rain. The yield, biomass, quality and irrigation water use efficiency were determined. The data collected were statistically analyzed using ANOVA. The variation in yield ranged from 34.4 ton/ha to 18.9 ton/ha and that of quality from 64 mm to 35 mm diameter for T100 and T50 respectively. The treatments T90, T80, T70, and T60 gave yields of 33 ton/ha, 32 ton/ha, 25 ton/ha and 23 ton/ha with corresponding bulb diameter of 60 mm, 58 mm, 53 mm and 40 mm. Water stress of 20% led to optimum yield with water saving of 10.7%. The results obtained from the field trial were used to calibrate and validate the performance of AquaCrop Model using separate data sets. Statistical indices, Model efficiency (E), root mean squared error (RMSE), coefficient of residuals (CRM) and coefficient of determination (R2) were used to evaluate the performance of AquaCrop model in simulating yield, biomass, canopy cover and soil moisture parameters. The model performance statistical index was found for R2 as 0.912 for canopy and 0.798 for soil moisture in confirming model calibration. Similarly, the index (R2) for confirming model validation for canopy and soil moisture was 0.892 and 0.616 respectively. The model was applied to derive full (T100) and deficit (T80) irrigation schedules for three weather regimes from October-March growing seasons between 2003 and 2012 giving rise to 34 and 30, 38 and 34, 45 and 40 irrigation events each of 13 mm respectively. It was concluded from the results that deficit irrigation (DI) at vegetative and late growth stages significantly influenced yields. DI influenced the size and size distribution of fresh Onion bulbs significantly (Fcalculated = 96.28, Fcritical = 3.12). However, it did not significantly affect the shape of onion. AquaCrop model performance in simulating yield, green canopy cover and soil moisture declined at higher stress levels. The model is useful in developing irrigation schedules for different weather regimes that can be applied by farmers through extension services. It was recommended that the model be used to simulate yield at lower stress levels and adopted for irrigation scheduling by farmers and field extension staff. DI technology should be adopted for optimum yield and maximum water productivity.