Crop water productivity models are important tools in evaluating the effect of different irrigation regime on crop yield. AquaCrop model is a crop water productivity model adopted by the Land and Water Division of FAO in the year 2009. It simulates yield response to water for herbaceous crops, and it is particularly suitable in addressing conditions where water is a key limiting factor in crop production such as in northern Ghana. The objective of this study was to calibrate the AquaCrop model for different irrigation regimes for onion (Allium cepa), to determine its effect on crop growth and yield parameters of the crop at the Bontanga irrigation scheme. To achieve these, the Randomised Complete Block Design (RCBD) was used on Red Creole onion variety. RCBD was made up of four irrigation treatment regimes, 117%, 100%, 80% and 60% crop water requirements (CWR) of onion, with five replicates. Results indicated that there was no significant variation in yield, dry bulb biomass and total biomass, but there was difference for dry leaf biomass of onion at 0.05 significance level. The AquaCrop model simulated satisfactorily the crop yield, biomass and evapotranspiration water productivity of onion. There was a strong correlation and a significant linear relation between the simulated and measured crop yield, biomass and evapotranspiration water productivity. Validation of AquaCrop model using Nash-Sutcliffe efficiency (E), Root mean square errors (RMSE) and index of agreement (d) showed that, AquaCrop model can be used to simulate CWR of bulb crops, such as onion.
The study evaluated the effect of irrigation regime on soil properties and onion yield at the Bontanga irrigation scheme. Randomised Complete Block Design was used on Red Creole onion variety using four treatments: 117%, 100%, 80% and 60% of the crop water requirement of onion with five replicates. The results indicated that mean bulb weight ranged from 3.167 to 4.213 t/ha with no significant difference observed among the various treatments (P > 0.05). However, the irrigation regime of 117% recorded the highest yield, with the yield decreasing with decrease in water application rate. There were significant differences between the soil moisture content at different weeks after transplanting for the various treatments. There was no significant difference among treatments for soil pH and nitrogen, but there was a decline in soil pH and nitrogen with decrease in water application. There was a significant increase in soil Potassium and Phosphorus contents between initial and after irrigation regimes of 60 and 117%. There was no significant difference among treatments for organic carbon content. After application of the various irrigation treatments, there was a significant increase in soil magnesium and calcium contents with respect to the initial contents of the soil, but with no significant difference among treatments. There was also a significant difference between each treatment and the initial cation exchange capacity. Farmers should adopt the minimum 60% irrigation regime in order to save water, while increasing yield. The study results suggested vegetable farmers could apply only nitrogen fertilizers or organic manure since phosphorus and potassium are available through irrigation.
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