Agbemabiese et al 1(1): 2017 1-12 1 Validation of Aquacrop for Different Irrigation Regimes of Onion (Allium Cepa) in Bontanga Irrigation Scheme Agbemabiese, Y. K., Shaibu A-G., Gbedzi, V. D. Department of Agric. Mech. and Irrigation Technology, Faculty of Agriculture, University for Development Studies, P. O. Box TL 1882, Tamale. Corresponding Author: yayra92@gmail.com ABSTRACT 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. INTRODUCTION Irrigation needs for onion, like any other crop are location-specific. The crop water requirement of onion is not evenly spread over the growing season, but depends largely on a number of factors, including the specie, growth stages, soil properties and climatic conditions. Ghanaweb Business News (2012) disclosed that about five million dollars is lost annually from the importation of onions from Niger and Burkina Faso to Ghana, which can be produced locally. Farmers in the Kumbungu District cultivate various crops by guessing the available moisture content of the soil by means of observation and feeling methods (Abagale and Tetteh, 2011). One of the major drawbacks with this approach is that the estimation of soil moisture is subjective (Schneekloth et al., 2007). In this regard, excess irrigation water supply may not only result in a yield decrease, but may also in turn produce other negative effects, such as leaching of nutrient and rise of groundwater. This situation requires modelling tools that support management decisions with regard International Journal of Irrigation and Agricultural Development IJIRAD Irrigation Science, Engineering & Development ISSN: 2616-1508 IJIRAD 1(1) [2017] 1-12 http://www.ijirad.org ARTICLE INFO Article history: Received: November 27, 2017 Received in revised form: December 24, 2017 Accepted: December 18, 2017 Keywords: Crop, Evapotranspiration, Irrigation, Productivity, Requirement, Water