Effects of regulated dry season irrigation on tree water use, root zone moisture dynamics and yield of cacao in a rainforest zone of Nigeria

Abstract A field trial was conducted to investigate the effects of regulated dry season irrigation on tree water use, root zone moisture dynamics and yield of cacao in a rainforest zone of Nigeria. Following cessation of rainfall in November, irrigation commenced from December 2017 to May 2018. Irrigation amount was computed based on cumulative class A Pan evaporation. Irrigation treatments were coded as IrT1, IrT2 and IrT3, consisting of water application using EPan *Pan coefficients (Kcp) of 1.0; 0.70 and 0.50 (9.6, 6.8 and 4.8 l/tree/day). Irrigation water applied at 5-days interval was discharged via point source emitters (2.8 l/h discharge rate ) on drip lines laterally installed per row of trees. Irrigation requirements were on the average, 4.49, 3.14 and 2.44 mm, total water applied per irrigation events were 1009.88, 706.92 and 504.94 mm per plot ( 225 m2 ), total seasonal water applied were 33858, 23701 and 16929 mm, and soil moisture contents were 52, 45 and 28% for the respective IrT1, IrT2 and IrT3. Tree evapotranspiration (ETc) were 4.54, 3.19 and 2.32 mm/day while seasonal sums were 809, 566 and 404 mm while the ratio of ETc to EPan were 0.9, 0.69 and 0.53 for IrT1, IrT2 and IrT3. Tree water use efficiencies were 0.3 and 0.04 t/mm for Y/ETc and 0.16 to 0.19 kg/mm for Y/Irrigation respectively. Cacao pod and bean yields were 35.4, 22.1 and 10.3 t/ha and 2.29, 1.37 and 1.03 t/ha while yields decreased by 60 and 40% under IrT3 and IrT2 compared with IrT1. The study identified suitable Pan coefficients for scheduling irrigation during the dry season for cacao, full irrigation (EPan*1.0) applied at 9.6 l/tree/day will be needed to replenish soil water depletion to satisfy crop consumptive water use ( transpiration and soil evaporation components). The low pressure gravity-drip irrigation system alleviated climate stress during the dry season and improved cacao performance in a tropical rainforest environment.