Spatial trends of rainfall and reference evapotranspiration (ETo) are crucial for crop water management and sustainable water resources management. Twenty-one years of climatic data were used to estimate ETo using FAO Penman Monteith method in Southeast Nigeria. Two spatial interpolation techniques, inverse distance weighting (IDW) and Kriging (K) in ArcGis were employed to distribute spatial monthly annual and seasonal rainfall and ETo data. Results showed that rainfall increased gradually from North to South while ETo increased from South to North of study area. ETo was found to be higher during dry seasons as average rise in temperature within study period stood at 1.1%. The coefficient of variation (CV) for monthly rainfall was found to be highest in December with a value of 66.6% and lowest in May with a value of 14.3%. Rainy and dry seasons had CVs of 20.7 and 45.7%, respectively. ETo had highest CV of 9.85% in March (dry season) and lowest of 4.30% in August (rainy season). There was no significant difference between IDW and K (observed and predictions) (P = 0.99 and 0.96, respectively). Average rainfall rate was higher than ETo during rainy season by 974 mm. However, dry season showed that ETo exceeded rainfall by 535 mm. The study suggests that the study area requires supplemental irrigation of 535 mm annually to sustain crop production. This study also provides background information on rainfall and ETo for efficient crop water and environmental management.
Keywords: Rainfall, evapotranspiration, spatial interpolation, inverse distance weighting, kriging