Actual Evapotranspiration Model Based on the Irrigation Volume of the Maize Fields on Alfisols

Evapotranspiration data are considered important to determine volume and schedule of the irrigation. The purpose of this study is to determine the actual evapotranspiration model based on the volume of the irrigation to obtain an accurate evapotranspiration value on Alfisols with maize plantation. This research is conducted in the experimental field Jumantono subdistrict, Karanganyar regency by the experiment of the maize (Zea mays) on Alfisols. The evapotranspiration model uses the soil correction factor (x) and the irrigation volume (% ETc). The soil correction factor (X) is calculated by linear regression on actual evapotranspiration (ETa) with crop evapotranspiration (ETc). ETc using reference evapotranspiration (ETo) using the Penman-Monteith model. The results showed that ETa was smaller than ETc in all treatments. The models that can be produced in this study are 3 models. All models applied to produce a determination coefficient > 90%, which all models have a positive relationship. The best actual evapotranspiration model was in total model uses ETa = {0.0403 + (0.0085 × Irrigation volume)} × ETc, for daily estimation and total one planting estimation; weekly estimation using the weekly model using ETa = {0.4428 + (0.0054 ×Irrigation volume)}× ETc. The errors of both models are ± 1%.