Evapotranspiration estimation by inverse soil water flow modelling

Crop evapotranspiration (ETc) and crop coefficient (K-c) estimation are important in irrigation scheduling. Evapotranspiration estimation is mostly done by Penman-Montieth method. Though Penman-Montieth method is adopted for drip irrigation, there are possibilities of uncertainty due to the non-availability of experimentally verified crop factor values. Hence the estimation of ETc directly in fields by installing soil moisture sensors is becoming one of the reliable methods. In the present work, soil moisture sensors were installed for paddy crop under drip irrigation. Richards' equation was used to represent the unsaturated flow in the root zone and a model namely Inverse Soil Water Flow Model (ISWFM) was developed. The average daily ETc and K-c for paddy were estimated using the model. The crop coefficient for drip irrigated Paddy estimated using ISWFM results were higher by 7-20% compared to the crop coefficient values reported in the literature (Allen et al. 1998). The developed model was validated by estimating soil water balance components in the modeled domain. Total water balance error of modeling was estimated to be 8%. The results proved that the soil moisture sensing methodology is useful in the determination of crop coefficient values.