In this paper, we present a new procedure using an observer for estimating soil water content (SWC), a key parameter of soils when dealing with the adaptation of water plant requirements within the framework of irrigation. Results showed that the estimation of SWC by means of the observer was good. On the one hand, the estimation of the unknown part of the soil-plant water balance equation (negative component) has been found to be noise-free and values of evapotranspiration ET, drainage Dr and runoff Ro were estimated after filtering the simulated data. On the other hand, SWC has been found to increase after each water supply and decrease in the interval separating two successive irrigations. In addition, crop water requirements of avocado were found to be as much as 622 mm (for the 2016 dataset) and 380 mm (for the 2018 dataset) during the growing season from June throughout September. The findings of this research helped accounting for the net water requirements of avocado with the aim to optimize irrigation supply and provide guidelines to growers and farmers on sustainable irrigation of this crop under Lebanon’s Mediterranean climate and limited water resources.