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In the semi-arid Haouz plain, located in central Morocco, agriculture still uses a tremendous amount of water and consumes about 85% of the available water resources. Therefore, accurate management of irrigation water is important to avoid both the water lost by direct soil evaporation and by deep percolation (DP) below the plant root zone. Estimating or measuring water demand (evapotranspiration) has been investigated by many studies in the Haouz plain, but DP has not been quantified yet in this area. It is one of the most important hydrological challenges in analyzing irrigation efficiency and understanding the potential contribution of irrigation returns to groundwater. Direct measurements of DP are difficult in both time and space. Therefore DP from irrigation is often indirectly estimated using the water balance equation. In this work, an orange site with drip irrigation technique was studied. DP was estimated by the water balance and then compared with the direct measurements of a flux-meter installed beneath the root zone. The obtained results showed that the variation of DP is the product of the combination of three factors: water supply, evapotranspiration and the changes in the soil water storage. The average value (about 15 mm week(-1)) of DP is obtained when the precipitation is absent and a high value (about 40 mm week(-1)) is recorded when the heavy rainfall occurs.