Effect of Periodic and Continuous Irrigation on Water Transport through Porous Media

Abstract

In this paper, we compare the effect of two modes of irrigation on water transport through an unsaturated porous medium. The two modes studied are (1) periodic and (2) continuous irrigation. The former process consists of two stages, i.e., imbibition and drainage. The entire volume of water is assumed to be added during the imbibition stage at a constant rate. The time during which imbibition occurs is taken as a fraction of the total time period. In the drainage stage, no fresh water is added and the water inside the soil redistributes itself and drains from the soil. In the continuous mode, water is added at a constant rate during the whole time period. To ensure a fair comparison, the rate of water addition in the continuous mode is kept the same as the average rate of water addition over a time period in the periodic mode that includes the drainage and imbibition steps. The recharge is calculated as the volume of water drained from the bottom of the soil during a time period. The transport of water in the unsaturated zone is studied in the presence of water uptake by plant roots. The two modes of operation were simulated using a mass conservative algorithm based on a modification of Picard’s iterative scheme. Predictions in the periodic mode were performed using direct simulation and the results obtained were compared with an algorithm based on a shooting method. The performances of these two modes have been evaluated by calculating the recharge amount. When the sink term due to the plant roots was included, it was found that the recharge is significantly higher for the case of periodic operation. A physical explanation for the results obtained is proposed. The effect of hysteresis in the water retention curves was simulated using an empirical method. We have found that the total recharge amount in the periodic operation calculated using a mean nonhysteretic curve is very close to that obtained when we include the effect of hysteresis.