Modelling percolation and lateral seepage in a paddy field-bund landscape with a shallow groundwater table

Abstract

Percolation in farmland increasingly is needed to be quantified to improve water use efficiency and mitigate accompanying nutrient loss. Experiments were conducted to measure the soil properties and moisture in a consolidated paddy field-bund landscape during the rice growing season in 2014–2015. The HYDRUS-2D model simulated soil water movement in the landscape with an excellent performance. In the study, the soil properties of the bund were similar to those of the paddy field after land consolidation. Modelling results showed that most percolation in the paddy field occurred along with water input (rainfall and irrigation). Groundwater capillary rise was detected in the coupled conditions of low soil moisture in the root zone and the shallow groundwater table. Percolation accounted for 23.2%–31.3% of water input, and groundwater capillary rise contributed 26.1%–31.2% to rice evapotranspiration. The variation of the soil water content of the bund indicated that lateral infiltration from the paddy field to the bund primarily occurred in the upper bund adjacent to the field, while the lateral seepage was only found in the lower bund below the groundwater level. Vertical percolation and lateral seepage accounted for 10.5%–14.8% of water input lost through the bund, in which the lateral seepage contributed 77.6%–88.4%. Scenario simulation with the calibrated model investigated the impacts of saturated hydraulic conductivity (Ks) and the irrigation amount on the percolation and lateral seepage in the landscape. The results implied that percolation increased with the increasing Ks of soil. The percolation in the paddy-bund landscape and lateral seepage were most influenced by the illuvial horizon layer of the paddy field. Furthermore, a higher irrigation amount decreased the groundwater capillary rise and aggravated percolation linearly, while it had few effects on lateral seepage. The research suggests a potential adjustment in making irrigation schedules and conducting land consolidation in the rice-planting areas with shallow groundwater depths.