Analysis of water movement in paddy rice fields (II) simulation studies

Part (I) of this study presented field and laboratory experimental data on soil characteristics and hydraulic parameters that are related to water infiltration in rice paddy fields. Part (II) includes a series of numerical simulations that are based on the in situ data, which three-dimensional finite element computer model for simulating density dependent flow and transport (FEMWATER) produced to identify and evaluate the controlling factors in the vertical percolation/lateral seepage processes. As well, ground water recharge that resulted from the fieldsʹ infiltration is estimated. FEMWATER was first compared with one-dimensional simulation algorithm for water flow in aquatic habitats, as well as experimental data that were in good agreement. Simulation results indicated that the plow sole was the major factor that controls infiltration rate. Notably, infiltration is primarily a vertical downward flow within flooded areas. Significant lateral movement occurred only at wet to dry boundaries. The length of the wet to dry boundary, the area of the flooded paddy rice fields, and the difference in initial water content between flooded and dry field soils determined the ratio of lateral seepage to vertical infiltration. If surrounded by dry land, simulated effective ground water recharge was estimated at 50–76% of the total infiltration within a typical 48 ha rice paddy. However, this was reduced to 39–71% within a 12 ha rice growing area. Breaking the hard pan enhances the infiltration rate but also increases the seepage ratio. Thus, it was concluded that a flooded rice paddy performs a significant function in ground water conservation. Experimental and simulation results elucidate water movement mechanisms in rice paddies and clarify the vertical and horizontal flow processes within an unsaturated region. This work also provides a scientific basis to re-evaluate water resource management in regions under irrigated rice.