Horizontal and vertical water and solute fluxes in paddy rice fields

Abstract

Water scarcity is an increasing problem in water-consumptive irrigated rice landscapes. This study quantified water losses in paddy rice fields due to vertical percolation through the plough pan and due to lateral water fluxes within the puddled layer. The average infiltration rates (geometric mean) for three paddy fields with a cultivation history of 3, 20 and 100 years were 28.0, 0.79 and 0.16 cm/day, respectively, demonstrating a strong dependence of the infiltration rate from the age of the field. Puddling reduced the percolation rate about 35-fold after 20 years and 175-fold after 100 years of paddy cultivation, confirming the importance of maintaining an undisturbed, permanent plough pan in order to increase water use efficiency. Lateral flux experiments revealed that horizontal preferential flow drove water and solute fluxes in the cracked topsoil of paddy fields over a flow distance of 50 cm, indicating high water losses during land preparation even with an existing plough pan. Lateral preferential transport of water and solutes towards unpuddled spots within the field or towards permanent cracks or root macropores in the plough pan is facilitated.