Importance of subsurface fluxes of water, nitrogen and phosphorus from rice paddy fields relative to surface runoff

Abstract

Rice paddy fields pose a high risk of water pollutions for the surrounding waterbodies through surface runoff and subsurface fluxes. Compared to surface runoff, subsurface flux from rice paddy fields has received less attention and is still poorly quantified, mainly due to low-frequency measurements at field scale and limited modeling capability at regional scale. Here we proposed a simplified modeling approach to estimate the subsurface fluxes of water, nitrogen (N) and phosphorus (P) from rice paddy fields and examined their relative importance compared to surface runoff. This model was established based on the high-frequency field measurements over two rice growing seasons in central China and the extended datasets across the East Asia. Two-year site-based observations indicate the significance of subsurface fluxes of water (737–785 mm season−1), N (28.5–40.0 kg N ha−1) and P (0.7–4.3 kg P ha−1) compared to those of surface runoff (178–199 mm season−1, 4.5–12.9 kg N ha−1, 0.5–2.6 kg P ha−1). Our regional estimations in the East Asia reveal that subsurface fluxes from rice paddy fields were comparable with surface fluxes, primarily controlled by the magnitude of seasonal precipitation. Subsurface fluxes were the dominant pathway of nutrient losses in drier rice cropping areas, while surface runoff was the more important process in wetter areas. In the light of the regional differences, we suggest that a spatially flexible set of policies for mitigating nutrient losses from rice paddy fields would be beneficial for the future water-quality improvements in the surrounding waterbodies.