| description abstract | The complexity of agricultural water management practices has challenged researchers attempting to determine how hydrological pathways affect water and nitrogen (N) losses. Field experiments in a typical rice agricultural catchment in the Zhanghe Irrigation District in China were conducted in 2014 and 2015 to evaluate and quantify the connections between hydrological pathways and water and N losses. Four water management periods—namely, the irrigation period (IP), the rainfall period (RP), the normal-drainage period (DP), and the nonirrigation-drainage period (NP)—were defined based on climate conditions and artificial irrigation and drainage periods. The experimental results indicated that most water and N losses occurred in DP and RP and that lateral seepage and tailwater were the main sources of the observed drainage. Low average concentrations of ammonia N (NH4+─ N, 0.373 mg L−1) and nitrate N (NO3−─ N, 0.285 mg L−1) and low proportions of dissolved N (42.2–54.0%) were detected in association with green manure application, whereas the average concentration of total N (TN, 1.376 mg L−1) was relatively high, which resulted in slight N pollution in the studied agricultural area. The concentrations and proportions of dissolved N were greastest in DP and RP. The average daily mass fluxes of NH4+─ N, NO3−─ N, and TN were 0.027, 0.020, and 0.095 kg ha−1 day−1, respectively, and the magnitudes in DP and RP were significantly greater than those in IP and NP. The average concentrations of NH4+─ N, NO3−─ N, and TN were not correlated to drainage discharge according to a Pearson correlation analysis, whereas a power function can quantify the relationship between N mass fluxes and discharge during various water management periods. The average thresholds of drainage discharge, at which N export from paddy fields equalled to zero, were 45.2 m3 day−1 ha−1 for NH4+─ N, 51.9 m3 day−1 ha−1 for NO3−─ N, and 55.5 m3 day−1 ha−1 for TN. These findings indicate that controlling drainage discharge is important for the reduction of water loss and N loss from paddy fields. | |
