LAUSR

In this study, nitrogen (N) loads in Day-Nhue River Basin (DNRB) in Vietnam were quantified (at river basin and province scale) via field observation and application of material flow analysis (MFA). By developing L(Q) curves, measured N loads were estimated from water level data and N content of rivers. N loads from DNRB to Tonkin Gulf were quantified by MFA. The gaps between MFA results and measurements were increased from 4.48 to 21.47% corresponding to the accumulative rainfall from year 2008 to 2010. Results illustrate that MFA can estimate potential N load across the river basin to surface water. Observed ranges in upstream and downstream river loads were different at 55–84% and 5–20% respectively, corresponding well with those estimated by MFA. MFA is effective at quantifying N load to surface water the downstream regions, which are mainly used for paddy agriculture. Original N sources were traced via MFA, identifying that decreasing chemical fertilizer application rates and pretreatment of drainage water before discharging to the surface water reduce N load from the DNRB to the surface water. Additionally, MFA identified a hidden flow in the upstream portion of the DNRB, which meant that measurement data were under-estimated.