the concentration were highest (nearly 1 mg/l) during the spring season (March and April) (Fig. S4). Such seasonal increase of ammonium in the precipitation samples in spring have also been observed in d15N-NH4, reported by Russel et al. (1998) in the Chesapeake Bay region in the U.S.A., who attributed it to increased spring-time agricultural emissions. The additional input of ammonium from precipitation has a greater influence in SC2 (in spring during high flow) than during SC1 (during low flow in summer).