Storm Event Exports of Dissolved Nutrients in a Large Agricultural River Basin in Arid Northwestern China

Abstract:

We evaluated watershed exports of dissolved organic matter (DOM) and inorganic C, N, P nutrients at storm flow and baseflow conditions in the Heihe River Basin (HRB). The HRB is the second largest inland river basin in China, and it is situated in an arid region where the annual precipitation is around 190mm and evaporation is ca. 1,700mm. In spite of the water scarcity, HRB supports a population over two millions relying heavily on irrigated agriculture. Storm events are concentrated in June and July in HRB, accounting for more than 65% of annual precipitation and leading to a considerable amount of overland flows that mobilize inorganic and organic nutrients from terrestrial landscape. In order to understand the impacts of storm events on river hydrology and biogeochemistry, we collected surface and hyporheic water along a 160 km river reach during a large storm event and under base flow conditions, in addition to groundwater sampling near the study reach. We found that storms mobilized top-soil DOM and ammonium, as shown by increases in the concentrations of DOC and ammonium as well as the proportions of soil-derived, humic-like DOM compounds in river water. For the study year, the loading of DOC, ammonium, and soil-derived humic fluorescence compounds were over 10 times higher during storm events than at baseflow. In contrast, dissolved inorganic carbon (DIC), nitrate/nitrite, and phosphate showed lower concentrations during storm events, and storm flow loads accounted for about 50% of the annual values. Based on mass balance calculations using δ¹³C-DIC and δ¹⁸O values of riverine water and groundwater, storm events can reduce the proportions of groundwater discharging to the river by three times. Together, our data demonstrate that storm events in arid regions have significant impacts on the sources and amount of riverine nutrients, and managing event-runoff associated impacts should be incorporated in regulatory efforts in this large agricultural basin.