Streamwater chemistry during rainfall events in a forested basin

Abstract

The hydrochemistry of streams has been measured during two water cycles in an experimental forested basin of 67.5 ha area. An itemized list of the water quality parameters includes pH, specific conductance (SC), Cl −, NH + 4-N, NO − 2-N, NO − 3-N, PO 3− 4-P, SO 2− 4, K +, Na +, Ca 2+, Mg 2+, and SiO 2. The chemical changes of streamwater measured during four rainfall runoff events allow to distinguish two types of solute element behavior. Until the peak flow, the concentration of each element decreases, due to a dilution effect by the surface runoff water. Whereas during the recession period the concentrations increase, following two major modes: (1) Type I, which shows approximately the same concentrations around the end of the runoff period as the baseflow level prior to the runoff event and (2) Type II, which overshoots the baseflow level. Type I includes SiO 2, SC, Cl −, Na +; and Type II, NO − 3-N, K +, Ca 2+, Mg 2+. During the recession period, the streamwater might be originating mainly from interflow, probably associated with surface soil water, therefore the soil water chemistry has been analyzed. SiO 2, originating from soil and bedrock showed nearly uniform concentration in the downward direction, and NO − 3-N concentration near soil surface was greater than that of deep soilwater and streamwater. Then two results correspond respectively to the runoff characteristics of Type I and Type II. The materials classified into Type II are revealed to be major essential elements closely involved in the metabolic activity of forest soil ecosystem and those of Type I to be minor or nonessential elements.