Atmospheric deposition, mass balances, and processes regulating streamwater solute concentrations in mixed-conifer catchments of the Sierra Nevada, California

Solute concentrations in atmospheric deposition and stream water were measured from 1984 through 1993 to determine the fate and mobility of solutes in two gauged mixed-conifer catchments (Tharp's and Log creeks) located in the Sierra Nevada, California. The two catchments contain mature forest stands dominated by Abies concolor (white fir), Sequoiadendron giganteum (giant sequoia), Abies magnifica (red fir) and Pinus lambertiana (sugar pine). Ammonium, Cl-, Ca2+ and NO3- were highest in concentration of the solutes measured in wet deposition; bulk deposition was highest in SO42-, NH4+, Cl- and H+. Net retention of H+, NO3-NH4+, SO42- and Cl- occurred in both catchments. Discharge was dominated by spring snowmelt with the largest export yields for acid neutralizing capacity (ANC), SiO2, and Ca2+. Export yields of H+, NO3-, NH4+ and PO43- were relatively small (0.5 kg ha(-1)). Discharge-concentration relationships for ANC, SiO2, Na+, K+, Ca2+ and Mg2+ were inverse and their concentrations in stream water were primarily influenced by discharge and annual differences in the relative contributions of snowmelt and groundwater. The mobility of these solutes is controlled by the rates of mineral weathering and ion exchange. The positive relationship of SO42- concentration with increasing discharge suggests that atmospherically deposited SO42- is temporarily stored and that its release is controlled by the extent of soil water flushing.