Atmospheric deposition and solute export in giant sequoia — mixed conifer watersheds in the Sierra Nevada, California

Abstract

Atmospheric depostion and stream discharge and solutes were measured for three years (September 1984 — August 1987) in two mixed conifer watersheds in Sequoia National Park, in the southern Sierra Nevada of California. The Log Creek watershed (50 ha, 2067–2397 m elev.) is drained by a perennial stream, while Tharp's Creek watershed (13 ha, 2067–2255 m elev.) contains an intermittent stream. Dominant trees in the area include Abies concolor (white fir), Sequoiadendron giganteum (giant sequoia), A. magnifica (red fir), and Pinus lambertiana (sugar pine). Bedrock is predominantly granite and granodiorite, and the soils are mostly Pachic Xerumbrepts. Over the three year period, sulfate (SO₄ ²⁻), nitrate (NO₃ ⁻), and chloride (Cl⁻¹) were the major anions in bulk precipitation with volume-weighted average concentrations of 12.6, 12.3 and 10.0 μeq/1, respectively. Annual inputs of NO₃-N, NH₄-N and SO₄-S from wet deposition were about 60 to 75% of those reported from bulk deposition collectors. Discharge from the two watersheds occurs primarily during spring snowmelt. Solute exports from Log and Tharp's Creeks were dominated by HCO₃ ⁻, Ca²⁺ and Na⁺, while H⁺, NO₃ ⁻, NH₄ ⁺ and PO₄ ³⁻ outputs were relatively small. Solute concentrations were weakly correlated with instantaneous stream flow for all solutes (r² <0.2) except HCO₃ ⁻ (Log Cr. r² = 0.72; Tharp's Cr. r² = 0.38), Na⁺ (Log Cr. r² = 0.56; Tharp's Cr. r² = 0.47), and silicate (Log Cr. r² = 0.71; Tharp's Cr. r² = 0.49). Mean annual atmospheric contributions of NO₃-N (1.6 kg ha⁻¹), NH₄-N (1.7 kg ha⁻¹), and SO₄-S (1.8 kg ha⁻¹), which are associated with acidic deposition, greatly exceed hydrologic losses. Annual watershed yields (expressed as eq ha⁻¹) of HCO₃ ⁻ exceeded by factors of 2.5 to 37 the annual atmospheric deposition of H⁺.