Streamwater elemental concentrations in this watershed show large fluctuations during storm events. Concentrations of Na and Ca decrease during storm events while NO3 concentrations increase with increasing stream flow volumes. The reason for this contrasting pattern appears to be due to changes in the hydrologic flowpath during storm events. During periods of baseflow, concentrations of Na and Ca are regulated by interaction of ground water with the bedrock. High rainfall events induce laterally flow of water through the upper soil horizons (A and AB horizons) above the rather impervious, clay-rich, argillic horizon (Bt horizon). This short-circuiting of the water flow prevents the water from interacting with the bedrock and therefore elements that have their concentrations regulated by ground water-bedrock interactions are diluted during storm events (e.g., Na, Ca). In contrast, NO3 concentrations increase during many storm events, especially those occurring in the late fall-early winter period when annual grasses are not actively growing. Nitrate concentrations are highest in the A and AB horizons as shown previously in the soil solution section. Nitrate accumulates in the A and AB horizons from the late spring through the onset of leaching events in the fall-winter due to its release from organic matter. Since annual grasses and oak trees are not appreciably active over the dry summer months, there is no significant nutrient uptake during this period. Once the upper soil profile becomes saturated and lateral flow is induced, mobile nutrients such as NO3 are easily transported laterally to the stream resulting in higher concentrations during storm events. Once these nutrients have been flushed from the soils and the annual grasses are actively growing, the magnitude of these nutrient peaks, during storm events, is greatly attenuated.
Elemental fluxes exiting the studied watershed in stream water are as shown. Losses of the most limiting nutrients (N, P, and K) were very low (<2 kg/ha/yr) indicating their retention in the watershed, primarily by biological processes. elements which are abundant in the bedrock (e.g., ca, mg, cl, na, and si) show the greatest loss from the watershed. suspended sediment fluxes are very low (<45 kg/ha/yr) relative to many rangeland watersheds where erosion rates may be several mg/ha/yr.