Watershed episodes

Base cations are mobilized by weathering and cation exchange reactions that neutralize acids in the watershed. They respond therefore indirectly to changes in sulphate and nitrate concentrations. In fact, if acid anion concentrations (mainly sulphate) decrease, base cations are also expected to decrease. However, in the last few years an increase of the occurrence of alkaline rain episodes (probably due to climatic effects) has been observed and it is likely that calcareous Saharan dust, rich in base cations, is responsible for it [27]. Accelerated weathering, resulting from recent climate warming may also contribute to higher base cation concentrations [28]. 

Figure 14. Stream-water chemistry for control watershed No. 4 at the Fernow Experimental Forest in West Virginia, a, Seasonal pattern in N03 indicates both elevated baseflow concentrations and episodic increases during high-flow events in the spring and thus suggests that this watershed has reached Stage 2 of watershed N loss, b, The long-term trend suggests that substantial changes in N03 export have occurred over the past 2 decades. (Data are from...

The state of episodic acidification in the Sierra Nevada Mountains (and the rest of the West) therefore remains uncertain because few data exist and the data that are available indicate ANC depressions to a value of 0 xequiv/L, but not below. For the time being, most western sites would be classified as Stage 0 watersheds. However, in systems as acid-sensitive as those at high elevations in the West, episodic acidification resulting from N03" may be possible in the watersheds that are only seasonally N-saturated. 

Hyer, K.E., G.M. Homberger, and J.S. Herman. 2001. Processes controlling the episodic streamwater transport of atrazine and other agrichemicals in an agricultural watershed. J. Hydrol. 254 47-66. 

Reinhardt R. L., Norton S. A., Handley M., and Amirbahman A. (2003) Mobilization of and hnkages among P, Al, and Fe during high discharge episodic acidification at the Bear Brook Watershed in Maine, USA. Water Air Soil Pollut. (in press). 

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