Comment on “A framework for understanding the hydroecology of impacted wet meadows in the Sierra Nevada and Cascade Ranges, California, USA”: paper published in Hydrogeology Journal (2009) 17:229–246, by Steven P. Loheide II, Richard S. Deitchman, David J. Cooper, Evan C. Wolf, Christopher T. Hammersmark, Jessica D. Lundquist Barry Hill & Sherry Mitchell-Bruker Meadow restoration in the Sierra Nevada, USA, and other western mountain ranges has been a goal of the US Department of Agriculture (USDA) Forest Service and other land-management agencies and organizations for over 70 years. Progress has been limited primarily by availability of grant funding. For some important sources of funding, grants for meadow restoration are generally based on expectations of downstream ecosystem benefits, as defined by beneficial changes in water quantity, quality, distribution, and timing of flow (Brauman et al. 2007). Recent efforts to expand the scope of wet-meadow restoration have provoked debate on the likely effects of restoration on groundwater discharge to streams from meadow aquifers. Restoration that elevates meadow water tables is generally acknowledged to increase both groundwater storage and evapotranspiration in meadows, but the overall effects on streamflow at the regional scale are uncertain and controversial. The recent article by Loheide et al. (2009), the subject article, presents a general hydrogeologic framework to explain patterns of groundwater flow in wet meadows in

Received: 18 September 2009 / Accepted: 14 July 2010 Published online: 10 August 2010 * U.S. Government 2010 B. Hill ()) USDA Forest Service, Pacific Southwest Region, 1323 Club Drive, Vallejo, CA 94592, USA e-mail: [email protected] Tel.: +1-707-5628968 Fax: +1-707-5629054 S. Mitchell-Bruker USDA Forest Service, Lassen National Forest, 2550 Riverside Drive, Susanville, CA 96130, USA Hydrogeology Journal (2010) 18: 1741–1743

the Sierra Nevada and Cascade Range. Using examples and models in which homogeneous meadow alluvium was more permeable than the surrounding bedrock, the paper has significantly advanced the general understanding of meadow hydrology by identifying the partitioning of basal and lateral groundwater flows into meadows as a major determinant of water-table configuration and vegetation composition. In most mountain meadows, however, meadow alluvium is highly heterogeneous (for example, Wood 1975) and often includes low-permeability strata that may strongly affect the discharge of groundwater to streams. Although evidence from mountain meadows is limited to a few locations (Wood 1975; Hill 1990; Jewett et al. 2004), several recent studies in other areas have illustrated the importance of low-permeability alluvial strata in controlling the direction and magnitude of groundwater discharge from aquifers to streams (for example, Salve and Tokunaga 2002; Wright et al. 2005; Andersen and Acworth 2009; Rosenberry and Pitlick 2009). Therefore, a representative hydrogeologic framework for mountain meadows should include cases in which the permeability of meadow strata is lower than that of surrounding bedrock. Results from previous studies of bedrock, alluvial, and organic-strata permeability are summarized in the following to provide general indications of situations in which meadow alluvium is likely to be less permeable than local bedrock. Bedrock permeability in the Cascade Range and Sierra Nevada ranges over several orders of magnitude (Table 1). Fractured basalt, common in the Cascades, is highly permeable (saturated hydraulic conductivity K as high as 1 cm/s), but unfractured and unweathered granite, more typical of the glaciated central Sierra Nevada, has very low permeability (K as low as 10–9 cm/s). Weathered and fractured granite, common in nonglaciated areas, has values of K on the order of 10–4–10–3 cm/s. Permeabilities of meadow alluvial and organic strata also vary widely (Table 1). Alluvial sands and gravels and DOI 10.1007/s10040-010-0634-9

1742 Table 1 Saturated hydraulic conductivity (Ks) results (cm/s) from previous studies of geologic materials representative of or similar to Sierra Nevada meadows and bedrock (rounded to nearest order of magnitude) Geologic material

Minimum Ks

Maximum Ks

References

Rhyolite Basalt Granite, unweathered and unfractured Granite, fractured

10–7 10–9 10–9

10–5 1 10–2

MacDonald 1987 Freeze and Cherry 1979; Nonner 2002 Williams et al. 1993; Katsura et al. 2008

10–5

10–4

Granite, weathered

10–4

10–3

Fine-grained meadow alluvium (sandy to silty loams) Peat acrotelm Peat, decomposed Meadow “topsoil”

10–1

10

Flint et al. 2008; Asano and Uchida 2004; Tromp-van Meerveld et al. 2007; Illman et al. 2009 Graham et al. 1997; Hubbert et al. 2001; Taylor and Eggleton 2001; Katsuyama et al. 2005; Katsura et al. 2005; Katsura et al. 2008 Wood 1975; Hammersmark et al. 2008

10–4 10–7 10–6

10 10–1 10–3

Holden and Burt 2003; Baird et al. 2004; Quinton et al. 2008 Richardson and Vepraskas 2001; Holden and Burt 2003; Quinton et al. 2008 Wood 1975; Hammersmark et al. 2008; Cornwell and Brown 2008

undecomposed peat can have values of K as high as 10 cm/s. In contrast, highly decomposed peat can have permeabilities as low as 10–7 cm/s. Decomposed peat, also known as fine-grained organic silt, commonly occurs as thick (generally>0.5 m) and extensive layers in meadows in the central and southern Sierra Nevada (Wood 1975; Anderson and Smith 1994). Meadow topsoils, which have likely been compacted by historical grazing, also have relatively low permeabilities (10–6–10–3 cm/s). Lacustrine deposits, which occur below meadow alluvium in some areas of the northern Sierra Nevada, also have very low permeabilities but are not generally breached by erosion and are therefore not considered here. The permeability ranges for bedrock and meadow strata summarized in Table 1 overlap substantially, but indicate that some meadow strata may be less permeable than surrounding bedrock. This situation is most likely for watersheds with relatively permeable bedrock such as basalt or weathered granite, and for meadows with extensive layers of decomposed peat. Upward flow of groundwater from bedrock has been recognized as an important component of meadow water budgets (Jewett et al. 2004; Cooper and Wolf 2006; Tennant et al. 2006; Loheide and Gorelick 2007; Loheide et al. 2009; Fogg and Trask 2009) and regional groundwater systems in mountainous terrain (Gleeson and Manning 2008). As noted by Loheide et al. (2009), the magnitude of upward (basal) groundwater flow varies between meadows based on hydrogeologic characteristics, including the relative permeabilities of meadow strata and surrounding bedrock. Vertically upward groundwater flow from bedrock through less permeable decomposed peat or meadow topsoil restricts groundwater discharge from meadows (Wood 1975; Hill 1990; Jewett et al. 2004; Cooper and Wolf 2006). Under stable or aggrading geomorphic conditions, meadows with low-permeability strata probably prolong the duration of baseflow by restricting discharge to streams early in the dry season and maintaining higher water tables and groundwater gradients late in the dry season. Gully erosion that breaches low-permeability strata is likely to allow more rapid early-season groundHydrogeology Journal (2010) 18: 1741–1743

water discharge and, therefore, reduce the duration and magnitude of late-season discharge. If future research confirms this hypothesis, the presence of low-permeability strata may provide a useful means of prioritizing meadows for protection and restoration. As noted by Loheide et al. (2009), a scientific basis for understanding meadow hydrologic processes is critical for land managers and restoration practitioners. The conceptual model proposed by Loheide et al. (2009) is a timely, important, and useful contribution to the scientific basis of meadow restoration, and shows that the relative permeabilities of meadow alluvium and surrounding bedrock can significantly affect meadow hydrology. Meadows with low-permeability alluvial or organic strata that restrict the discharge of groundwater to streams are likely to be especially important for streamflow regimen, particularly in areas with higher permeability bedrock such as weathered granite or fractured basalt. Therefore, such meadows should be included in a general hydrogeologic framework for mountain meadows. Acknowledgements This manuscript benefited substantially from review comments on an earlier draft by Dave Stonestrom of the US Geological Survey.

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DOI 10.1007/s10040-010-0634-9