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WATERSHED HYDRO-BIOGEOCHEMISTRY

Permafrost has a dominant control on watershed hydrology and biological processes in the boreal forest of interior Alaska.  Permafrost forms an impermeable barrier and restricts subsurface flows to the shallow active layer of soils (the shallow soil layer above permafrost that freezes and thaws each year; Fig. 1).  Over summer, as depth of thaw increases, subsurface flowpaths through soils migrate deeper and can transition from organic to inorganic dominated soil horizons.  In regions with discontinuous permafrost, such as interior Alaska, north facing slopes and valley bottoms are commonly underlain with permafrost, whereas warmer south facing slopes and ridge tops typically lack permafrost.  Consequently, ground water flowing through north versus south facing slopes travels along different subsurface flowpaths, which has important implications for stream discharge (Bolton et al. 2000).

Conceptual model of permafrost and watershed hydrology

Conceptual model of the influence of permafrost on watershed hydrology and biogeochemistry.

Boreal forests are thought to be limited by nitrogen, yet store vast amounts of organic nitrogen in cold or frozen soils (Post et al. 1982).  Soil warming could stimulate mineralization of stored nitrogen, potentially altering terrestrial ecosystem productivity and species composition or leading to increased nitrogen export to freshwaters and coastal areas (Shaver et al. 1992).  Despite this key role for nitrogen, surprisingly little is known about nitrogen cycling in watersheds of boreal forests that are completely or partially underlain by permafrost.  What we do know from long-term measurements of nitrogen output in stream flow seems to contradict theory and empirical results derived in temperate watersheds.  In contrast to the retentive nature of temperate watersheds, fluvial export from boreal forest watersheds of interior Alaska is up to seven times greater than atmospheric input on an annual basis (Stottlemyer 1992, 1997, 2001, Jones et al. 2005, Petrone et al. 2006).

Export of nitrogen from boreal forest watersheds

Nitrogen fluxes from three subcatchments of CPCRW for four years (from Jones et al. 2005).

Approach:  We are using watershed hydrologic and solute transport studies as a tool to investigate catchment-scale responses to changes in climate and disturbance.  Using end-member mixing models, we can distinguish at least four major flowpaths:a shallow flowpath through the surface organic mat that predominates in high-permafrost watersheds, two flowpaths through mineral soil,and a deeper groundwater flowpath that predominates during low- and winter-flow conditions.  Seasonal changes in the relative contribution of the four flow paths and discharge dynamics reflect responses to hydrologic inputs (including snowmelt) and the depth of seasonally frozen soils.  Long-term monitoring of discharge and stream chemistry should show responses to changes in permafrost extent and integrity.  For example, as permafrost is lost from the high permafrost watershed, we anticipate that watershed flowpaths, storm hydrographs, and nitrogen exports will transition towards patterns similar to lower permafrost catchments.  The lower permafrost watersheds, in turn, will likely change, and the smaller stream channels may become ephemeral.  In addition to three watersheds that have been our primary focus, we are studying a medium-permafrost watershed burned extensively in 2004.  We have several years of pre-fire stream discharge and chemistry data providing the rare background information to study fire disturbance effects on stream hydrology and nutrient fluxes.

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