The 25 participants in the Modeling Forest-Stream Interactions workshop outlined four main zones of a forested watershed which must be conceptually and quantitatively linked with respect to hydrology and biogeochemistry in order to effectively model the dynamics of forest-stream interactions. These are:
- The watershed vegetation,
- The upland soils and groundwater,
- The interface zones between uplands and streams -- i.e. riparian and hyporheic zones
- The stream channel
All agreed that the hydrologic coupling of these zones represents an important, yet difficult, component of any modeling effort. A considerable number of models currently exist for simulating aspects of watershed hydrology; however, none of these was considered entirely adequate for addressing the specific time, space, and process dimensions of forest-stream interactions.
Therefore, combining the most appropriate components of existing models was discussed as a possible approach. A specific example mentioned was the combination of PROSPER (with its rigorous treatment of evapotranspiration) with PRMS (with its variable time-step capability for simulating processes over the hydrographic time scale). A particularly useful document by van der Heijde et al.* for evaluating and comparing the strengths of existing hydrology models was noted.
Also addressed were approaches to interpreting nutrient chemistry and solute transport in evaluating the major biotic and geochemical transformations from hillside to stream. Although empirical relationships may suggest the relative importance of specific processes of nutrient and organic matter processing at different soil depths and lateral positions along the hillside, emphasis was placed on direct measurement of specific processes (i.e., nitrogen fixation, denitrification, nitrification, decomposition, etc.) for deriving rate parameters in forest-stream interaction models.
The hydrology and biogeochemistry of the riparian and hyporheic zones was discussed as especially complex and less well understood at present, although several sites are providing new information which could be incorporated into developing models.
The roles of riparian vegetation in transmitting light and exporting woody material was discussed as a key component of forest-stream interactions, determining hydrologic and biotic characteristics of the stream ecosystem.
Participants agreed that individual investigator collaborations were well served and that sufficient interest exists for convening additional working groups at future LTER meetings.
* van de Heijde et al. 1988. Groundwater modeling: an overview and status report. GWMI 88-10, The Groundwater Modeling Institute, Butler University, Indianapolis, Indiana 46208.