Discussion1

TL;DR: The proposed leveraged approach aims to assess the effects of agricultural conservation programs on stream habitat quality by integrating existing hydrologic models to predict key habitat quality attributes.

Abstract: Shields e t al. (2006) proposed a “leveraged” approach to be used for assessment of effects of agricultural conservation programs on stream habitat quality as requested by the Conservation Effects Assessment Project (CEAP). While important, assessment of stream habitat is fraught with difficulty due to the poor understanding of intricate relationships between watershed practices and degradation of stream habitats. Stream habitats are complex and interconnected, being composed of stream channel, contiguous habitats in shallow ground water, adjacent wetlands, other riparian habitats, and floodplain water bodies (Shields et al., 2006). The relationships among these components are further complicated by weather, climate, and climate change. Also, our limited understanding of the link between watershed practices and stream corridor habitat quality may be partially attributed to the subject boundary between hydrology and ecology, which developed as separate disciplines. As a result, outputs from models focusing on the hydrologic aspects of an ecosystem may be limited as direct inputs into models focusing on the ecologic aspects of the ecosystem. This may be one reason for Shields et al. (2006) t o propose the leveraged approach to adapt existing models to conduct detailed analyses of conservation effects within selected watersheds, which is the second component of CEAP. Adhering to the authors’ proposal, we herein extend discussion of, and suggest some strategies for appropriately applying, the proposed leveraged approach. The leveraged approach involves conjunctively using several hydrologic models to predict six key habitat quality attributes governing stream habitat quali ty (Shields e t al., 2006). Among these six attributes, watershed land use is taken as a model input both by AnnAGNPS (Bingner and Theurer, 2001) and SWAT (Neitsch et al., 2002). These two models simulate stream discharge and loadings of constituents (e.g., sediment, nitrogen, and phosphorus) as affected by land use change, while baseflow is only simulated by SWAT. Subsequently, the outputs from either AnnAGNPS or SWAT would be taken as inputs into SNTEMP (Bartholow, 1990), REMM (Lowrance et al., ZOOO), and CONCEPTS (Langendoen, 2000, 2002) to predict water temperature, large wood density, and bed material composition. The effects of land retirement, conservation buffers, and tillage management on stream habitat quality would be assessed in terms of responses of the latter five attributes to changes of the first attribute, land use. The approach is somewhat novel and has the primary advantage that no additional effort to develop new models is needed. However, stream habitat assessment requires bridging the subject boundary between ecology and hydrology and implementing the approaches of ecohydrology (McClain, 2006), an emerging discipline. The main focus of ecohydrology is t o understand the interdependence among functions of an ecosystem, instream structures, and hydrologic and ecologic factors that is, to understand how the ecologic