and messages to better understand how to encourage stronger engagement on water quality issues. It also would be useful to better understand how to apply and learn from results from effective land use and nutrient management actions.
Models are used to interpret water quality monitoring data and are thus critical for any monitoring program. Modeling exercises and results also are a useful complement to data collection. For example, areas that are remote or otherwise difficult to access (e.g., hilly terrain) present challenges for implementing data collection stations or collecting samples, and data collection always will have some limits.
There are many different kinds of water quality models, from simple conceptual models to quantitative, process-based models that can consider loads and concentrations in domains of different scales. Process-based land-water interaction models in common use, such as the SWAT, APEX, and EPIC models (referenced earlier), are useful for identifying sources of pollutants, evaluating the effectiveness of conservation practices, identifying optimal locations to target for conservation practices, and identifying factors responsible for temporal changes in water quality. The statistical land-water interaction model SPARROW from the USGS (Alexander et al., 2008) has been used for identifying watersheds that contribute the largest pollutant loads and for assessing the causal factors for these loadings. Models focused on in-stream water quality, such as HSPF (Singh et al., 2005), are useful for establishing load allocations to point and nonpoint sources, but are less useful in evaluating the effectiveness of agricultural practices, or factors responsible for trends.
There was some discussion among workshop participants about building models that can help bridge from small scale to large watershed scale. Currently, “mechanistic” models are limited to small scale, and several participants mentioned the need to build mechanistic models that can “scale up” to the larger, watershed scale.
To assist design and implementation of monitoring programs at the watershed and basin scales, additional process models that can be advantageous at larger scales would be helpful. To date process-based models for land-water interactions largely have been focused at the field and small-watershed scale; additional attention on process-based models at the large-watershed and basin scale would complement the field and watershed-scale efforts. To represent the diverse processes and pathways for pollutant transport that operate across scales and to account for the changing importance of differing drivers across scales, routine model