development of quantitative measures of watershed condition has proceeded rapidly, empirical studies that test for significant relationships between watershed metrics and ecological condition (e.g., presence or abundance of species, water quality) are still few in number (Johnston et al. 1990). There is a clear need to identify the most important watershed metrics to monitor. In addition, it is essential to be aware of the assumptions and constraints that are implicit in the metrics, a problem that is not unique to watershed studies. For example, the selection of the land cover categories to be used in the analysis partially determines the results and the spatial scale of the data—both the total extent of the area and the grid cell size—and thus can strongly influence the numerical results (Turner et al. 1989a, b).
In agriculture-dominated watersheds, monitoring and management practices take on their own unique character. The long-term monitoring of various soil and water indicators is essential to document the status and trends of nutrient sources and coastal impacts. Monitoring is also essential to document any changes in nutrient inputs, system response to best management practice implementation, and impact of any land management changes. Effective monitoring strategies, however, must be spatially extensive as well as sufficiently frequent to detect real and statistically valid changes. Unfortunately, the outcomes and benefits of most monitoring programs will not be manifested for several years before yielding useful information. Monitoring programs are costly, labor intensive, and in most cases will need to be in place for several years. Overcoming these challenges, through documentation and education, is critical to the continued support of existing monitoring programs.
The long history of watershed monitoring in the United States provides examples of programs that have experienced varying degrees of success. These programs can be grouped into those that focus on nutrient sources and those that concentrate on coastal water impacts. It must be emphasized that the choice of both soil and water quality indicators will vary from situation to situation. Soil and water quality monitors will have to decide what they need to measure and how often (Sparrow et al. 2000).
Environmental concerns have forced many state and federal agencies to consider adopting standard soil phosphorus fertility tests as indicators of the potential for phosphorus release from soil and its transport in runoff. Environmental threshold levels range from two (Michigan) to four (Texas) times agronomic thresholds. In most cases, agencies proposing these thresholds plan to adopt a single threshold value for all regions under their jurisdiction. However, threshold soil phosphorus levels are too limited to be the sole criterion to guide manure management and phosphorus applications. For example, adjacent fields having similar soil