In its study, "Comparison of Load Estimates for Cultivated Cropland in the Chesapeake Bay Watershed," LimnoTech recommended suspending implementation of the Chesapeake Bay Total Maximum Daily Load (TMDL) until differences between the CBP watershed model and the USDA-NRCS model could be resolved. The Chesapeake Bay TMDL allocates pollution limits for nitrogen, phosphorus and sediment to protect water quality standards in the Chesapeake Bay and its tributaries. Agricultural trade groups and state and federal legislators have cited LimnoTech’s findings as evidence in opposition to the CBP's watershed model and the Chesapeake Bay TMDL. STAC recently convened a panel of scientific experts to conduct an independent review of LimnoTech’s findings.
The independent scientists found that LimnoTech's comparison of the two models was scientifically flawed and did not provide sufficient evidence to suspend implementation of the Chesapeake Bay TMDL. The experts cite numerous problems with LimnoTech's interpretation of the two models and their results, and suggest that LimnoTech's recommendations were based on unrealistic expectations for watershed model performance and agreement between models.
According to the panel, LimnoTech failed to acknowledge that fundamental differences between models are unavoidable because of differences in model objectives and the availability of data and modeling resources. The CBP watershed model and the USDA-NRCS model were developed for different purposes. The two models use different modeling methods, make different simplifying assumptions and take advantage of different data sources.
The experts report that when factual errors in LimnoTech’s analysis are corrected, the two models' predictions of agricultural loads delivered to the Bay are closer than reported by LimnoTech and are in approximate agreement. More importantly, results from both models indicate that more management practices on cropland are needed to protect the Chesapeake Bay.
The panel also notes that the USDA-NRCS model provides new information and approaches that could improve the CBP watershed model. The reviewers encourage the CBP and USDA to continue, and expand, their cooperative efforts to share data and modeling results. Finally, the reviewers suggest that future restoration efforts could be enhanced by the application of multiple models.