Submitted to: Transactions of the ASABE
Publication Type: Book / Chapter
Publication Acceptance Date: 12/15/2007
Publication Date: 9/1/2008
Citation: Mulla, D.J., Birr, A.S., Kitchen, N.R., David, M.B. 2008. Limitations of evaluating the effectiveness of agricultural management practices at reducing nutrient losses to surface waters. In: Laing, G., editor. Final Report: Gulf Hypoxia and Local Water Quality Concerns Workshop. St. Joseph, MI: American Society of Agricultural and Biological Engineers. p. 189-212.
Technical Abstract: Hypoxia in the Gulf of Mexico is a serious problem. Excessive transport of nitrogen and phosphorus to the gulf from the Mississippi River contributes to growth of the hypoxic area. The Gulf of Mexico Hypoxia Task Force set a goal for a 30% reduction in the area of the hypoxic zone, and a variety of ways to reduce the area of the hypoxic zone by 30% have been proposed. These measures include reductions in nitrogen applications to cropland, restoration of wetlands, installation of riparian buffer strips, and improvements in nitrogen treatment processes at wastewater treatment plants. However, it has been difficult to document the effectiveness of these measures at the field, and more specifically, the watershed scales. This is due to five factors we’ve identified and discuss in this paper. One, climate is highly variable, which has a dominant effect on the transport of nutrients and sediment. Change in precipitation patterns lead to highly variable nutrient and sediment exports from one month to another, and one year to another. Therefore without long-term data, it is difficult to know when a change in nutrient and sediment export has occurred. Long-term data sets of sufficient monitoring intensity are generally not available, and short-term (1 to 5 year) data sets can give false impressions of the response. A commitment to long-term, baseline, monitoring can help differentiate between climate effects and documented management changes. Two, our inability to measure reduction of nutrient and sediment losses in conjunction with specific management practices once smaller watersheds are aggregated into larger watersheds. Nutrient and sediment concentrations can be highly variable in surface runoff and tile drainage from one watershed to the next, and intensive measurements are needed to obtain accurate loss rates. Most monitoring studies do not have sufficient intensity for long enough time periods and in enough locations to allow statistically significant differences to be determined. Three, long lag times are common in response to changes in management. Because of large and dynamic pools of soil nitrogen and phosphorus, agricultural landscapes are buffered and therefore response to implementation of altered management practices can take many years to elicit a change in water quality. In addition, stream or river response may be obscured by previous accumulation and transport of in-stream sediments and nutrients, that mask reduced export from fields. Four, implementation of many improved management practices at watershed scales has been sparse. Most management programs at the watershed only involve some of the fields and often do not target the most critical areas. Given the previous points, this can greatly reduce our ability to document change. And five, modeling limitations make projections uncertain. Limitations include uncertainty in many parameters (e.g., soil hydraulic properties, denitrification, mineralization rates, biological N fixation), incomplete representations of field and watershed processes, and limited data for validation. Future assessment of agricultural watersheds will need to address these five issues in order to understand the potential benefit current and future management practices will have on water quality.