Evaluating the Influences of Climate Change and Land Use Change on River Discharge and Water Quality in Minnesota

In the last four decades, agricultural management practices and precipitation pattern in the Midwest USA have simultaneously gone through significant changes. Changes in either can affect nutrient export from agricultural landscapes to freshwater or marine environments. Remedies to improve water quality require a better understanding of the relative roles of changes in agricultural management practices versus changing precipitation. This paper investigates the extent to which the increasing export of nitrate-N from agricultural landscapes in the Upper Midwestern USA is due to changes in agricultural management practices versus an increasingly wetter climate.

The Agricultural Drainage and Pesticide Transport (ADAPT) model was applied on the 7,719 ha Seven Mile Creek, an agricultural watershed in southern Minnesota, USA. Annual flow and nitrate-N losses were simulated using agricultural management practices typical of three different periods (1967, 1978, and 2001) in combination with two climatic scenarios (actual versus recent). The actual climatic pattern scenarios involved weather data for the four years surrounding the date of agricultural management data (e.g. 1965-1969 for 1967). The recent climatic pattern scenarios involved weather data from 1999-2003, which is a wetter period than the 1965-1969 or 1976-1980 climatic periods.

Simulation results showed that the recent wetter climatic would have caused a 70 and 51% increase in flow for management conditions typical of 1967 and 1978, respectively. Similarly, the wetter climate would have caused a 62 and 137% increase in nitrate-N losses for 1967 and 1978 management conditions, respectively. We conclude from the analysis that the most important factor that increased nitrate-N losses from agricultural landscapes to surface waters during 1978-2001 periods was an increasingly wetter climate rather than changes in agricultural management practices. In contrast, the most important change that increased nitrate-N losses from agricultural lands to surface waters during 1967-1978 periods was changes in agricultural management practices.