This paper investigates the causes for increasing export of nitrate-N from agricultural landscapes in the Upper Midwestern USA over the last 40 years. The agricultural drainage and pesticide transport (ADAPT) model was used to separate the effects of changing agricultural management from the effects of an increasingly wetter climate on discharge and nitrate-N losses from the 7,719 ha Seven Mile Creek Watershed in southern Minnesota, USA. Discharge and nitrate-N losses were measured from 1999-2003 and were simulated using agricultural management and land use data from three 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 and land use data (i.e., 1965-1969, 1976-1980 or 1999-2003). The weather from 1999-2003 is wetter than the other climatic periods. Results showed that discharge and nitrate-N losses were sensitive both to changes in agricultural management and land use as well as climate. An increasingly wetter climate would have caused a 62% increase in nitrate-N losses for 1967 management conditions (relative to 1965-1969 weather), and a 137% increase in nitrate-N losses for 1978 management conditions (relative to 1976-1980 weather). In contrast, using 1999-2003 climatic conditions, changes in agricultural management and land use would have increased nitrate-N losses by 183.9% between 1967 and 1978, but would have decreased nitrate-N losses by 13.5% between 1978 and 2001. The specific types of changes in agricultural management that are most important include increasing rates of N fertilizer application, increases in the area devoted to corn, and increases in crop yield. We conclude from this analysis that the most important factor driving increased nitrate-N losses from agricultural lands to surface waters between 1978 and 2001 was an increasingly wetter climate.