Field- and watershed-scale evaluation of water quality trends due to changes in landscape and management practices.

摘要

Large N losses from the Upper Mississippi River Basin contributing to hypoxia in the Gulf of Mexico are due to high concentrations of nitrate-N and large tile drain discharges from row crop agriculture. A water quality simulation model was calibrated and validated at the field and watershed scales for tile drainage and associated nitrate-N losses in the Seven Mile Creek watershed of Nicollet County under various scenarios. The first series of simulations involved N fertilizer rate and timing. At the field scale, reductions in N fertilizer rate (from 176 to 121 kg/ha) resulted in a 14% reduction in nitrate-N losses, while at the watershed scale reducing N fertilizer rate (from 179 to 112 kg/ha) resulted in a 23% reduction in nitrate-N losses. Shifting N fertilizer application from fall to spring produced a 5.5% reduction in nitrate-N losses at the field scale (N rate 121 kg/ha), while a similar shift at the watershed scale produced a 12% reduction (N rate 112 kg/ha). A series of long-term simulations (1955-2004) were performed at the watershed scale to investigate impacts of changes in climate, cropping systems, N fertilizer rates, area of wetlands and tile drain spacing. Using constant 2003 baseline conditions, nitrate-N losses increased 185% over the last 50 years due to an increasingly wetter climate. Historical records of land use, wetland area, crop rotations, crop yields, N fertilizer application rates and tile drain spacings were then used to investigate nitrate-N losses for 1955, 1967, 1978 and 2003 conditions. When accounting for these changes, but excluding nitrate transformations in wetlands, nitrate losses were 14.5, 19.0, 23.1 and 36.7 kg/ha for the 1955, 1967, 1978, and 2003 periods, respectively. Wetlands covered 15% of the study area in 1955, while none exist at present. Water budgets were calculated considering the direct effects of wetlands 72.6 cm deep, along with changes in area of agricultural land, cropping systems, crop yields, N application rate and tile drain spacing. Discharges were reduced by 29% in 1955 relative to 2003, while nitrate-N losses were reduced by 73% in 1955 relative to 2003.