R. D. Libra, K. E. Schilling, and C. F. Wolter
Iowa Department of Natural Resources, Geological Survey Bureau
44th Annual Midwest Groundwater Conference
October 13-15, 1999
St. Paul, MN
The relationships between row-crop land use and the nitrate-N concentrations and loads in surface water were evaluated for 15 watersheds ranging from 390-1100 mi2 (8-digit HUC), and for nine smaller watersheds ranging from 20 to 300 mi2 (11- and 14-digit Hydrologic Unit Codes [HUCs], for the period 1996 to 1998). The percentage of land in row crop varied from 24% to more than 80% in the 8-digit HUCs, and average NO3-N concentrations ranged from 0.5 to 10.8 mg/L. In the 11- and 14-digit HUC watersheds, row crop percentage varied from 28% to 87% and average NO3-N concentrations ranged from 3.0 to 10.5 mg/L. In both cases, nitrate-N concentrations were higher in watersheds that were more intensely row-cropped. Linear regression shows similar slopes for both sets of watersheds (0.092 for 11-digit HUCs and 0.113 for 14-digit HUCs) although data from small watersheds had a higher degree of correlation (r 2 = 0.52 and 0.94, respectively). The data suggest that average surface water NO3-N concentrations in Iowa, and possibly similar agricultural areas, can be estimated by multiplying a watershed's row crop percentage by 0.1.
The data indicate loads and concentrations relate to hydrogeologic factors as well as land use. In northeast Iowa watersheds underlain by shallow, highly permeable aquifers, movement of water and transport of nitrate is efficient from cropped fields to streams, resulting in relatively high stream discharge and nitrate loads. Comparing a fractured rock watershed like the Upper Iowa River to a glacial till-dominated watershed like the Floyd River (northwest Iowa) indicates nearly twice as much runoff from the Upper Iowa River watershed. While nitrate-N concentrations in the Floyd River (80% row crop) were 2.2 times the Upper Iowa River (40% row crop), the lower discharge for the Floyd River resulted in an NO3-N load only 1.2 times that of the Upper Iowa River.
Thirteen of the 8-digit HUC watersheds have complete monthly nitrate-N monitoring records for 1987 to 1996. These watersheds are 63% row crop, close to the state average of 60%, indicating they are representative of state conditions. Extrapolating from these watersheds, data from 1987-1996 suggest Iowa contributes about 225,000 tons of NO3-N to the Mississippi River system annually.