Decadal surface water quality trends under variable climate, land use, and hydrogeochemical setting in Iowa, USA

Abstract

Understanding how nitrogen fluxes respond to changes in agriculture and climate is important for improving water quality. In the midwestern United States, expansion of corn cropping for ethanol production led to increasing N application rates in the 2000s during a period of extreme variability of annual precipitation. To examine the effects of these changes, surface water quality was analyzed in 10 major Iowa Rivers. Several decades of concentration and flow data were analyzed with a statistical method that provides internally consistent estimates of the concentration history and reveals flow-normalized trends that are independent of year-to-year streamflow variations. Flow-normalized concentrations of nitrate1nitrite-N decreased from 2000 to 2012 in all basins. To evaluate effects of annual discharge and N loading on these trends, multiple conceptual models were developed and calibrated to flow-weighted annual concentrations. The recent declining concentration trends can be attributed to both very high and very low discharge in the 2000s and to the long (e.g., 8 year) subsurface residence times in some basins. Dilution of N and depletion of stored N occurs in years with high discharge. Reduced N transport and increased N storage occurs in low-discharge years. Central Iowa basins showed the greatest reduction in flow-normalized concentrations, likely because of smaller storage volumes and shorter residence times. Effects of land-use changes on the water quality of major Iowa Rivers may not be noticeable for years or decades in peripheral basins of Iowa, and may be obscured in the central basins where extreme flows strongly affect annual concentration trends.

River chloride trends in snow-affected urban watersheds: increasing concentrations outpace urban growth rate and are common among all seasons

“…Estimation by the WRTDS model was performed using measured chloride and corresponding daily streamflow values. This tool has not previously been used to examine chloride trends, but applications using WRTDS have primarily been used for examination of nutrients Medalie et al, 2012;Green et al, 2014).…”

Evaluating the impacts of climate change and crop land use change on streamflow, nitrates and phosphorus: A modeling study in Bavaria

“…In basins where agricultural activities dominate, it is common for the quality of water to be compromised (Green et al, 2014;Patoine et al, 2012;Volk et al, 2009). For example, in Denmark, Nielsen et al (2012) found a high correlation between the amount of agricultural land and the total nitrogen (N) and total phosphorus (TP) concentrations in adjacent lakes.…”

“…However, cleanup efforts at the larger scale may only show results after a few decades. A study examining NO 3 − and NO 2 − changes in Iowa (U.S.) from 1970 to 2012 suggested long-term N transport in watersheds due to fertilizer inputs on maize (Green et al, 2014). Since agricultural land can have such a variety of effects on water quality, investigating potential land use and management changes in a basin is necessary to achieve the WFD objectives which will be evaluated during planning cycles ending in 2015, 2021 and 2027.…”

The environmental effects of crop price increases: Nitrogen losses in the U.S. Corn Belt

“…However, the timing of nitrate-nitrogen concentration in surface water varies substantially across space and crop years due to weather (Jayasinghe et al, 2012) and other biophysical factors (Green et al, 2014). Thus, it would be difficult to obtain meaningful estimates of the effect of crop prices on observed nitrate-nitrogen concentrations, especially if one were to model the dynamics and spatial heterogeneity.…”