Iowa Stream Nitrate, Discharge and Precipitation: 30-Year Perspective

Abstract

We evaluated Iowa Department of Natural Resources nitrate (NO-N) and US Geological Survey hydrological data from 1987 to 2016 in nine agricultural watersheds to assess how transport of this pollutant has changed in the US state of Iowa. When the first 15 years of the 30-year water-quality record is compared to the second 15 years (1987-2001 and 2002-2016), three different metrics used to quantify NO-N transport all indicate levels of this pollutant are increasing. Yield of NO-N (kg ha) averaged 18% higher in the second 15 years, while flow-weighted average concentrations (mg L) were 12% higher. We also introduced the new metric of NO-N yield (g ha) per mm precipitation to assess differences between years and watersheds, which averaged 21 g NO-N ha per 1 mm of precipitation across all watersheds and was 13% higher during the second half of the record. These increases of NO-N occurred within a backdrop of increasing wetness across Iowa, with precipitation and discharge levels 8 and 16% higher in the last half of the record, indicating how NO-N transport is amplified by increasing precipitation levels. The implications of this are that in future climate scenarios where rainfall is more abundant, detaining water and increasing evapotranspiration within the cropping system will be necessary to control NO-N losses. Land use changes that include use of cover crops, living mulches, and perennial plants should be expanded to improve water quality and affect the water balance within agricultural basins.

Assessment of Spatial Nitrate Patterns in An Eastern Iowa Watershed Using Boat-Deployed Sensors

“…Schilling and Libra [29] used grab sample monitoring to report average Cedar River concentrations of 5.4 mg L −1 near the watershed outlet. Jones et al [30] reported 1987-2016 flow weighted average concentrations for the West Fork of the Cedar River of 8.0 mg L −1 using a monthly grab sample database.…”

Supplementary material to "Hydrologic regimes drive nutrient export behavior in human impacted watersheds"

“…For detailed QA/QC protocol see (Garrett, 2019) for USGS sites and (Jones et al, 2018) for IIHR sites. Instruments were calibrated for bias and sensor drift before deployment and during periodic site visits in which hand samples were collected and analyzed to check the sensor accuracy.…”

A novel approach to spatial assessments of surface water nitrate trends in selected Iowa rivers and lakes

“…Previous studies have quantified nitrate trends at high temporal resolution over extended periods of time, giving consideration to varying seasonal and yearly precipitation (Jones et al 2018a. ;Jones et al 2018b. ;Jones et al 2018c.…”

“…A movement toward high frequency measurements using in-situ sensors has occurred in recent years, but NO3-N loads are also still measured traditionally with grab samples(Jones et al 2018a. ;Jones et al 2018b. ;Jones et al 2018c.…”

“…) NO3-N concentration and discharge must be measured if researchers wish to calculate NO3-N load data using grab samples. Because these measurements require staffing resources, time, and money, they are often collected at coarse temporal resolutions(Bowes et al 2009;Jones et al 2018b.). A degree of error results when both NO3-N concentration and discharge are measured at course temporal resolutions.…”