Nonpoint Source Nitrogen and Phosphorous Loads at Implementation Scale: Direct Agricultural Nutrient Loads to Surface Waters in Relation to Land Use and Management

Date: 
Feb 2013

Issue

The Iowa Nutrient Reduction Strategy estimated potential reductions in nitrogen and phosphorous loads that could be achieved by a wide range of in-field and edge-offield practices. The estimates were based on a review of published research on the effectiveness of various practices and potential applicability. However, most of the studies were conducted at plot scale. The report highlighted the need for studies that scale up to the area of practice implementation to better assess water quality impacts across landscapes and with multiple practices.

Objective

For most of the cultivated cropland in Iowa, the most appropriate scale for assessing agricultural nonpoint source loads to surface water would be from a few hundred to a few thousand acres. This project focuses on measuring N and P loads at this scale, and in relation to land use and management. In addition to better characterizing loads at implementation scale, this research will improve the predictability of practice performance and the understanding of practice uncertainty.

Approach

Several dozen potential monitoring sites have been identified in the upper Des Moines River, Iowa River, Raccoon River, Skunk River and Beaver Creek watersheds. Instruments will be installed at a selected set of sites for close interval, automated sampling and flow measurement. Land management and watershed conditions will be characterized by working with cooperating landowners/farmers to collect information on nutrient management practices, crop yield and soil test phosphorus. Nutrient concentrations and flow data will be used to calculate mass nutrient load from the contributing watersheds for evaluation against the land use and management information and GIS-based load estimates.

Project Updates

March 2016

Selected sites have been monitored for water quality samples and discharge during this quarter. Water samples collected daily during this quarter are being assayed for total nitrogen (TN), nitrate, total phosphorus (TP), and total reactive phosphorus (TRP). Discharge data have been collected and are being evaluated for discharge for each study site. Annual nutrient yields from the exporting watersheds and annual loads delivered to receiving waters for the study sites monitored during 2014 and 2015 have been calculated.

December 2015

Selected sites were monitored for water quality samples and discharge during the fourth quarter. Water samples collected daily are assayed for total nitrogen (TN), nitrate, total phosphorus (TP) and total reactive phosphorus (TRP). Discharge data have been collected and are being evaluated. Discharge data will be merged with nutrient concentration data to calculate nutrient yields from the exporting watersheds, plus annual loads delivered to receiving waters for the study sites monitored during 2015. 

September 2015

Daily water sample collection and flow monitoring are ongoing, with samples assayed for total nitrogen (TN), nitrate, total phosphorus (TP) and total reactive phosphorus (TRP). Nitrate, TN, TP and TRP assays have been completed for samples collected through June 30, 2015. Discharge data have been collected and are being merged with nutrient concentration data to calculate loads delivered to receiving waters and nutrient yields from the watersheds for the sites monitored this year. 

June 2015

Daily water sample collection and flow monitoring is ongoing, with samples assayed for total nitrogen (TN), nitrate, total phosphorus (TP) and total reactive phosphorus (TRP). Nitrate, TN, TP and TRP assays have been completed for samples collected during 2014. The 2014 concentration and discharge data have been merged and nutrient loads delivered to receiving waters and nutrient yields from the exporting watersheds have been calculated for the study sites monitored during 2014. 

March 2015

This project is assessing direct agricultural loads to surface waters in relation to land use and management. It is focused on loads at implementation scale, meaning the scale at which loads are delivered to surface waters.  Automated water sampling and flow-monitoring equipment was installed at sites selected for this study. Daily water sample collection and flow monitoring is ongoing, with samples assayed for total nitrogen (TN), nitrate, total phosphorus (TP) and total reactive phosphorus (TRP). Nitrate assays are nearly complete but TN, TP and TRP assays are still in process. Raw flow measurement data files have been evaluated and discharge calculations completed for each of the study sites. Once the 2014 nutrient assays are complete, the discharge and nutrient concentration data will be used to determine nutrient loads delivered to receiving waters from the exporting watersheds.

December 2014

A project focuses on measuring nitrogen and phosphorus loads on a scale from a few hundred acres to a few thousand acres. Automated water sampling and flow-monitoring equipment was installed at selected sites. Daily water sample collection and flow monitoring are ongoing, with samples assayed for total nitrogen, nitrate, total phosphorus and total reactive phosphorus concentration. Stream water depth and velocity are measured at five-minute intervals, with this data used to determine discharge. The discharge and nutrient concentration data will be used to determine nutrient loads delivered to receiving waters from the exporting watersheds.

September 2014

Another project focuses on measuring nitrogen and phosphorus loads on a scale from a few hundred acres to a few thousand acres. Infrastructure to house automated water sampling and flow-monitoring equipment was installed at sites selected for this study. Daily water sample collection and flow monitoring is ongoing to monitor water draining from farmland ranging from 68 to 95 percent row crop agriculture (based on 2012 NASS land use data). The water samples have been assayed for total nitrogen, nitrate, total phosphorus and total reactive phosphorus concentration. Discharge will be determined on the basis of water depth, water velocity data and stream profile data. The discharge and nutrient concentration data will be used to determine nutrient loads delivered to receiving waters from the exporting watersheds. Data collection, analysis and load estimation work is ongoing.

June 2014

Another project focuses on measuring nitrogen and phosphorus loads on a scale from a few hundred acres to a few thousand acres. Infrastructure to house automated water sampling and flow monitoring equipment was installed at sites selected for this study. Nutrient concentrations and flow data will be used to calculate mass nutrient load from the watersheds for evaluation against land use and management information. Water samples collected during the previous reporting period have been analyzed for nitrogen and phosphorus fractions. During the winter, the monitoring equipment was removed from the field, cleaned, serviced and stored until temperatures rose sufficiently above freezing to redeploy. The sampling program will be fully operational this summer.

March 2014

Another project focuses on measuring nitrogen and phosphorus loads on a scale from a few hundred acres to a few thousand acres. Eighteen monitoring sites have been established in several central Iowa watersheds, where water quality monitoring will begin this spring. Nutrient concentrations and flow data will be used to calculate mass nutrient load from the watersheds for evaluation against land use and management information.

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