2004 Progress Report: Measuring and Modeling the Source, Transport and Bioavailability of Phosphorus in Agricultural Watersheds

EPA Grant Number: R830669
Title: Measuring and Modeling the Source, Transport and Bioavailability of Phosphorus in Agricultural Watersheds
Investigators: Lathrop, Richard C. , Armstrong, D. E. , Hoopes, John A. , Karthikeyan, K. G. , MacKay, David Scott , Nowak, Peter , Panuska, John C. , Penn, Michael R. , Potter, Kenneth W. , Wu, Chin H.
Institution: Wisconsin Department of Natural Resources , The State University of New York at Buffalo , University of Wisconsin Madison , University of Wisconsin - Platteville
EPA Project Officer: Packard, Benjamin H
Project Period: December 17, 2002 through December 16, 2005 (Extended to December 16, 2006)
Project Period Covered by this Report: December 17, 2003 through December 16, 2004
Project Amount: $749,307
RFA: Nutrient Science for Improved Watershed Management (2002) RFA Text |  Recipients Lists
Research Category: Water , Water and Watersheds


The objectives of this research project are to:

  1. quantify effects of manure management and crop production systems on runoff phosphorus (P) losses, particularly related to the portion that is biologically available;
  2. determine spatial patterns of sediment and associated P in streams;
  3. determine in-stream fate and transport processes of P, including bioavailable P (BAP);
  4. evaluate and improve modeling tools used to assess P transport in agricultural watersheds over a wide range of spatial scales;
  5. determine relation of P losses with the scale of animal operation;
  6. and integrate outreach into on-going research efforts.

Progress Summary:

Upland Research

Part I: Quantify Effects of Manure Management and Crop Production Systems on Runoff P Losses (Objective 1)

This phase of the research project is intended to improve our understanding of the impact of levels and sources of organic matter (residue or manure) for a conservation-tillage corn management system on aggregate stability and runoff particulate P (PP) content. As its primary objectives, the project will attempt to integrate the results of previous studies using a systems monitoring approach to characterize the edge-of-field PP losses with the following sub-objectives:

  • Investigate the size distribution of primary particles and soil aggregates in runoff.
  • Determine the total P mass distribution across particle-size fractions in runoff.
  • Investigate the physical stability of aggregates and the factors impacting aggregate stability in runoff.
  • Develop general regression relationships to predict PP transport in similar soil environments with different sets of physical (e.g., rainfall intensity, runoff volume, slope length, gradient) and chemical (e.g., organic matter content, soil test P) factors.

Field Design Modifications

After the completion of field sample collection in 2003, the sample collection system was evaluated and modified to improve performance as outlined below.

  • In the spring of 2004, the drainage area tributary to each collector was defined more precisely using 10 cm high berms. The bermed area to each collector was standardized to be 36.6 m long by 4.57 m wide, encompassing an area of 0.017 ha.
  • In the summer of 2004, it was determined that the sloping-approach floor H-flume design was inadequate to keep sediment in suspension to allow automatic sampling and flow measurement to function properly. The flume was replaced with a pipe section designed to contain a bubble port for flow measurement, and automatic sampling was discontinued.
  • After completing a detailed field investigation of the area tributary to the Field AR-2

(corn-silage, no manure) collector, it was determined that the bedrock was located closer to the surface than previously known. To ensure that the shallow bedrock depth did not adversely impact the comparisons between crop management systems, the collector was moved 24.4 m up slope.

Sample Collection and Analysis Activities Completed During Year 2 of the Project

  • Samples were collected from five runoff events from June–November 2003. The November 2003 event included runoff 4 days after liquid manure was applied on frozen ground.
  • Sixteen precipitation events (including snow-melt) were collected between February and August 2004.
  • Analysis of samples collected in Year 1 of the project (2003) is complete, and analysis of the Year 2 (2004) samples is approximately 90 percent complete.
  • Field sample collection is anticipated to continue through May 2005.
  • Preliminary runoff data for the growing season (May–August, 2004) are summarized in Table 1.

Table 1. Annual Edge-of-Field Sediment and Phosphorus Loads Under Different Agricultural Management Systems



Field ID*












% VS


































* AR-1 = Corn harvested as grain, no manure; AR-2 = Corn harvested as silage, no manure; AR-3 = Corn harvested as silage w/8,700 gal/acre, liquid dairy manure.

** TS – total solids; TVS – total volatile solids; TP – total P; DRP – dissolved reactive P

Review of Preliminary Results

  • Sediment loss is significantly greater from the silage systems than the corn grain system.

This is attributable to surface residue cover differences.

  • Total P (TP) and sediment losses are highly correlated, indicating that the majority of P transport occurred in the particulate form.
  • Contributions to P losses in the dissolved form (dissolved reactive P [DRP]) is similar for all treatments.
  • Despite generating similar sediment loads (within 5%), field AR-3 (the only field receiving manure) had a significantly higher TP loss (37%) when compared with field AR-2.

Part II: Determine Relation of P Losses With the Scale of Animal Operation (Objective 5)

A stage-discharge relationship for the two monitoring stations that are needed to study runoff and P delivery at the watershed scale was completed in 2004. This will enable flow-paced sampling at both sites that were chosen to monitor P delivery differences between subwatersheds dominated by small and large animal feeding operations (AFOs).

Preliminary results from grab sampling indicated average DRP concentrations of 1.09 and 1.15 mg L-1 for the small AFO and large AFO watersheds, respectively. The similar concentrations mask the fact that discharge from the small AFO watershed ranged between 0.05 and 1.15 m3s-1, whereas the large AFO watershed delivered flows ranging from 0.35 to 1.71 m3s-1. Conclusions drawn from preliminary results will be confirmed with continuous monitoring, but these early readings suggest that DRP loadings from the large AFO watershed are much greater. Monitoring goals for Year 3 of the project include: (1) continuous flow monitoring at both locations; (2) automated sample collection for baseflow and storm runoff; (3) processing of samples to determine DRP, total dissolved P (TDP), BAP, and TP; and (4) determination of relationships between P delivery and upland or streambed soil P levels.

Runoff samples have been processed from the sheet flow monitoring stations installed to study P delivery at the hillslope scale. These results have been compiled as part of an exploratory data analysis on sediment and P losses from alfalfa swards (Cabot et al., in review, 2005). Findings of this analysis included the observation of significantly greater DRP concentration during events occurring in the late season (November), most likely due to lysing of plant cells. Declines in both DRP and TDP concentration with increased TS delivery were also observed. These observations support the findings of other researchers, who have observed that particulate-bound P tends to dominate overall P delivery as erosion and sediment movement increases. This pattern is attributed to the sorption of dissolved phase P to transported sediments. Lastly, results indicated declines in the ratio of DRP to TP during successive events following broadcast manure placement. This pattern suggests that water P extractability from soils receiving surface-applied manure is progressively decreased following successive flushing runoff events.

Stream Research

Stream Survey Work

In Year 1 of the project, the focus was to quantify and characterize the sediments in the perennially flowing section of Dorn Creek. In Year 2 of the project, the focus shifted to the upper portion of the subwatershed, which historically was ditched to drain wetlands for crop production. Sediment depth was measured at a minimum of 4 locations along cross-sections of the ditches as 13 sites over the approximately 1 km length of the upper section of the stream. Cores were collected and sub-sampled to produce profiles of TP, volatile solids, and water content.

From the cross-sectional information, estimates were generated of total sediment mass (dry weight [DW]) and P mass within the sediment deposits throughout the entire creek channel: 21.6 H 106 kg DW of sediment and 26,000 kg of P, yielding an average sediment P concentration of 1200 mg kg-1. Few such estimates of P inventories in stream channels have been reported, and this information is being prepared for publication.

Previously, Dane County Land Conservation Department had developed estimates of sediment and P exported from each of the major subwatersheds to Lake Mendota, including Dorn Creek. Using these estimates and soil loss and sediment delivery estimates, a preliminary mass balance was determined for the stream (Table 2). Currently, estimates are being refined to proportion these mass estimates among the smaller subwatersheds of Dorn Creek with varying topographic characteristics (i.e., land and stream slope).

Table 2. Preliminary Sediment and P Mass Balance for Dorn Creek.

Mass delivered to

stream (kg/yr)

Stream export to

lake (kg/yr)


accumulation (kg/yr)


3.3 H 106

5.6 H 105

2.8 H 106





Stream Sediment Chemistry Work

We are investigating the levels and dynamics of bioavailable phosphorus in the sediments of a stream (Dorn Creek) draining an agricultural watershed. Our goal is to determine whether the stream sediments are a sink for BAP transported from the watershed or a conduit for transport to the lake (Mendota) receiving the stream discharge. BAP is measured as NaOH-extractable inorganic P, a relationship developed earlier using algal bioassays. Other P fractions (i.e., chemical forms) of P in the sediments also were measured. Sediment cores along the stream at sites chosen to represent contrasts in depositional environments and associated sediment characteristics are related to accumulated sediment depth, stream dynamics (stream velocity, width, and depth), and nearby watershed characteristics.

To obtain information on the influence of discharge on P transport by the stream, we sampled stream water during several runoff events in 2004. Stream P concentrations rose to very high levels (over 1 mg/L) during events, falling to background levels of about 0.09 mg/L during base flow. Most of the P at peak concentrations was BAP, largely filterable DRP, but the excess of BAP over DRP indicated substantial particulate BAP also was transported. This was supported by concurrent high suspended particulate matter levels during these discharge events. These data show substantial transport of BAP during precipitation events. An important question is whether the P transported during these events is from external (watershed) or internal (stream sediments) sources. We believe the high DRP levels are indicative of a local watershed source, such as livestock confinement areas, while the particulate BAP may reflect stream sediment mixing and resuspension.

We used 7Be (beryllium) as a measure of sediment dynamics. With a half-life of 53 days, 7Be should be found only in “recent” sediments. Sediments obtained after a major discharge event in June 2005 showed low surficial 7Be levels at some sites, indicating sediment scouring, and significant levels several centimeters below the sediment surface at one site, indicating burial of surficial sediments by eroded sub-soil or stream-bank sediments. Sediments obtained several weeks later showed moderate 7Be levels in the surficial sediments with inventories in the range expected based on the flux of 7Be from the atmosphere. In general, the distribution of 7Be during this time interval indicated sediment mixing over the upper few centimeters, possibly associated with several moderate discharge events. The sediment mixing may account in part for the high levels of BAP carried by the stream during these events.

BAP in stream sediments varied among stream sites, but always represented a large (25–70%) fraction of the total P in the sediments. Overall, we found large amounts of BAP stored in the stream sediments. The variations among sites generally reflected the site characteristics. Levels were highest in sites where sediment accumulation was favored (lower stream velocity areas) and lower in areas more conducive to sediment scouring. Variations may also be related to the adjacent local watershed characteristics (e.g., wetland versus cropland or livestock holding areas) and associated local P transport to the stream.

Although levels of BAP and other forms of P varied with depth (time) in the sediment cores, profiles were surprisingly uniform with depth over the upper 15 cm. Organic P was especially uniform, showing little evidence of organic P mineralization after sediment deposition in the stream. This suggests that either that labile organic P has been mineralized on the watershed before sediment delivery to the stream or that the mixing indicated by the 7Be data leads to near-surface mineralization of labile organic P before the sediments are buried.

Stream Hydrodynamic Work

During Year 2 of the project, we conducted the second phase of in-stream measurements, including real-time water levels and suspended sediment concentrations, velocity profiles, and morphology changes. A video monitoring system, Automatic Control Timer Object Recorder (ACTOR), was developed and installed. In addition, a rating curve (discharge versus water surface elevation) was established. A preliminary study of sediment residence times over riffle-pool sequences was conducted. Details are described below:

To examine stream pools and riffles in response to hydrological storm events, two instrument packages were installed, upstream and downstream of the in-stream research site. Each instrument package consisted of a water level pressure sensor, an optical backscatter sensor, and a water temperature sensor with a Campbell data logger. A number of rainfall events were recorded during 2004, ranging from small storms (< 1 in rainfall) to mid-sized (1–2 in) events. A bimodal behavior in the hydrograph, with a primary water peak coming shortly after an intense rainfall, followed by a second, broader peak some time later was found. The lag time between the two peaks was generally around 13–14 hours.

Possible mechanisms to cause the bimodal hydrograph are: (1) runoff from areas with different travel times to the stream; (2) groundwater storage; and (3) surface marsh area storage. The suspended sediment concentration (sedigraph) also was bimodal, but with an increased lag between the two peaks. The first sediment peak consistently leads the first water level peak, and the second sediment peak consistently lags the second water level peak. The first sediment peak is attributed to the movement of loose sediments in the channel. The occurrence of the second sediment peak could be caused by either a rise in stream velocity as flow transitions from overbank to channelized flow or by suspended sediment delayed in surface storage. The lag time between the first and second sediment peaks in small storm events was much smaller than for the larger events in 2003, suggesting that the time scale is affected by the magnitude of storm events. Estimations of discharge and sediment load show that the second peak can carry as much as 50 percent of the total suspended load. Direct runoff coefficients are low, ranging from about 0.01 for small (< 1 in) rainfalls to over 0.10 for large (> 2 in) rainfalls. Estimated sediment fluxes ranged from 2.5 kg to 11,000 kg for two fall 2003 storms. These results show that the magnitude of hydrological storm event magnitude significantly affects the pattern of sediment transport through the riffle-pool sequences.

The video monitoring system, ACTOR, was developed with the purpose of studying bank morphology change and water level change in dry pools. The system was deployed in September 2004 at the pool-riffle site for preliminary testing. It was oriented to provide a view of the staff gauge, providing a secondary source of water surface elevation data. The results are promising. We are in the process of writing a paper on the development of this system.

To quantify the residence time of suspended sediments through riffle-pool sequences, Rhodamine WT dye was injected both continuously and as a slug to determine the discharge, mean velocity

(or mean travel time), and longitudinal dispersion coefficient. To examine the channel bed in response to storm events over time, surveys of the channel cross section at the pool-riffle site were made three times throughout the summer of 2004. Continuous channel morphology measurements are being conducted to understand short-term and long-term sediment dynamics.

Modeling Activities

Beginning in the summer of 2004, a new Ph.D. graduate student of Mackay’s (State University of New York [SUNY]- Buffalo) joined the research project as a Ph.D. Research Assistant. In June, the student spent 10 days in Madison, Wisconsin assisting the other research teams both in the field and in the lab to become familiar with all data collection efforts. This student’s work on the project will include coordination with each of the data collection groups to establish a protocol for simulations. In 2004, SWAT modeling tools were used to generate a database of Dorn Creek to support simulations with APEX.

Work also began in 2004 to begin the APEX model running on Dorn Creek. Results show the tradeoffs between dissolved and particulate P, and their relative dominance at low and high flows, respectively. During the summer of 2004 and into the fall, Mackay continued his work developing a series of analytical tools designed for quantifying errors and uncertainty in models (results were presented at a conference). During the reporting period, Mackay was co-author of a manuscript that deals directly with internally drained watersheds associated with sinks. The work examines large watersheds throughout Dane County, Wisconsin. The work demonstrates the need for consideration of internal drainage associated with glaciated topography in the establishment of regulatory watershed boundaries. The work has direct implications for the project, although at a much larger spatial extent. A logical site to explore more fully the impact of sinks on P transport is in one of the small sub watersheds being studied—a goal for Year 3 of the project.

Future Activities:

We will continue field research activities to allow for at least 2 full years of data collection at all research sites representing a wide range of watershed scales. A number of interesting questions related to shallow field depressions and their role in affecting sediment and P delivery will be pursued. The stream sediment reconnaissance work will move to new areas of the watershed, including the upper intermittent stream reaches and drainage ditches. Future work will include quantifying the contribution of sediment P resuspension and desorption to the transport of BAP during discharge events as related to sediment properties, P forms, and environmental conditions. Lead210 will be used to investigate long-term mixing, sedimentation rates, P burial rates, and sediment age in zones of sediment accumulation.

The stream hydrodynamic measurement data collected during Years 1 and 2 of the project allow us to relate the movement and storage of sediment within the riffle-pool sequences in Dorn Creek to short-term hydrologic events. The new instrument package (ACTOR) we developed will allow us to monitor the sediment dynamics of dry pools in Year 3 of the project. We will also examine the residence time for fine grained sediments in the stream bed and banks and determine the critical condition for sediment resuspension. Finally, a new University of Wisconsin-Madison Ph.D. student will join the stream hydrodynamic research team to address the research question involving the effects of stream pool and riffle, and bank morphology on sediment transport and the converse.

Modeling activities will pick up during Year 3 of the project as field research analyses are being completed. Work will be conducted to address analytical techniques for quantifying model errors and uncertainty. Improved uncertainty analysis associated with simulations of BAP is an important goal of the project and will become part of the SUNY-Buffalo student’s Ph.D. dissertation research. It is anticipated that the student will start to work with a series of analytical tools in connection with APEX modeling as he works towards a dissertation proposal (summer 2005).

Journal Articles on this Report : 3 Displayed | Download in RIS Format

Other project views: All 50 publications 9 publications in selected types All 9 journal articles
Type Citation Project Document Sources
Journal Article Cabot PE, Nowak P. Planned versus actual outcomes as a result of animal feeding operation decisions for managing phosphorus. Journal of Environmental Quality 2005;34(3):761-773. R830669 (2003)
R830669 (2004)
R830669 (2005)
R830669 (Final)
R828010 (Final)
  • Abstract from PubMed
  • Full-text: American Society of Agronomy-Full Text HTML
  • Abstract: American Society of Agronomy
  • Other: American Society of Agronomy-PDF
  • Journal Article Cabot PE, Karthikeyan KG, Miller PS, Nowak P. Sediment and phosphorus delivery from alfalfa swards. Transactions of the American Society of Agricultural and Biological Engineers (ASABE) 2006;49(2):375-388. R830669 (2004)
    R830669 (2005)
    R830669 (Final)
  • Abstract: ASABE
  • Journal Article Chen E, Mackay DS. Effects of distribution-based parameter aggregation on a spatially distributed agricultural nonpoint source pollution model. Journal of Hydrology 2004;295(1-4):211-224. R830669 (2003)
    R830669 (2004)
    R830669 (2005)
    R830669 (Final)
  • Full-text: ScienceDirect-Full Text HTML
  • Abstract: ScienceDirect
  • Other: ScienceDirect-PDF
  • Supplemental Keywords:

    upper Midwest, EPA Region 5, phosphorus, manure management, dissolved reactive phosphorus, bioavailable phosphorus, sediment transport, APEX model, SWAT model, agricultural community, bioindicators, conservation, ecosystem indicators, nutrient transport, watershed assessment,, RFA, Scientific Discipline, INTERNATIONAL COOPERATION, ECOSYSTEMS, Water, Waste, Ecosystem Protection/Environmental Exposure & Risk, Aquatic Ecosystems & Estuarine Research, Water & Watershed, Bioavailability, Aquatic Ecosystem, Water Quality Monitoring, Terrestrial Ecosystems, Environmental Monitoring, Watersheds, anthropogenic stress, bioassessment, anthropogenic processes, watershed classification, nutrient transport, fate and transport, model, ecosystem monitoring, watershed management, biodiversity, nutrient flux, conservation, nitrogen inputs, diagnostic indicators, ecosystem indicators, aquatic ecosystems, bioindicators, watershed sustainablility, water quality, biological indicators, ecosystem stress, watershed assessment, conservation planning, nitrogen uptake, bioavailable phosphorus, transport modeling, ecosystem response, aquatic biota, land use, restoration planning, watershed restoration, biological impairment

    Progress and Final Reports:

    Original Abstract
  • 2003 Progress Report
  • 2005 Progress Report
  • Final Report