Linking Agricultural Practices to Water Quality Improvement: The Importance of Scale in Accurately Characterizing Nonpoint Source Nutrient Loads in Iowa Streams
The Iowa Nutrient Reduction Strategy (INRS) estimated the potential reductions in nitrogen and phosphorous loads that could be achieved by a wide range of infield and edge-of-field practices. However, most prior work on practice performance and nutrient loads in Iowa has been done at either plot scale or larger watershed scale (HUC 10 and greater). It has been challenging to properly scale up plot-level measurements to the area of practice implementation and to larger watersheds to better assess water quality impacts across landscapes and with multiple practices. Nutrient loads and load reductions at plot scale can differ substantially from loads actually delivered to surface waters. Nutrient loads at larger watershed scales (HUC 12 and above) can also differ substantially from loads actually delivered to surface waters due to the effects of in-stream processes (for example the effects of bed and bank erosion and phosphorous exchange with stream sediments). Few studies have focused on the scale at which the load is actually delivered to surface waters or how nonpoint source contributions to nutrient loads change with increasing scale. This information is critical for accurately characterizing the impacts of management practices on stream nutrient loads and for the development, calibration and validation of landscape-scale models of nutrient delivery and transport.
This project will examine the relationship between land use and management practices and stream nutrient (N and P) concentrations and loads across spatial scales spanning two orders of magnitude (from delivery scale at less than 1000 acres to approximately HUC 10 scale at greater than 100,000 acres). The primary objectives of this work are to:
- Collect close-interval surface water nutrient samples at locations along stream channels that represent a span of watershed scales within a catchment in central Iowa with a mix of agricultural and non-agricultural land use patterns.
- Measure stream flows at critical locations within this same watershed to develop data for estimating nutrient loads.
- Analyze acquired water quality and flow data in conjunction with appropriate geographical information, to improve understanding of how land use, agricultural practices, hydrology and in-stream processes affect water quality across watershed spatial scales.
Researchers will perform regular synoptic and event-based surface water sampling at selected locations in the Squaw Creek watershed of central Iowa. Collected water samples will be analyzed for total P, reactive P, total N, nitrate, and total reduced N (TRN, equivalent to TKN) at the Iowa State University Wetlands Research Laboratory (ISU-WRL). Stage-discharge rating curves will be developed for selected river stage monitoring stations based on measured discharge over a range of flow conditions (Figure 1). Point measurements of stream discharge will be obtained and correlated with continuous measurements of river stage at existing monitoring stations. Additionally, if feasible and warranted, complementary flow monitoring locations and stage recorders will be established in stream channels farther into the uplands of the catchment to better document water yields from these landscapes and improve load estimates. Field and laboratory data will be collated into a database and Geographic Information System (GIS) for further analyses and to support future modeling efforts. We will also integrate into the project database information available from existing monitoring efforts within the basin.
Note: Project reports published on the INRC website are often revised from researchers' original reports to increase consistency.
Fieldwork for the season continued through mid-November. In July, one additional sampler and stage recorder were placed at another tributary location (site WC13), bringing the total number of sampling locations to six. As with sampling from March through June, automated water samplers were maintained at each sampling location and collected samples analyzed for common nutrients at the ISU Wetland Research Laboratory. Manual flow measurements continued to be routinely collected at all sampling sites during this reporting period.
Researchers began collating the data collected to date into a comprehensive flow and nutrient database. Reasonable stage-discharge rating curves have also been developed for each monitoring location for both the 2021 and 20222 sampling seasons and researchers began calculating constituent loads from available data.
Other activity during this project period included one presentation.
Fieldwork for this field season began in early March 2022. As with sampling during the 2021 season, automated water samplers were deployed at five distinct locations along the main stem of Walnut Creek and at one tributary location. QCEC automated samplers were programmed to collect samples every two hours into a composited six-hour sample. Turbidity sensors were also installed at WC6 and WC10 and set to take readings at five-minute intervals. Grab samples were collected from each location at weekly intervals.
All samples were analyzed for nitrogen and phosphorus species (nitrate, total nitrogen, total reactive phosphorus and total phosphorus) at the Wetlands Research Lab at Iowa State. Grab samples from sites WC6 and WC10 were also analyzed for suspended sediments.
Stream stage was monitored at each location using pressure transducers set to measure at five-minute intervals. Bi-weekly flow measurements were made at each sampling location using a Sontek Flowtracker handheld Doppler velocimeter, or a Sontek Acoustic Doppler Current Profiler (ADCP). The measured discharge values were co-related to corresponding stream stage values and were subsequently used to develop stream stage discharge rating curves.