Linking Agricultural Practices to Water Quality Improvement: The Importance of Scale in Accurately Characterizing Nonpoint Source Nutrient Loads in Iowa Streams

Date: 
Sep 2019

Issue

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. 

Objective

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:

  1. 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.
  2. Measure stream flows at critical locations within this same watershed to develop data for estimating nutrient loads.  
  3. 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. 

Approach

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.  

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