Water Quality Evaluation of Prairie Strips Across Iowa
The STRIPS project (Science-based Trials of Rowcrops Integrated with Prairie Strips) conducted at Neal Smith National Wildlife Refuge near Prairie City showed integrating tallgrass prairie vegetation into row-cropped watersheds can provide water quality benefits and increase biodiversity across several taxa. Because these studies were conducted on small watersheds at one location, there is a need to evaluate the water quality benefits of prairie strips when implemented on a full-farm scale across various locations in Iowa.
The objectives of this project are to assess the effects of prairie strips on the quantity and quality of surface water runoff from cropped watersheds, assess the effects of prairie strips on dissolved nutrient concentrations in shallow groundwater, and disseminate project information through field days, Iowa State University Extension Outreach fact sheets, and presentations throughout the state.
In the past 36 months, prairie strips have been installed on more than 34 sites in Iowa. Some of these sites allow for detailed surface runoff water quality and quantity evaluation using a paired approach in which catchments with prairie strips are compared with catchments without prairie strips. This project will be conducted at seven of these study sites throughout Iowa.
Note: Project reports published on the INRC website are often revised from researchers' original reports to increase consistency.
The greatly disproportionate benefits from incorporation of minimal acreage of tallgrass prairie vegetation into row-cropped agriculture has propelled the STRIPs team to develop the long-term goals of 1) implementing prairie strips on farmers’ fields throughout Iowa and 2) evaluating the impacts of these systems over watersheds and landscapes. The overall objective of this prospectus was to evaluate and compare water quality benefits from sites with newly installed variable width prairie strips to those with no prairie strips and existing CRP CP-15 contour strips. The questions this project examined include:
What are the effects of variable width prairie strips when compared to similar fields without contour strips?
Are there differences in water quality benefits of CRP contour strips and variable width contour prairie strips relative to erosion and sediment, nitrogen, and phosphorus export?
Are there modifications to the CRP contour strip practice that would make the practice more beneficial for reducing erosion and sediment, nitrogen, and phosphorus leaving fields?
Key findings are outlined in the attached document.
Groundwater and surface water runoff monitoring continued in 2019. Groundwater depth data followed by groundwater samples were collected once per month and submitted for nutrient analyses. Surface water runoff monitoring has been successful, with at least one runoff event at each site having been significant enough to collect water samples from each monitored watershed. Runoff is generally similar or lower for the treatment site, except at the McNay farm where runoff has been significantly greater than at the treatment watershed. This has been noted in previous years as well and is due to there being a side-slope seep that keeps the treatment watershed soil relatively more wet and therefore less likely to absorb rainfall when compared to the control watershed.
The 2018 runoff monitoring season concluded on October 8. A long period of unusually cold weather in the early part of October caused monitoring to cease early, as the freezing temperatures can be harmful to field equipment. Results of total nitrogen and total phosphorus have yet to be returned from the lab.
Prairie strips seem to have reduced runoff volume, with the exception of the McNay site. The field where prairie strips are located at McNay is noticeably wetter than the field without prairie strips due to side-slope seeps. We suspect the wet conditions reduce infiltration and therefore contribute to the higher amount of runoff in the treatment field compared to the control field.
Nutrient loss summaries show variable results between the treatment and control fields. These will require further and deeper investigation to better understand and will most likely require analysis on a single runoff event scale, not the entire monitoring season.
Groundwater depth monitoring shows that groundwater tends to be closer to the surface in the fields without prairie strips. Groundwater nutrient analyses results have yet to be returned from the lab.
There were significantly more runoff events this monitoring season in comparison to the previous two years. We have collected water samples from multiple runoff events from every site, and these have been submitted to our laboratory for sediment and nutrient analyses. Groundwater continues to be monitored monthly, and will also be summarized at the end of the monitoring season along with the runoff data.
Surface water runoff monitoring for the 2018 season began during the second and third week of April. There were several rain events across the state significant enough to have caused runoff events at all research sites, except the Armstrong research farm. These runoff data will be summarized at the end of the monitoring season. Water samples from these events have been collected and submitted to a laboratory for sediment and nutrient analyses. Groundwater continues to be monitored on a monthly basis.
Groundwater continues to be collected on a monthly basis. Surface water runoff monitoring has not yet commenced in 2018 due to cooler than normal conditions, yet equipment has been delivered to the sites. Progress has been made with nutrient analyses. Early in 2017, groundwater at most sites was relatively shallow and tended to slowly get deeper until about September, when levels began to rebound. Nitrate-N concentrations are consistently in the 10 mg/L range and reductions in concentrations of nitrate-N at the downstream side of the bottom prairie strip are seen at three of the sites. Overall, the level of dissolved P is less than 0.1 mg/L at most sites and little treatment effect of prairie strips is seen. The experimental sites, both in 2016 and 2017, have experienced somewhat rare conditions in terms of a relatively few number of significant surface runoff producing rain events. Generally speaking, when there is a noticeable difference within sites and between the control (watershed with no prairie strips) and treatment (watershed with prairie strips), the cumulative export of total suspended solids and nutrients is less in the treatment watersheds.
Surface water runoff monitoring for the 2017 season concluded in early November due to freezing temperatures. The automated water samplers were shut down and brought in from the field until next monitoring season. In early October, there was a single runoff sample collected from the Rhodes site (experimental control watershed). Groundwater continues to be monitored on a monthly basis. Minor maintenance and upkeep to the flumes and water samplers continues throughout the season. Progress has been made with nutrient analyses.
Surface water runoff monitoring continued. However, none of the sites received enough rain intensity to produce samples for nutrient analyses this quarter. Groundwater continues to be monitored on a monthly basis. Minor maintenance and upkeep to the flumes and water samplers continues throughout the season.