Evaluation of Stacked Conservation Practices on Phosphorus and Sediment Loss
Accurate assessment of phosphorus (P) sources and the effect of multiple best management practices on P loss is necessary for the successful implementation of the Iowa Nutrient Reduction Strategy. The positive effects of single nutrient reduction practices are well-documented, but little research is available on the effects of multiple practices used in unison.
Field studies of stacked practices are difficult to conduct, while modeling offers low-cost, credible insight into interactions between multiple simultaneous practices. Modeling also can expedite efficient planning of field trials and demonstrations most likely to succeed across a wide range of soil, topography and weather conditions, which may serve as a farmer decision support system.
This project will add sediment-bound P modeling capabilities to the current USDA Water Erosion Prediction Project (WEPP) and Iowa’s Daily Erosion Project (DEP) computer models, enabling identification of highest potential P source areas and benefits of stacking management practices. It also will map structural and buffer practices on 27 subwatersheds to provide baseline data on practices; determine spatially appropriate scenarios for stacked management practices to estimate possible P loss reduction scenarios; analyze the validity of assumptions of additive P reduction effects; and analyze the propensity for and consistency of P and sediment export from subwatersheds statewide using the DEP and rainfall data covering 2008 through June 2015.
Data from several agencies and organizations were gathered for use in the modeling process. Work continues with the WEPP and DEP computer models to improve the framework for these to handle phosphorus modeling.
An undergraduate student was hired to begin work mapping structural and buffer practices on 27 subwatersheds to provide baseline data on practices. Work also is underway to analyze the current USDA Water Erosion Prediction Project (WEPP) computer model, and how other models and modules could be linked to it.
A team members presented details of Iowa’s Daily Erosion Project (DEP) at the Soil and Water Conservation Society meeting. DEP is one of the computer models to which this project eventually will be able to add sediment-bound P modeling capabilities.
The algorithm has been updated to extend the current flowpaths from watershed boundaries to areas of concentrated flow. Work continued on including the use of the subwatershed boundaries already developed for the Daily Erosion Project (DEP) as a second methodology. Further algorithm development also was done to increase the number of flowpaths in each subwatershed without having the flowpaths overlap. Discussion on the details of practice stacking methods continued. It was decided different sets of management files will be developed that feature individual practices singly and in different combinations, rather than attempting to develop a methodology to assign practices to watersheds.
The goals of the GIS portion of this project were to inventory structural and buffer best management practices in 27 select HUC 12 watersheds in Iowa. This work was completed last fall, and now has been reviewed by staff for inclusion in the completed datasets collection. These 27 watersheds were put on an ISU server for other staff to use in the modeling portion of the grant. The data creation, quality assessment and posting to a public data download portion of the grant are completed. Work is being done to update the algorithm to extend the current flowpaths from watershed boundaries to areas of concentrated flow.
During this reporting period, students completed 56 watersheds worth of data. Eight students worked on digitizing six specific conservation practices in the Upper Chariton, Maple and Boyer HUC 8 watersheds. Portions of the English River area in the Lower Iowa also were completed. The students evaluated the presence of terraces, wascobs, pond dams, grassed waterways, contour buffer strips and strip cropping using hillshade, CIR imagery, summer imagery and a high resolution land cover dataset. The inventory is based on the years 2007-2010, which coincides with the years Lidar was collected across Iowa. The total number of watersheds completed from all funding sources was 106. The draft completed watersheds were sent to the Iowa Department of Natural Resources staff for a final accuracy check.
The goals of the GIS portion of this project were to inventory structural and buffer best management practices in 27 select HUC 12 watersheds in Iowa. This work was completed by the end of August and is being reviewed by staff for inclusion in the completed datasets collection. These 27 watersheds make up a small portion of the 460 completed as of September 30, 2016.
Work is progressing towards the project goal of doing an inventory of riparian vegetation and grassed waterways in 27 selected HUC 12 watersheds in Iowa. Methods for estimating sediment bound phosphorus (P) losses continue to be studied. Preliminary sensitivity analysis of the Daily Erosion Project (DEP) framework using two different methods for P-loss estimations has been done, and ideas for using remote sensing to aid the DEP framework have been discussed. Work continues on the extension of current flowpaths from watershed boundaries to areas of concentrated flow.
Applicable methods for estimating sediment bound P losses continue to be studied. Two previously identified methods based on enrichment ratios were compared—an empirical method that uses total sediment loss to determine P loss, and a specific surface area (SSA) based method that incorporates a ratio of the different particle size classes found in the sediment and their physiochemical properties. The investigation found that soil type and position on the landscape have significant influences on the P losses predicted from each method. Further investigation of methods for predicting total soil P (TSP) was conducted. Additional methods that have been identified are using soil properties as proxies for different P pools that make up TSP, and using remote sensing to estimate TSP.
This project will add sediment-bound P modeling capabilities to the current USDA Water Erosion Prediction Project (WEPP) and Iowa’s Daily Erosion Project (DEP) computer models, to help identify the highest potential P source areas and benefits of stacking management practices. Methods for estimating sediment-bound P losses are being studied. Two likely candidates are a method that uses total sediment loss to determine P and a surface-area-based method that incorporates a ratio of the different particle size classes found in the sediment. Methods for estimating total soil P also are under investigation with potential ideas including using one average value for the state, tying total P values to soil type or using county level data to set an average value for each county.