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ARS Home » Pacific West Area » Kimberly, Idaho » Northwest Irrigation and Soils Research » Research » Research Project #441130

Research Project: Contribution of Legacy Phosphorus to Watershed Outcomes, Phase 1 & 2

Location: Northwest Irrigation and Soils Research

Project Number: 2054-13000-010-004-I
Project Type: Interagency Reimbursable Agreement

Start Date: Sep 1, 2021
End Date: Jul 30, 2026

The overall goal of Phase 1 is to understand the contribution and processes of legacy phosphorus to loads at the edge-of-field and in small watersheds to develop conservation strategies to mitigate for legacy phosphorus. Specific goals are to 1. develop common data base and use state-of-the-art analytical techniques to quantify the contribution of legacy phosphorus to runoff loads at field and small watershed scales; 2. characterize legacy phosphorus sources and mobilization processes within study watersheds; and 3. develop watershed-specific recommendations for the mitigation of legacy phosphorus associated with agriculture. Phase II: 1. Build a SWAT model to accurately represent water distribution and reuse within a watershed with highly managed hydrology. 2. Develop furrow irrigation erosion simulation routine for SWAT. 3. Simulate the recommended watershed strategy for cost-effective mitigation of legacy phosphorus sources developed at the conclusion of Phase 1 of the project. 4. Modify SWAT routines to improve representation of legacy phosphorus processes for final assessment of watershed outcomes.

Research for the Snake River Watershed will focus on irrigation return flow from the Twin Falls Canal Company. Detailed analysis of individual return flow phosphorus concentrations and loads will be conducted to identify seasonal or annual trends. Return flow data will also be correlated with crop type, irrigation method and field topography to identify practices and conditions that lead to greater sediment and phosphorus returning to the river. Since irrigation runoff from one field is often used for irrigation on another field, correlation analysis will compare land use from a defined radius from sampling point, a defined distance from the return flow channel, and the entire HUC12 watershed. Phosphorus sorption parameters will be measured on field soil and channel sediment to estimate potential soluble phosphorus contribution to return flow from channel sediment. Prior data from rainfall simulations and furrow irrigation studies at Kimberly will be synthesized to estimate potential edge-of-field phosphorus losses. Phosphorus removal with water quality ponds will be evaluated with detailed monitoring to document impacts of pond design and management on sediment and soluble phosphorus removal. Chemical treatment, such as alum, will be evaluated to enhance phosphorus removal in water quality ponds. Research will also investigate the fate of alum and phosphorus downstream from treated ponds, the fate of precipitated P when water starts flowing through the ponds each spring, and the impacts of growing crops in alum-treated sediment. Phase II: New knowledge learned from Phase 1 data analysis will identify knowledge gaps and limitations of current models. Solutions to address those limitations will be proposed and, where feasible, implemented and tested in models. This will primarily be accomplished through SWAT simulation in the Twin Falls irrigation tract. Model development will include the entire irrigation tract and select sub-basins within the tract. Modeling hydrology in this watershed is particularly challenging because it is mainly influenced by water flowing in irrigation canals, ditches and drains, not precipitation and natural topography. Furthermore, furrow and sprinkler irrigation are the primary causes of erosion, not precipitation. Soluble phosphorus concentrations also increase as water flows over soil during furrow irrigation. In addition to modifying SWAT to accurately simulate hydrology, irrigation runoff and erosion prediction routines will be added to SWAT.