Phosphorus Loss from Ephemeral Gully Formation and Sediment Transport

Date: 
Feb 2014

Issue

Phosphorus in streams can impact water quality. Determining the source of the phosphorus is difficult, since some comes from riverbanks and decaying plants, while some comes from farm fields. Until recently, tracking how much phosphorus is lost from fields has been difficult. But a new area of research estimates soil loss and associated soil materials from ephemeral gully formation.

Objective

This project seeks to determine the quantity of phosphorus loss associated with ephemeral gully source sediment in 12 Iowa watersheds, plus the proportion of total phosphorus loss from these watersheds that originate from ephemeral gully formation.

Approach

Twelve Neal Smith National Wildlife Refuge watersheds (the STRIPS project) will be studied. The watersheds already are equipped with flumes and active functioning water/sediment sampling equipment. As part of another project, ephemeral gully development and quantity of eroded sediment is currently being determined, as is the sediment and phosphorus (Total P) moving through each flume and leaving the watershed. This project will support a detailed analysis associated with each contributing ephemeral gully, allowing estimation of sediment and P export.

Project Updates

December 2015

FINAL REPORT

This study was conducted on 12 small watersheds prone to the development of ephemeral gullies within the Neal Smith National Wildlife Refuge. The watersheds are under no-till management and in a two-year corn-bean rotation. In watersheds where there were no management practices such as grass strips, evidence suggests an average of 66% of Total Phosphorus (TP) and 36% of Water Soluble Phosphorus (WSP) that escaped the watersheds and entered tertiary waterways originated from the development of ephemeral gullies. In watersheds where a portion was seeded to native prairie grass strips either at the foot slope or split between the foot slope and mid slope, there was a significant reduction in the amount of TP loss. When 10% of the watershed was put into grass strips, the ephemeral gullies eroded 150% more TP than what was lost from the watershed. The strategically placed grass strips were able to capture the ephemeral gully soil sediment containing phosphorus before it left the watershed. Within the three years the ephemeral gully developed after being filled in 2012, for the watersheds without grass strips, the ephemeral gully area lost an average of 151 lbs/ac TP and 2.3 lbs/ac WSP from the soil used to fill the ephemeral gullies alone. In watersheds where 20% of the production area was put in grass strips, the average amount of TP and WSP loss from the ephemeral gully area was reduced to 77 lbs/ac and .99 lbs/ac respectively. It is clear ephemeral gullies are main contributors to phosphorus loss in Iowa, and management practices such as grass strips or waterways placed in strategic field locations can significantly reduce nutrient loss.

September 2015

The gully channel volumes of each of the 12 study watersheds were calculated using digital elevation models (DEM) of the channels. The DEM were created from data collected in the third year of channel development. Mass of phosphorus content within each channel was calculated. The estimated phosphorus mass originating from the ephemeral gullies was compared with the total phosphorus mass leaving the field from all sources of erosion. The effect of perennial grass waterways at reducing sediment and phosphorus transport out of the watersheds was evaluated. In watersheds with 100 percent row crop, approximately half of the total phosphorus loss is from the formation of ephemeral gullies. The contribution of phosphorus loss from ephemeral gullies is reduced by an average of 50 percent in watersheds that contain grass waterways. Average loss of water-soluble phosphorus from ephemeral gullies is reduced by 40 percent when grass waterways are in place.

June 2015

Twelve Neal Smith National Wildlife Refuge watersheds (the STRIPS project) are being studied to determine the quantity of phosphorus loss associated with ephemeral gully source sediment, plus the proportion of total phosphorus loss that originate from ephemeral gully formation. Data from 570 soil samples analyzed by the Soil and Plant Analysis Laboratory have been collated and entered into Excel sheets corresponding to their sampling location. A method of calculating the estimated gully channel volume has been designed. The next step is to run the calculations across all 12 of the gully channels where samples were collected.

March 2015

Determining the quantity of phosphorus loss associated with ephemeral gully source sediment in 12 Iowa watersheds, plus the proportion of total phosphorus loss from these watersheds that originate from ephemeral gully formation, is the focus of this project. Twelve Neal Smith National Wildlife Refuge watersheds (the STRIPS project) are being studied. A total of 570 soil samples have been obtained, logged and submitted to the Soil and Plant Analysis Laboratory. Results have come back for 320 of the samples, and 150 samples are currently at the lab. The remaining 100 will be submitted when the results of the third batch have been returned. 

December 2014

This project seeks to determine the quantity of phosphorus loss associated with ephemeral gully source sediment in 12 Iowa watersheds, plus the proportion of total phosphorus loss from the watersheds that originate from ephemeral gully formation. Twelve Neal Smith National Wildlife Refuge watersheds (the STRIPS project) are studied; 570 soil samples have been obtained, logged and are being submitted to the Soil and Plant Analysis Laboratory for analysis.

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