Phosphorus Loss with Surface Irrigation
Surface irrigation uses the soil to convey water across a field, contrary to sprinkler or drip irrigation that uses pipe or tubing for conveyance. Water flows across the field in small streams for furrow irrigation or in a sheet for border and basin irrigation.
In some surface irrigation systems, runoff is desirable to improve the uniformity of water infiltration between the inflow and lower ends of the field. Containing all runoff from sloping fields (>1 percent) is also impractical. Two main mechanisms influence phosphorus (P) transport in surface irrigation: erosion and P desorption into water. During the irrigation event, water flowing over soil detaches, transports and deposits sediment and the nutrients attached to sediment. Phosphorus can also desorb from the soil and suspended sediment, increasing soluble P in surface irrigation runoff.
How Does This Practice Work?
Water flowing in furrows tends to detach sediment from the upper (or inflow) end of a field with uniform slope. As the water moves across the field, the flow rate decreases due to infiltration, which not only decreases sediment detachment, but causes sediments to deposit in lower portions of the field. As water infiltrates, dissolved P is carried into the soil so the total amount of dissolved P leaving the field may be less than the amount in the inflow.
Total P in runoff water is highly correlated to sediment concentration. Roughly two pounds of total P are transported with each ton of sediment. When row crops are surface irrigated and there is significant erosion, approximately 90 percent of the total P is particulate (attached to sediment). In close-seeded crops like alfalfa and small grains or fields with minimal slope (<0.2 percent) where there is little erosion, typically less than 20 percent of the total P in runoff is particulate.
Conservation practices that control erosion and minimize runoff reduce P loss from surface irrigation. Controlling erosion reduces contact between water and detached sediment, which can reduce soluble P concentration in runoff.
One of the most effective practices is converting to sprinkler or drip irrigation to eliminate the need for water to flow over the soil. Other practices such as good inflow management, conservation tillage, cover crops, polyacrylamide (PAM) application, filter strips and sediment ponds can reduce sediment and nutrient losses from fields.
Careful inflow management will minimize the amount of runoff while achieving acceptable uniformity. In addition to reduced runoff, reducing inflow rates also decreases erosion. Inflow management is subjective and varies by field and irrigation. The main cost associated with inflow management is the labor required to check runoff and adjust flow rates periodically during irrigation.
Conservation tillage can reduce soil erodibility and increase residue in furrows, both of which reduce soil loss with surface irrigation. Cover crops in cotton in the MidSouth can reduce runoff P concentrations by 50%.
Applying PAM with irrigation water or directly to furrow soil can reduce sediment losses in runoff by 50-80 percent from fields with highly erodible soil. PAM stabilizes the soil surface and therefore is less effective in more stable, less erodible soil. Decreased erosion translates to a decrease in total P losses from treated fields. Retail cost of PAM in 2022 is $5-10/lb. Applying 1 lb/acre with each irrigation after the field is cultivated should cost less than $20/acre.
Vegetative filter strips (10 to 30 feet wide) on the bottom end of the field reduce erosion in the tail ditch and filter out 40 to 70 per- cent of transported sediment. Effectiveness is variable and excessive sediment deposition can cover and kill vegetation in the filter strip. Therefore, filter strips should be used in combination with other practices that reduce erosion on the field. Filter strips can be established with no cost by eliminating herbicide application on the bottom end of the field, which allows annual grasses to grow in the tail ditch, or they can be established for the cost of seeding a grass-type crop.
Sediment ponds placed at the end of irrigated fields remove suspended material from the irrigation water by reducing water velocity and allowing particles to settle. Sediment ponds also remove nutrients associated with sediment particles. Well-constructed ponds can potentially remove 50-80 percent of the sediment and 20-40 percent of total P entering the pond. Total P reduction will depend on the relative amounts of dissolved and particulate P in the pond inflow. Dissolved P reductions are typically minimal when ponds have continuous inflow and short retention times (<12 h).
Avoiding over-application of P, both as fertilizer and manure, will prevent soil test P from increasing and therefore reduce the potential for P losses. Nutrient management plans should ensure that P application does not exceed crop removal for extended periods of time. This will not only save money on P inputs, but will reduce the potential for elevated soil test P concentrations that can lead to P losses through erosion and soluble P in runoff.
Aryal, N., M.L. Reba, N. Straitt, T.G. Teague, J. Bouldin, and S. Dabney. 2018. Impact of cover crop and season on nutrients and sediment in runoff water measured at the edge-of-fields in the Mississippi Delta of Arkansas. Journal of Soil and Water Conservation 73:24-34.
Berg, R.D. and D.L. Carter. 1980. Furrow erosion and sediment loss- es on irrigated cropland. Journal of Soil and Water Conservation 35:267-270.
Bjorneberg, D.L., A.B. Leytem, J.A. Ippolito and A.C. Koehn. 2014. Phosphorus losses from an irrigated watershed in the northwestern United States: case study of the upper snake rock watershed. Journal of Environmental Quality 44:552-559.
Bjorneberg, D.L. and R.D. Lentz. Sediment pond effectiveness for removing phosphorus from PAM-treated irrigation furrows. Applied Engineering in Agriculture 21:589-593.
Westermann, D.T., D.L. Bjorneberg, J.K. Aase and C.W. Robbins. 2001.Phosphorus loss- es in furrow irrigation runoff. Journal of Environmental Quality 30:1009-1015.
For Further Information
Contact your local soil and water conservation district, USDA-NRCS or Cooperative Extension Service office. To find your local USDA Service Center, visit https://www.nrcs.usda.gov/contact/find-a-service-center.
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Bjorneberg, D., A. Leytem, K. Nouwakpo and M. Reba. 2023. Phosphorus Loss with Surface Irrigation. SERA17 Phosphorus Conservation Practices Fact Sheets. https://sera17.wordpress.ncsu.edu/phosphorus-loss-with-surface-irrigation/