Assessment of phosphorus retention in irrigation laterals

Abstract

Irrigation laterals transport irrigation return flow, including water, sediment, and dissolved nutrients, such as phosphorus (P), back to surface water bodies. Phosphorus transformations during transport can affect both P bioavailability and the best management practices selected to minimize P inputs to waters of the United States. The objective of this study was to determine P retention in three irrigation laterals. Soluble reactive P (SRP) concentrations in lateral waters were increased from 0.08 to 0.25 mg L -1 (0.08 to 0.25 ppm) by constantly injecting a phosphate (PO 4 ) solution for 2.5 hours. Bromide (Br) was used as a conservative tracer to determine dilution effects. Water was sampled at 10-minute intervals, beginning 30 minutes prior to injection and 120 minutes following injection, at one upstream location and various downstream locations to approximately 1,550 m (~1 mi) from injection sites. When at steady state, SRP concentrations only decreased by 5% over the lengths studied, equating to P uptake lengths of over 18 km (11.2 mi), which was one to two orders of magnitude greater than natural streams; the linear SRP uptake rate was 0.011 mg L -1 km -1 (0.018 ppm mi -1). Longer P uptake lengths and lower uptake rates in irrigation laterals, as compared to natural streams, may be due to the elevated sediment equilibrium P concentration, greater water velocities, and removal of vegetation causing a reduction in frictional resistance. Reducing water velocities should optimize irrigation lateral conditions to reduce uptake length and maximize P uptake. Key words: coulee-irrigation lateral-retention-soluble reactive phosphorus-transportAquatic resources in the United States are among its most valuable assets, yet approximately 40% of assessed stream miles, 45% of assessed lake acres, and 50% of assessed estuary acres are impaired (USEPA 2003). The transport of excess nutrients is one of the leading causes of waterway impairment (USEPA 2003). In particular, soluble reactive phosphorus (SRP) transport in aquatic systems is of interest because phosphorus (P) is commonly regarded as the limiting nutrient governing primary production in fresh water systems (Foy 2005). Specifically, P transport in return-flow irrigation laterals may be a significant P source to river systems such as the Snake River in Idaho. Thus, determining and understanding the uptake distance SRP travels prior to being removed from solution during source-tosink water movement could help identify whether current regional soil and water management strategies are having as significant of an impact on water quality improvement as intended.Soluble reactive P travel distance during water transport can be quantified in terms of P spiraling length, or the distance SRP travels as it cycles through dissolved to particulate (organic or inorganic forms) and back to the dissolved state (Davis and Minshall 1999). Total spiraling length is the sum of uptake length (distance traveled in the dissolved form) and turnover length (distance traveled in...

Mechanisms Responsible for Soil Phosphorus Availability Differences between Sprinkler and Furrow Irrigation

“…In fact, the primary P loss pathway from agricultural soils is through surface runoff (Vadas et al, 2004), which may be enhanced by the use of certain irrigation systems. Furthermore, once in irrigation return flow waters, P may be transported distances greater than 18 km (Ippolito and Nelson, 2013). Thus, managing irrigation practices may help control runoff and consequently reduce P losses from agricultural systems.…”

Phosphorus Losses from an Irrigated Watershed in the Northwestern United States: Case Study of the Upper Snake Rock Watershed

“…The concentration of DP in furrow irrigation runoff is related to soil test P concentration and the amount of sediment detached by flow in irrigation furrows (Bjorneberg et al, 2006). Research investigating the transformations and transport of DP through the return flow system has suggested that DP in runoff from furrow-irrigated fields likely remains in the water until it returns to the Snake River (Ippolito and Nelson, 2013).…”