Contributions of systematic tile drainage to watershed scale phosphorus transport

Authors: King, Kevin; Williams, Mark; Fausey, Norman

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/28/2014
Publication Date: 10/3/2014
Publication URL: http://handle.nal.usda.gov/10113/61311
Citation: King, K.W., Williams, M.R., Fausey, N.R. 2014. Contributions of systematic tile drainage to watershed scale phosphorus transport. Journal of Environmental Quality. 44:486-494. DOI:10.2134/jeq2014.04.0149.

Interpretive Summary: Harmful algal blooms (HABs) and nuisance algae continue to plague freshwater systems throughout the world. The extent and severity of the HABs often leads to undue hardship on the surrounding communities that depend on the water for a drinking water supply and for the community’s economic viability. Much of the focus is on agricultural phosphorus, particularly, that delivered through subsurface (tile) drainage systems. Using 8 years of surface and subsurface water quality data from the Upper Big Walnut Creek watershed in central Ohio, we determined that a significant portion of agricultural phosphorus measured at the watershed outlet originated in the subsurface drainage network. We also found that the delivery of phosphorus in subsurface drainage was seasonal with greater losses in the winter and spring. The unit area phosphorus losses were consistent with the amounts identified as problematic for freshwater systems. However, the measured losses represented less than 2% of application amounts. So, from an agronomic standpoint the losses are negligible; however, from a water quality perspective the losses are significant. The findings point to the need for conservationists and policy makers to jointly consider the economic and environmental benefits associated with the development and implementation of best management practices, programs, and policies.

Technical Abstract: Phosphorus (P) transport from agricultural fields continues be a focal point for addressing harmful algal blooms (HABs) and nuisance algae in freshwater systems throughout the world. In humid, poorly drained regions, attention has turned to P delivery through subsurface tile drainage. Research on the contributions of tile drainage to watershed scale P losses, however, is limited. The objective of this study was to evaluate P movement through tile drainage and its manifestation at the watershed outlet. Hydrology, along with associated P concentrations were collected for 8 years (2005-2012) from 6 tile drains and the watershed outlet of a headwater watershed within the Upper Big Walnut Creek watershed in central Ohio, USA. Results showed that tile drainage accounted for 51% of the discharge, 47% of the dissolved P, and 43% of the total P exported from the watershed. Average annual total P loss from the watershed was 0.98 kg/ha and annual total P loss from the 6 tile drains equaled 0.48 kg/ha. Phosphorus loads in tile and watershed discharge tended to be greater in the winter and spring, while P concentrations were greatest in the summer. Over the 8 year study, P transported in tile drains represented less than 2% of application rates and greater than 90% of all measured concentrations exceeded recommended levels (0.03 mg/L) for minimizing HABs and nuisance algae. Thus, the results of this study show that in systematically tile drained headwater watersheds, the amount of P delivered to surface waters via tile drains cannot be dismissed. Given the amount of P loss relative to application rates, development and implementation of best management practices must jointly consider both economic and environmental benefits.