Location: National Soil Erosion ResearchTitle: Dissolved organic carbon loading from the field to watershed scale in tile-drained landscapes Author
Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/21/2017
Publication Date: N/A
Citation: N/A Interpretive Summary: In Midwestern agricultural watersheds, best management practices have typically focused on decreasing nutrient, sediment, and pesticide losses, but few studies have examined how these practices affect dissolved organic carbon (DOC) transport. Decreasing DOC losses from agricultural landscapes is important because DOC can influence the transport and bioavailability of organic pollutants and heavy metals, which pose a risk to downstream drinking water quality. In this study, we measured DOC concentrations and losses in a watershed in Ohio, USA to better understand how agricultural practices influence the amount and timing of DOC loss. Results showed that tile drains account for, on average, 33% of monthly watershed DOC loss, which suggests that they are a significant source of DOC to streams. We also tested the effect of drainage water management on decreasing DOC concentration and load. Managing drainage water with a control structure at the end of a tile outlet resulted in a 22% decrease in tile discharge and a 26% decrease in DOC load compared to a free draining tile outlet. Study results indicate that drainage water management is an effective best management practice in tile-drained Midwestern landscapes to decrease DOC losses to streams.
Technical Abstract: Dissolved organic carbon (DOC) is an integral part to the functioning of aquatic ecosystems; yet, there is a paucity of data on DOC delivery and management in tile-drained agricultural headwater watersheds. The objective of this study was to quantify the contribution of subsurface tile drains to watershed DOC export and to evaluate the effect of drainage water management of DOC concentrations and loads in tile-drained fields. Discharge and DOC concentration were measured at the outlet of an agricultural headwater watershed (3.9 km2) in Ohio, USA and all of the subsurface tile drains (6 total) within the watershed from 2005 through 2012. Results showed that DOC concentration in both subsurface tile drains and stream water were highly variable (0.1 to 44.4 mg L-1), with mean DOC concentrations ranging from 5.7 to 8.2 mg L-1. Intra-annual variability in subsurface tile drain and watershed hydrology yielded seasonal differences in DOC loading. Over the study period, 81.7% and 92.4% of watershed and subsurface tile drain DOC loading, respectively, occurred during 20% of the time, typically during winter and spring high flow events. Mean annual DOC loading from the drainage network was 19.6 kg ha-1, while mean annual DOC loading at the watershed outlet was 43.9 kg ha-1. On average, subsurface tile drainage comprised 33% of monthly watershed DOC export (<1 to 82%). Implementing drainage water management at one of the subsurface tile drains decreased discharge (179 mm; 22%) and DOC loading (6.8 kg ha-1; 26%) compared to an adjacent free draining subsurface tile drain. Findings from this study demonstrate the utility of simultaneously monitoring solute fluxes from both field and watershed scales, and indicate that subsurface tile drains are a significant source of DOC to headwater agricultural streams. Further, results suggest that drainage water management can significantly decrease DOC losses from tile-drained fields.