Estimation of tile drainage contribution to streamflow and nutrient loads at the watershed scale based on continuously monitored data

Abstract

Nitrogen losses from artificially drained watersheds degrade water quality at local and regional scales. In this study, we used an end-member mixing analysis (EMMA) together with high temporal resolution water quality and streamflow data collected in the 122 km Otter Creek watershed located in northeast Iowa. We estimated the contribution of three end-members (groundwater, tile drainage, and quick flow) to streamflow and nitrogen loads and tested several combinations of possible nitrate concentrations for the end-members. Results indicated that subsurface tile drainage is responsible for at least 50% of the watershed nitrogen load between April 15 and November 1, 2015. Tiles delivered up to 80% of the stream N load while providing only 15-43% of the streamflow, whereas quick flows only marginally contributed to N loading. Data collected offer guidance about areas of the watershed that should be targeted for nitrogen export mitigation strategies.

Assessment of Spatial Nitrate Patterns in An Eastern Iowa Watershed Using Boat-Deployed Sensors

“…Lowest NO3-N concentrations were observed in August for all reaches except the Middle Cedar. Our data suggest either more in-stream processing of NO3-N is occurring in the warm water condition of August, or dilution by lower-NO3-N groundwater is disproportionately larger during that time, or both, consistent with existing literature [8,23,25]. Our data also demonstrate that a fixed-location study based in the uppermost 60 km of either stream would show little seasonal variation (Figures 7 and 8).…”

“…Concentration declines were observed in the Cedar River despite the absence of major tributaries from 325 km-305 km and 180 km-0 km, indicating in-stream processes were likely consuming the nitrogen. Whereas other fixed-location monitoring studies have reported lower NO3-N concentrations in August compared to May, June, and July [8,23,25], our boat-deployed longitudinal analysis showed how these relationships evolve over long reaches from month to month in the growing season. Lowest NO3-N concentrations were observed in August for all reaches except the Middle Cedar.…”

“…The average Iowa River concentration generated by the boat-sensor system was 7.8 mg L −1 . Sprague et al [31] modeled a 2008 flow normalized average concentration at the Iowa River outlet of Whereas other fixed-location monitoring studies have reported lower NO 3 -N concentrations in August compared to May, June, and July [8,23,25], our boat-deployed longitudinal analysis showed how these relationships evolve over long reaches from month to month in the growing season. Lowest NO 3 -N concentrations were observed in August for all reaches except the Middle Cedar.…”

“…However, NO 3 -N concentrations lower than that of the main channel were observed in Prairie Creek, Sugar Creek, and Wapsinonoc Creek despite these three tributaries having over 80% of their land area grown to those crops. Wapsinonoc Creek and Sugar Creek are both in the SIDP, whereas the South Fork, Beaver Creek, Black Hawk Creek, and Wolf Creek, are in the DML and IS, demonstrating that tile drainage (DML and IS) is also a strong driver of stream nitrate [23,29]. It is likely that tile drainage required for row crop production in the low-relief landforms and the percentage of corn and soybean grown in the watershed both contribute to higher stream NO 3 -N concentration [23,29].…”

“…Downstream from headwater areas, several Cedar River tributaries extend west into land with The Iowa and Cedar Rivers have differing geology and land use in their headwaters. The Iowa River headwaters are in the DML, a region of intense row crop agriculture and artificially drained soils that efficiently deliver NO 3 -N to streams [17,23]. Most of the major tributaries of the Iowa River, with the exception of the South Fork and Minerva Creek, lie within the SIDP where tile drainage and agricultural intensity is less.…”