Assessing the effectiveness of alternative tile intakes on agricultural hillslopes

Authors: WILSON, CHRISTOPHER - University Of Tennessee; STREETER, MATTHEW - University Of Iowa; ETTEMA, WILLIAM - Ducks Unlimited; ABBAN, BENJAMIN - Us Bureau Of Reclamation; GONZALEZ, ADRIAN - University Of Tennessee; SCHILLING, KEITH - University Of Iowa; Papanicolaou, Athanasios - Thanos

Submitted to: Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/14/2024
Publication Date: 1/17/2024
Citation: Wilson, C.G., Streeter, M.T., Ettema, W.D., Abban, B.K., Gonzalez, A., Schilling, K.E., Papanicolaou, A.N. 2024. Assessing the effectiveness of alternative tile intakes on agricultural hillslopes. Water. 16(2):309. https://doi.org/10.3390/w16020309.
DOI: https://doi.org/10.3390/w16020309

Interpretive Summary: Three demonstration projects were established in southeast Iowa to assess the effectiveness of Alternative Tile Intakes, or ATIs, at reducing sediment and nutrient exports from intensively managed agricultural fields. There is a need to expand the current knowledge about ATIs through proper field-based demonstrations to increase their acceptance state-wide. The demonstration projects used ATIs as a support practice for terraces and WASCOBs (water and sediment control basins). This coupling increases the overall efficiency at retaining soil that would otherwise pass freely through conventional open or slotted intakes. In addition, an ATI can be a stand-alone practice in low-relief areas with poorly drained soils. Due to their low profile, farmers can pass freely over ATIs, unlike with the slotted-pipe intakes. The performance of ATIS was complemented by a cost-benefit analysis that compared the price for installing an ATI as a support practice for a terrace or WASCOB against the monetary equivalent of the lost soil, if no practice was there. The total cost for installing an ATI was estimated at $883. The benefit of the ATI was determined using the range of erosion rates determined at the Iowa demonstration sites in Iowa, Keokuk, and Wapello counties from collected core data. The Iowa site experience annual losses between $171 to $473. The Keokuk site has annual losses between $456 to $1,260 and the Wapello site has annual losses between $970 and $2,678 per year. Thus, in less than 5 years a farmer would save enough money to recoup the costs of installing an ATI. These findings can help land managers and NRCS guide conservation planning.

Technical Abstract: Existing surface inlets behind terraces and water and sediment control basins (WASCOBs) were replaced with Alternative Tile Intakes (ATIs) in agricultural fields of southeast Iowa. These ATIs consist of a buried column of gravel sitting atop a layer of woodchips. Computational, experimental, and field methods were used to design and evaluate the performance of the ATIs at reducing sediment and nutrient export from the fields. Single-storm simulations using the Watershed Erosion Prediction Project (WEPP) established boundary conditions for permeameter experiments. The simulations provided a ponding depth of 20 cm, which was used for constant head tests with the permeameter experiments. The hydraulic conductivity of the layered gravel-woodchip ATI configuration was 4.59 cm/s ± 0.36 cm/s. Additionally, the simulations provided a sediment delivery concentration of 17 g/L, which was used to determine the trapping effectiveness of the ATI. A proportional amount of sediment was retained in the permeameter (42%) compared to the amount that settled on the permeameter surface (58%) highlighting the benefit of the ATI compared to open or slotted pipe inlets. Event monitoring of field-installed ATIs during the 2013, 2019, and 2021 growing seasons measured the delivery to and flow through of sediment, nitrogen, and phosphorus. The sediment trapping efficiency of 86 ± 12% led to deposition rates of 5.44 ± 3.77 cm/yr quantified with 210Pb profiles. Percent reduction values were 43% for nitrate and 17% for ortho-phosphate. Finally, additional modeling using continuous-storm WEPP simulations suggested that these ATIs could withstand at least 75 25-year events before clogging. Modeling using the Agricultural Conservation Planning Toolbox extrapolated their performance to the watershed scale. Load reductions of 1.6% for NO3 and 1.4% for total P were determined for ATIs draining 6.8% of the modeled watershed. Using ATIs in conjunction with WASCOBs and terraces, or as standalone practices, can be a cost-effective means for keeping sediment and nutrients on the landscape.