Water storage, mixing, and fluxes in tile-drained agricultural fields inferred from stable water isotopes

Authors: Williams, Mark; McAfee, Scott

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/13/2021
Publication Date: 4/17/2021
Citation: Williams, M.R., McAfee, S.J. 2021. Water storage, mixing, and fluxes in tile-drained agricultural fields inferred from stable water isotopes. Journal of Hydrology. 599. Article 126347. https://doi.org/10.1016/j.jhydrol.2021.126347.
DOI: https://doi.org/10.1016/j.jhydrol.2021.126347

Interpretive Summary: Understanding how water moves through the soil to subsurface tile drains in agricultural fields is important for assessing nutrient loss such as nitrogen and phosphorus losses and determining conservation practices that can decrease these losses. In this study, we used naturally occurring stable isotopes of water (oxygen-18 and deuterium) to (1) determine how rainfall mixes with water stored in the soil and groundwater, (2) assess the average age of water stored in the soil and groundwater, and (3) examine the primary sources of water discharged from tile drains. We monitored a field in northeastern Indiana over a 2-year period. We found that mobile soil water in the top 8 inches of the soil profile was reflected seasonal rainfall patterns in isotopic signature, while groundwater data suggested that water stored below tile depth was largely recharged during the winter months. Water discharge from the subsurface drainage network had an average age of 245 days and was primarily comprised of groundwater and water stored in the soil profile. Findings show that stable water isotopes are a useful tool for understanding water fate and transport in tile-drained fields. The results also provide valuable information for understanding nutrient transport and for improving models used to simulate water and nutrient loss in drained landscapes of the U.S. Midwest.

Technical Abstract: Quantifying hydrological processes that control the upper critical zone water balance and contaminant transport in drained landscapes is needed, especially as precipitation patterns driving water balance dynamics continue to shift due to climate change. Here, hydrometric data are integrated with stable isotope signatures to quantify water storage, mixing, and fluxes to subsurface tile drainage at an agricultural field located in Indiana, USA. Over a 2-yr period, precipitation, soil water sampled with suction lysimeters (10–80 cm depth), groundwater (below tile depth; >1 m), and subsurface tile discharge were sampled 97 times. Results showed that isotopic variability in near-surface soil water (10–20 cm) reflected the seasonality of the precipitation input signal, while groundwater values were relatively consistent indicating that water stored below tile drain depth was recharged during winter. Soil water between 20–80 cm depth was a mixture of near-surface water and groundwater that varied seasonally depending upon groundwater hydrodynamics. Mean transit time of water ranged from 12–20 d for 10-cm soil water to 225–334 d for groundwater, with tile drainage exhibiting a mean transit time of 245 d. Both two- and three-component hydrograph separation indicated that groundwater was the primary source of water to the tile drain followed by soil water. Tile drain hydrograph response (i.e., celerity) was largely controlled by antecedent wetness. Comparison of tile drain celerities and velocities revealed however varying mechanisms controlling hydrograph response across a range of environmental conditions. Data sets of both water and tracer flux were, thus, useful to track the spatiotemporal variability of water fluxes within and from the critical zone. Such data provide valuable information to improve the representation of critical zone processes in these landscapes within spatially distributed hydrological models.