Preferential flow in the shallow vadose zone: Effect of rainfall intensity, soil moisture, connectivity, and agricultural management

Authors: Williams, Mark; FORD, WILLIAM - University Of Kentucky; MUMBI, ROSE - Purdue University

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/6/2023
Publication Date: 12/27/2023
Citation: Williams, M.R., Ford, W.I., Mumbi, R. 2023. Preferential flow in the shallow vadose zone: Effect of rainfall intensity, soil moisture, connectivity, and agricultural management. Hydrological Processes. https://doi.org/10.1002/hyp.15057.
DOI: https://doi.org/10.1002/hyp.15057

Interpretive Summary: Quantifying water movement through the soil profile is important for assessing nutrient, pesticide, and sediment transport to subsurface tile drains. In this laboratory study, we collected intact, undisturbed soil columns (1 ft3) from an agricultural field in northeastern Indiana. A total of 11 rainfall simulations were conducted as part of 3 experiments to evaluate the effect of rainfall intensity, soil moisture, and agricultural management (no-till vs. till) of water storage, mixing, and transport. Resulted showed that flow rate through the soil profile increased with increased rainfall intensity, but the dominant source of water remained similar across the intensities tested. Wetter soil resulted in greater mobilization of stored soil water compared to drier conditions and resulted in less preferential flow. Similarly, tillage of the soil surface decreased the amount of preferential flow compared to undisturbed soil conditions. Findings are crucial for predicting when, where, and how water moves through the soil profile over a range of environmental and management conditions. Future work will examine how differences in observed hydrology affect nutrient (nitrogen and phosphorus) and sediment transport through the subsurface.

Technical Abstract: Preferential flow is ubiquitous in soil, and it affects water infiltration, runoff, and contaminant transport. The objective of this study was to quantify the effect of climate (rainfall intensity), soil (antecedent moisture), and agricultural management practices (tillage) on water transport through the shallow vadose zone. Undisturbed soil lysimeters (n=10; 900 cm3) collected from an agricultural field were subjected to three laboratory rainfall simulation experiments (n=11 rainfall simulations) with varying rainfall intensity (10-60 mm hr-1), antecedent soil moisture (3-21 d rainfall interruption duration), and management practice (no-tillage vs. tillage). Stable water isotopes (d18O and d2H) were used to separate leachate into event (Qe) and pre-event water (Qpe). During the second experiment, an artificial macropore was added to n=3 lysimeters. Results showed that event water transport in leachate was not affected by rainfall intensity (Qe/Q = 49±21% to 50±24%); however, event water decreased from 65±5% to 23±28% with increasing soil moisture. Addition of an artificial macropore resulted in leachate that was nearly all event water (85±12% to 92±4%) irrespective of soil moisture. Tillage of the soil surface resulted in decreased event water transport for both lysimeters with and without an artificial macropore by ~30%. Findings show how varying climate, soil, and management practices produce a continuum of preferential flow, which has implications for nutrient and sediment transport. Water and tracer flux data collected in the current study are therefore essential for predicting conditions with high relevance of preferential flow when assessing or modeling long-term hydrographs where these conditions are only met during a small proportion of the flow time.