Testing a Water Redistribution Model in a Cracked Vertisol at Two Scales

Abstract

Core Ideas Time for water to fully drain from cracks and mesopores was 24 to 72 h. In moist, cracked soil, water moved to 60 cm after 2 h. On dry, cracked soil, water moved to >120 cm in <1.5 h. The mesopore infiltration module improved estimation of soil profile water content. At the pedon scale, the mesopore module improved ponding predictions. Water is preferentially conducted away from the soil surface through large cracks formed in shrink–swell soils, which complicates our ability to calculate the partitioning of water into infiltration and runoff. Preferential flow paths affect the hydrology of a landscape but often are not included in hydrology models. The Precision Agricultural‐Landscape Modeling System (PALMS) contains a Mesopore and Matrix (M&M) module that allows preferential flow and was tested on cracking soil at the pedon and small watershed scale for this study. Four irrigation events were conducted on 10‐m by 10‐m plots of a cracking soil, and volumetric water content (VWC) output for PALMS with and without the M&M module was compared with that measured by a neutron moisture meter. Additionally, measurements of VWC on a 4.4‐ha small watershed were compared with PALMS predictions. At both scales, the M&M module simulated water movement down the soil profile more quickly and eliminated unobserved ponding at the pedon scale relative to the PALMS matrix only. Simulations of water content of the soil profile were generally improved when the M&M module was used. Furthermore, PALMS M&M was relatively easy to parameterize using obtainable and physically relevant parameters, rendering it applicable to shrink–swell soils in a variety of systems.

Modeling Tillage Effects on Plant-Available Water by Considering Changes in Soil Structure

“…The shallow layers showed greater r 2 values and smaller RMSE, indicating better performance of the model for shallow layers than the deeper layers. This trend is consistent with that of Bagnall et al (2019), as the model drains water out of the soil profile too quickly for it to move from the macropores to the soil matrix. The PALMS model was previously evaluated for the Blackland Prairies of Texas by Bagnall et al (2019) and used to assess the effect of soil structure parameters on VWC and water redistribution in the soil profile.…”

“…The SW17 field was a 1.2-ha perennial grassland that was managed for improved grasses (e.g., coastal Bermuda grass) and was rotationally grazed by cattle. We selected this field due to the availability of measured VWC data (Bagnall et al, 2019) for validation of PALMS's ability to simulate VWC. The second field, W13, was a 4.7-ha conventionally tilled field where corn (Zea mays L.) was planted during the 2008-2009 period, and this field was selected to model the effects of tillage practices.…”

“…Volumetric water content was measured using a neutron moisture meter (TH2O portable soil moisture probe; Delta-T Devices) at five different locations in the field at roughly two-week intervals between July 2008 and December 2009. Details about the calibration of the moisture meter can be found in Bagnall et al (2019). The coefficient of determination (r 2 ) and the root mean squared error (RMSE) were used to evaluate the model's performance in predicting VWC.…”

“…Both cases use Richard's equation for water movement, but when the M&M model is on, water moving in the macropores becomes a source for all layers of the soil profile, whereas the Green & Ampt method (1911) only supplies soil water to the surface layer. As a result, when the M&M model is used in PALMS, water is distributed quickly throughout the profile, and soil wetting behavior is less Darcian (Lepore et al, 2009;Bagnall et al, 2019).…”

Current Insights into Nonuniform Flow across Scales, Processes, and Applications

“…Moving from soil columns to the pedon and plot scales, shrinkage cracks that form in Vertisols and other soils with vertic properties often act as conduits for preferential flow to occur. Bagnall et al (2019) assessed the impact of the initial hydration condition on infiltration and wetting front characteristics. Through a series of irrigation experiments, they showed that the wetting front moves faster and deeper under dry initial conditions, than in wet soils.…”