Re-conceptualizing the Soil and Water Assessment Tool to Predict Subsurface Water Flow Through Macroporous Soils

More water and nutrients from artificially-drained agricultural land reach surface waters by leaching through macropores than by percolating through the soil matrix. However, the Soil and Water Assessment Tool (SWAT) describes water flows poorly in land with subsurface drainage because it does not partition water between macropore and matrix transport processes. We produced a new percolation algorithm to distinguish the macropore flow pathway, which was integrated in the SWAT-MAC model and used to predict water flows in a 30 km2 agricultural subwatershed in southern Quebec, Canada. Partitioning of subsurface flow between macropore and matrix components was reasonable, compared to a chemical-based hydrograph separation of streamflow in this subwatershed. The macropore flow algorithm also improved water allocation between the annual surface runoff and subsurface flow in the SWAT-MAC model. We predict more macropore flow into tile drains under fine-textured soils than coarse-textured soils, which is consistent with experimental observations. However, macropore flow was underestimated in the non-growing season and over-predicted during the growing season, which can be adjusted in the macropore flow algorithm by accounting for dynamic macropore connectivity or effective macroporosity. There are too few observations of regional-specific effects of soil moisture and management practices on macropore flow to correct the algorithm at this time. We conclude that the percolation algorithm of SWAT-MAC represents the macropore flow pathway and improves the description of water movement through agricultural soils with subsurface drainage systems, which are important for transferring water and nutrients to downstream aquatic systems in cold, humid temperate regions.

Spatial heterogeneity of soil properties and solute transport characteristics and their correlations in degraded wetland soils

Soil properties have a significant influence on solutes redistribution in the soil vadose zones. The aim of this study was to assess the relevance of soil properties for solute transport characteristics in degraded wetland soils using 72 undisturbed soil columns from two experimental fields located in Robinia pseudoacacia (CH) and Tamarix chinensis (CL) communities. Combining soil column tracer experiments, all experiments were conducted under the same initial and boundary conditions using Brilliant Blue FCF as a conservative tracer. Solute transport characteristics were described by four measures of dye solution steady infiltration rate of effluents, dye solution concentration of effluents, soil column dye staining patterns, and cumulative dye solution leaching. Numerical modeling by the dual-permeability model in HYDRUS-1D was used to simulate the proportion of cumulative dye solution leaching from soil macropore flow. This study showed that basic soil properties exhibited a significant difference at CH site and at CL site. Dye solution steady infiltration rate of effluents at CH site decreased with soil depth, but increased at first and then decreased with soil depth at CL site. Dye solution concentration of effluents both at CH site and at CL site decreased nonlinearly with soil depth. Soil column dye staining patterns were significantly different among different soil locations, indicating the largest dark blue staining domains from soil depth of 0-10 cm at CH site and 20-40 cm at CL site. The proportion of cumulative dye solution leaching from soil macropore flow was from 37.6 to 61.1% at CH site, whereas from 0 to 99.9% at CL site. Basic soil properties played inconsistent roles in solute transport characteristics. The understanding of soil properties and its correlation with solute transport characteristics is the first step for degraded wetland restoration and development. Some alternative solutions of wetland restoration are proposed for managers.

HESS Opinions: Unsaturated infiltration – the need for a reconsideration of historical misconceptions

Briggs (1897) deduced capillary flow from deviation of the equilibrium between capillarity and gravity. Richards (1931) raised capillary flow to an unproven soil hydrological dogma. Attempts to correct the dogma led to concepts of non-equilibrium flow, macropore flow, and preferential flow during infiltration. Viscous film flow is proposed as an alternative approach to capillarity-driven flow during unsaturated infiltration.

Investigating the impact of exit effects on solute transport in macroporous media

The effect of macropore flow on solute transport has spurred much research over the last forty years. In this study, non-reactive solute transport in water-saturated columns filled with porous media crossed by a macropore was experimentally and numerically investigated. The emphasis was put on the study of exit effects, whose very existence is inherent to the finite size of any experimental column. We specifically investigated the impact of a filter at the column outlet on water flow and solute transport in macroporous systems. Experiments involving breakthrough measurements and magnetic resonance imaging (MRI) showed that solute transport displayed some significant non-unidirectional features, with a strong mass exchange at the interface between the macropore and the matrix. Fluid dynamics and transport simulations indicated that this was due to the non-unidirectional nature of the flow field close to the outlet filter. The flow near the exit of the column was shown to be strongly impacted by the presence of the outlet filter, which acts as a barrier and redistributes water from the macropore to the matrix. This impact was apparent on the breakthrough curves and the MRI images. It was also confirmed by computer simulations and could, if not properly taken into account, impede the accurate inference of the transport properties of macroporous media from breakthrough experiments.

DRAINMOD-P: A Model for Simulating Phosphorus Dynamics and Transport in Drained Agricultural Lands: I. Model Development

HighlightsDRAINMOD-P has been developed to simulate phosphorus (P) dynamics in drained croplands.Key hydrological and biochemical processes affecting P cycling are represented in the model.The model predicts surface and subsurface P losses as affected by weather, soil, and management factors.Abstract. High phosphorus (P) loads to streams and lakes can promote harmful algae blooms and cause water quality deterioration. Recent research has identified subsurface drainage as an important pathway for the transport of dissolved P from drained croplands to receiving surface water bodies, particularly when macropore flow contributes a considerable portion of the subsurface drainage outflow. Currently, a few models are capable of simulating P dynamics in poorly drained soils with artificial drainage systems. The objective of this study was to develop DRAINMOD-P, a field-scale, process-based model that simulates P cycling and transport in drained croplands. Processes represented in the model include atmospheric deposition, organic and inorganic fertilizer applications, plant uptake, sediment-bound and dissolved P losses in both surface runoff and subsurface drainage, tillage practices, and P mineralization and immobilization. The model predicts P losses under different management practices, climatic conditions, drainage systems, and crop rotations. The model is an extension to the nitrogen model DRAINMOD-NII, with full integration of the nitrogen and P model components. DRAINMOD-P uses the recently modified hydrology component that simulates macropore flow. A soil erosion component, based on the RUSLE approach, has been incorporated into the model to estimate sediment loss and associated particulate P loss. Sediment deposition in tile drains is considered to quantify particulate P settling in the drainage system. In this article, we review the approaches used in DRAINMOD-P for simulating P-related processes. Model testing against field-measured data from a subsurface-drained field in northwest Ohio is presented in a companion article. Keywords: Best management practices, Phosphorus model, Phosphorus processes, Soil erosion, Water quality modeling.

DRAINMOD-P: A Model for Simulating Phosphorus Dynamics and Transport in Drained Agricultural Lands: II. Model Testing

HighlightsDRAINMOD-P was tested using a dataset from a drained field with desiccation cracks.Surface and subsurface phosphorus losses were mainly in the particulate form.Surface runoff was a major pathway for phosphorus loss in this field.The model performance in predicting edge-of-field phosphorus loss is promising.Abstract. The recently developed phosphorus (P) model DRAINMOD-P was tested using a four-year dataset from a subsurface-drained field in northwest Ohio with significant potential for desiccation cracking or preferential flow. The model satisfactorily predicted subsurface drainage discharge, with a monthly Nash-Sutcliffe efficiency (NSE) of 0.59 and index of agreement (IOA) of 0.89. Lack of annual water budget closure was reported and was likely caused by uncertainty in measured surface runoff and/or modeling approaches representing macropore flow. More than 80% of predicted surface and subsurface P losses were in the particulate form. Surface runoff was the major pathway for P loss, contributing 78% of predicted total P (TP) load. On average, predicted macropore flow represented about 15% of drainage discharge and contributed 21% of DRP loss via subsurface drains. The performance of DRAINMOD-P in predicting monthly dissolved reactive P and TP losses through subsurface drains can be rated as poor (NSE = 0.33 and IOA = 0.60) and very good (NSE = 0.81 and IOA = 0.95), respectively. DRAINMOD-P demonstrated potential for simulating P fate and transport in drained cropland. More testing is needed to further examine newly incorporated hydrological and biogeochemical components of the model. Keywords: Agricultural drainage, Edge-of-field phosphorus load, Macropore flow, Phosphorus model, Sediment yield, Water quality modeling.

Comparative Non-Darcian Modeling of Subsurface Preferential Flow Experimental Observations in a Riparian Buffer

HighlightsHigh ecohydrological activity drives macropore prevalence in riparian buffers.An abundance of macropore flow (MF) was confirmed in a field riparian buffer in Kenya.Source-response (SR) and multilayer kinematic diffusive wave (MKDW) MF models are compared.A novel MKDW modeling framework efficiently identifies and predicts preferential flow in riparian buffers.Abstract. The significant ecohydrological activity typical of riparian buffers makes them potential hotspots of macropores, i.e., structured preferential flow pathways, through the soil vadose zone. The prevalence of these preferential pathways can allow transported contaminants to bypass the soil matrix and quickly reach a seasonal shallow water table and the adjacent surface waterbody. This quick transport can ultimately limit the role of riparian buffers for runoff pollution control. Currently, there are no management tools that incorporate macropore flow (MF) when assessing riparian buffer performance. The objective of this study was to experimentally quantify and mathematically simulate macropore flow and arrival time in a riparian buffer under field conditions. Three infiltration experiments were conducted with a grid of 20 time-domain transmission (TDT) dielectric soil moisture sensors along a field riparian buffer transect in Kenya to quantify the presence of macropore flow and to test two non-Darcian soil MF models, including the source-responsive (SR) model and the modified kinematic-dispersive wave (MKDW) model developed in this study, by adding a user-defined multilayer convection scheme and a new hysteresis function between water flux and content. The abundance of MF in the riparian buffer was corroborated experimentally. Modeling results showed that the MKDW model was an efficient (average NSE of 0.937 and 0.721 for calibration and testing, respectively), flexible, and robust method to identify and represent non-linear and non-sequential MF signals at any soil depth and antecedent conditions. The SR model was computationally inexpensive and provided good calibration results (NSE = 0.867) but required piecemeal recalibration of the travel time and maximum water content at each layer and yielded lower performance in testing. The Akaike (AIC) and Bayesian (BIC) information criteria showed that MKDW outperformed SR when accounting for the trade-off between model complexity and efficiency. The results support further research focused on independent characterization of model parameters at the field scale, and the inclusion of MKDW in holistic riparian buffer management and decision-support tools such as VFSmod. Keywords: Kinematic-dispersive wave, Macropore flow, Numerical modeling, Preferential flow, Riparian vadose zone.

HESS Opinions: Unsaturated infiltration: from unproven capillary flow to viscous flow

Briggs (1897) deduced capillary flow from deviation of the equilibrium between capillarity and gravity. Richards (1931) raised capillary flow to an unproven soil hydrological dogma. Apparent corrections of the dogma led to non-equilibrium flow, macropore flow, and preferential flow during infiltration. Viscous flow is proposed as alternative to capillary flow during unsaturated infiltration.