Preferen al fl ow in soils and similar permeable media refers to processes of faster than average... more Preferen al fl ow in soils and similar permeable media refers to processes of faster than average water movement through only a fraction of the pore space, thereby bypassing most of the matrix. Moreover, it is commonly accepted that preferential fl ow is dominantly gravity-driven, thus dealing with rapid water movement from higher to lower locations and excluding capillary rise. Preferential fl ow has become an important issue in environmental research and has gained increasing attention over the last three decades in the hydrology of soils and fractured rocks mainly-but not only-with respect to contaminant leaching and unintended pollution of subsurface and surface waters in agricultural landscapes, and most recently in the context of hydropedology . Preferential fl ow has frequently been reviewed in the hydrology of soils (e.g., and fractured rocks (e.g., Neuman, 2005), while , for instance, emphasized the relationships between preferential fl ow and ecosystem services. Th e increasing number of research articles from around 100 in 1999 to more than 160 articles per year in 2009, and even more the number of citations from 1000 in 1999 to over 4500 in 2009 (according to ISI Web of Science for water resources and environmental sciences, and for soil and geo-sciences; see http://thomsonreuters.com/products_services/science/ [verifi ed 23 Apr. 2010]) indicate the increasing awareness of the topic.
Some drainage ditches in the intensively managed row-crop agricultural region of southern Minneso... more Some drainage ditches in the intensively managed row-crop agricultural region of southern Minnesota evolved from a trapezoidal form to multi-staged channel forms similar to natural streams. Older ditches constructed in cohesive sediment of the Des Moines Lobe till tend to follow a channel evolution model developed by Simon and Hupp. Site cross sections, longitudinal water and bed profiles and bed material particle size were determined according to Harrelson and others at 24 older ditch reaches, 5 newly constructed ditch reaches and 13 natural stream reaches. Morphological features were hypothesized to change from trapezoidal form to flat bench banks, similar to benches found in natural stream channels. All data were statistically analyzed with respect to drainage area using regression, because channel form is directly related to drainage area for a given climate, geology and land use. Results show similar regression slope and intercept for bankfull channel width and bankfull cross-sectional area (CSA) of older ditches and natural streams compared to typical trapezoidal designed ditches. Evolved ditches developed a small floodplain bench above the ditch bed and adjusted their bankfull widths similar to natural stream channels with respect to drainage area. Old ditches showed a relatively strong R 2 (0.82, 0.68) for channel CSA and width, a weaker R 2 (0.45) for water surface slope, and little to no correlation with bed particle size. Channel form appears to have adjusted more quickly than bed facets and/ or bed particle size distribution. However, stepwise regression determined that D 84 , width/depth ratio and mean bankfull depth explained 83 % of the variability of channel features across varying drainage areas. Findings suggest a possible reduction of long-term maintenance costs if older ditches are allowed to evolve over time. A stable ditch form similar to natural streams is typically self-sustaining, suggesting that prior to a scheduled clean-out, the ditch should be examined for hydraulic capacity, sediment transport and bank stability.
Many field observations have led to speculation on the role of piping in embankment failures, lan... more Many field observations have led to speculation on the role of piping in embankment failures, landslides, and gully erosion. However, there has not been a consensus on the subsurface flow and erosion processes involved, and inconsistent use of terms have exacerbated the problem. One such piping process that has been the focus in numerous field observations, but with very limited mechanistic experimental work, is flow through a discrete macropore or soil pipe. Questions exist as to the conditions under which preferential flow through soil pipes results in internal erosion, stabilizes hillslopes by acting as drains, destabilizes hillslopes via pore-pressure buildups, and results in gully formation or reformation of filled-in ephemeral gullies. The objectives of this article are to review discrepancies in terminology in order to represent the piping processes better, to highlight past experimental work on the specific processes of soil pipeflow and internal erosion, and to assess the state-of-the-art modeling of pipeflow and internal erosion. The studies reviewed include those that examined the process of slope stability as affected by the clogging of soil pipes, the process of gullies forming due to mass failures caused by flow into discontinuous soil pipes, and the process of gully initiation by tunnel collapse due to pipes enlarging by internal erosion. In some of these studies, the soil pipes were simulated with perforated tubes placed in the soil, while in others the soil pipes were formed from the soil itself. Analytical solutions of the excess shear stress equation have been applied to experimental data of internal erosion of soil pipes to calculate critical shear stress and erodibility properties of soils. The most common numerical models for pipeflow have been based on Richards' equation, with the soil pipe treated as a highly conductive porous medium instead of a void. Incorporating internal erosion into such models has proven complicated due to enlargement of the pipe with time, turbulent flow, and episodic clogging of soil pipes. These studies and modeling approaches are described, and gaps in our understanding of pipeflow and internal erosion processes and our ability to model these processes are identified, along with recommendations for future research.
There have been significant increases in stream flow in many rivers of the Upper Midwestern Unite... more There have been significant increases in stream flow in many rivers of the Upper Midwestern United States since 1980. Increased summer flows may negatively impact ecological processes, including aquatic organisms' life cycles. The smooth softshell (Apalone mutica) and wood turtle (Glyptemys insculpta) are threatened by alteration of stream flow regime and other changes to river ecosystems in the Upper Midwest. We hypothesized that prolonged duration of high summer flows would reduce time available for nesting. We assessed hydrologic change using the Indicators of Hydrologic Alteration program and stream gauge data, characterized physical properties of sandbars, surveyed turtle nesting sites and assessed historical channel change using aerial photos in GIS on five Upper Midwest rivers. A river stage-sandbar area relationship was developed to determine the effect of prolonged summer flow duration on turtle nesting opportunity for the 1940-2009 time period. Suitable water levels have declined since 1980 in the agricultural watersheds of southern Minnesota likely delaying hatching and reducing survival, particularly for aquatic turtles such as A. mutica. In contrast to the agricultural watersheds, there was no significant change in the northern forested rivers' stream flow and sandbar availability during the nesting season. Management to reduce summer stream flow in agricultural watersheds and protection of known nest sites could benefit threatened aquatic turtle populations.
The Minnesota River carries the largest load of sediment to the Mississippi River in Minnesota, m... more The Minnesota River carries the largest load of sediment to the Mississippi River in Minnesota, most of which comes from channel sources. This study investigates bank retreat in the lower Minnesota River since 1938. Specifically we asked, How have changes to river form influenced sediment transport and deposition in the lower Minnesota River and how did hydrological and ecological processes affect channel change? It was hypothesized that channel straightening, reduction in floodplain access, and streamflow increases contribute to increased channel-derived sediment load and decreased point bar deposition. Secondly, it was hypothesized that hydrologic changes have reduced woody riparian vegetation on sandbars, further promoting channel widening. To quantify channel sediment and phosphorus loading rates in the lower Minnesota River, we analyzed historic aerial photos for evidence of channel change, we performed long-term monitoring of erosion and deposition rates within the river corridor, and we calculated channel sediment transport rates. Results from this study showed that the Minnesota River has widened by 52% between Mankato and St. Paul since 1938, on average contributing 280,000 Mg of sediment per year and 153 Mg total phosphorus. The river also shortened by 7% since 1938, increasing bankfull shear stress and stream power. Sediment deposition rates in the floodplain have increased since European settlement by an order of magnitude. Ecohydrological studies showed that establishment of woody riparian plants has been inhibited on sandbars by prolonged summer flow duration and scour at high flow, reducing potential point bar growth. Findings from this study will be useful in prioritizing sediment and vegetation management actions.
Traditionally, flows that did not cause flooding were thought to be inconsequential for agricultu... more Traditionally, flows that did not cause flooding were thought to be inconsequential for agricultural watershed management. However, flow volume plays an important role in flow duration and Total Maximum Daily Loads (TMDLs), particularly for nitrate-nitrogen. Prolonged below-bankfull flows may also increase bank saturation and the frequency of mass wasting, leading to increased sediment and phosphorous loading and reduced index of biotic integrity scores. Low, mean, and high flows below the bankfull elevation have increased in many upper midwestern watersheds in the past 30 years, although large floods have not increased significantly at most of our study sites. Using the indicators of hydrologic alteration suite of statistical metrics, we found that streamflow has increased in agricultural watersheds (> 67% agricultural land use) in annual mean flow, most monthly median values, and many flow duration metrics during the 1980-2009 time period compared to . As a percentage, flow has increased most in December and least in August through October. At the same time, the streamflow-to-precipitation (Q:P) ratio has increased in the past three decades compared to the previous several decades. The overall change in Q:P, the timing of increased flow, and the reduced streamflow variability, as measured by the coefficient of variation, suggest a mechanism of subsurface tile flow and/or increased groundwater flow. Management actions are needed in agricultural watersheds of the Upper Midwest to reduce water volume as well as peak flow to meet TMDL requirements.
The borehole permeameter technique can produce erratic saturated hydraulic conductivity (K,J in s... more The borehole permeameter technique can produce erratic saturated hydraulic conductivity (K,J in soils with macropores and abrupt layers because operating theory assumes homogeneous and isotropic conditions. Dye application during an infiltration test demonstrated water movement in macropores and erratic K M or matric flux potential (<t> m )-To evaluate the effects of macropores, cracks, and layered soil on K, a and <A m , a finite element solution of the Richards equation was used to simulate infiltration from a borehole (0.03-m radius and 0.50 m deep) with a constant head (H) of 0.05 or 0.10 m. Borehole infiltration (for 2 h) was simulated for a Rozetta silt loam (fine-silty, mixed, mesic Typic Hapludalf) with four configurations: homogeneous, layered, a cylindrical macropore centered at the borehole base, and a crack intersecting the borehole wall. Simulated flow rates were increased by 29% (with H = 0.05 m) and 21% (with H = 0.10 m) when a cylindrical macropore (4 mm by 0.10 m) was located at the borehole base. Respective increases were 25% (H = 0.05 m) and 20% (H = 0.10 m) when a crack extending 0.1 m laterally intersected the borehole wall. Three methods were tested for calculating K M . The simultaneous-equations approach (SEA) using either the Guelph or the Philip model for a homogeneously configured borehole estimated K M within a factor of 2 from input K M , but the Laplace analysis method overestimated input K,, t by a factor of 5 to 12. The fixed a value (a = K a J<j>J method with either the Guelph or Philip model estimated K M close to input K M when a proper a value was chosen, but the proper a value differed by soil and model. A negative K M was computed using the SEA with the Guelph model when macropores intersected the base of a borehole; negative K,, t or <t> m were produced when cylindrical macropores laterally intersected the borehole wall, depending on the vertical locations of the macropores. Soil with layered hydraulic properties also produced unrealistic K M .
The flow of liquid water in a snowpack is complex because of the coupled processes involved, incl... more The flow of liquid water in a snowpack is complex because of the coupled processes involved, including the phase change between liquid and solid, and the latent and sensible heat transfer processes. To properly describe the details of spatial and temporal changes in a snowpack it is necessary to include these coupled processes. This paper presents a numerical model of coupled liquid water flow and heat transport in a snowpack. The model is intended to quantify infiltration into a snowpack, and evaluate the potential for the formation of distinct heterogeneities in liquid water and heat transport properties in a snowpack. The numerical model solves the two-dimensional form of the governing coupled equations using a finite difference scheme. The governing equations assume thermodynamic equilibrium between the solid and liquid phases in the snowpack. Equations describing the metamorphosis of ice grains during liquid water flow are applied within the model, and the heat and liquid water transport properties of the snow are treated with relations identical to those used for mineral porous media. Sample solution results for an alternative formulation taken from the literature are used to test the present solution, and it is found that the present model yields similar results but with some distinct differences. The effect of direct coupling of the temperature with the liquid water pressure is presented in a simple horizontal freezing simulation, which is compared with the Stefan problem where liquid water is not redistributed. Overall the direct coupling and water redistribution is found to lead to greater front penetration in comparison to the Stefan formulation. For infiltration with gravity it is shown that grain size growth during infiltration leads to increased wetting front penetration.
This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues.
Cross sections of a 203-mm diam. soil core taken from a site under Kentucky bluegrass (Poa praten... more Cross sections of a 203-mm diam. soil core taken from a site under Kentucky bluegrass (Poa pratensis L.) were scanned using x-ray computed tomography (CT) to determine if macropores such as cracks, earthworm holes and root channels could be distinguished and characterized. The core was physically sectioned at the same locations as the CT scans to verify the size and location of pores seen on the CT images. Macropores 1 mm and larger could be easily and quickly distinguished by the method. By manipulation of the scanner images, air-filled pores, roots and stones were identified within the soil matrix. A wet bulk density plus an air-filled macroporosity were calculated for scans. Prior to sectioning, a dye solution was ponded on the surface of the core and allowed to infiltrate to assess the continuity of pores through the core.
and is dependent on pore geometry, water content, and water distribution in a porous medium. Netw... more and is dependent on pore geometry, water content, and water distribution in a porous medium. Network models of porous media offer the ability to investigate the influence and interaction of pore-scale porous media properties and fluid properties on macroscopic properties of the medium. This study was conducted to investigate the macroscopic relative gas diffusion coefficient vs. air-filled porosity relationship (diffusion characteristic) of porous media using a network modeling approach. A cubic sphere-and-tube network model of porous media was adapted from petroleum engineering using Pick's law and the principle of conservation of mass to simulate one-dimensional, steadystate, isothermal, isobaric, molecular diffusion of a dilute binary gas in a nonadsorbing porous medium containing a single nonwetting fluid (air) and a single wetting fluid (water). The network model simulates hysteresis in air and water distributions in porous media for boundary drying and wetting curves of the soil water characteristic and demonstrates the effect of air-filled porosity, Henry's law liquid-gas partitioning coefficient, the ratio between gas-and liquid-phase diffusion rates, and pore geometry on the diffusion characteristic.
Gas diffusion often dominates constituent transport in porous media (PM) and is dependent on pore... more Gas diffusion often dominates constituent transport in porous media (PM) and is dependent on pore geometry, water content, and water distribution in PM. Network models of PM offer the ability to investigate the influence and interaction of pore-scale PM properties and fluid properties on macroscopic properties of the system. This study was conducted to investigate the macroscopic relative gas diffusion coefficient vs. air-filled porosity relationship or diffusion characteristic (DC) of PM using a network model. The network model was used to simulate DCs in wetting and drying PM containing air and water. A network size of nine by nine by nine spheres was used; increasing the network size to 19 by 19 by 19 produced essentially no change in the DC. The DC was independent of Henry's law gas-liquid partition coefficient (H) for H values of 0.1, 1.0, and 5.0. The product HRv,, where A w is the ratio between the bulk gas-and liquid-phase diffusion coefficients, strongly influences the DC when H values of 1000 to 10 000 are considered; this indicates that certain organic compounds have DCs independent of air-filled porosity. Hysteresis in DCs was found in selected network cases, with the wetting DCs being greater than the drying DCs for most air-filled porosities, in accord with some experimental results reported in the literature. Spatial correlation of network pore space was shown to bring the simulated DCs into better agreement with some experimental DCs.
Reliable estimates of soil water tension in field soils using few tensiometers are often difficul... more Reliable estimates of soil water tension in field soils using few tensiometers are often difficult to obtain because of the large spatial variability of soil water tensions. Therefore, the objective of this study was to evaluate the effect of tensiometer cup size on soil water tension variability. We installed three sets of tensiometers with different effective outside cup surface areas (4.8, 42.3, and 88.3 cm 2 ) in a silty clay loam that was flood-irrigated for sorghum [Sorghum tricolor (L.) Moench] production. During a period of 10 mo, soil water tensions were measured with a tensiometer under fallow and cropped conditions on 71 nonconsecutive days. We found that an increase in tensiometer cup size reduced the variability of soil water tension measurements. This reduction in variability suggests that the medium cup size (42.3 cm 2 ) extensively used for irrigation scheduling and environmental monitoring, is rather small and consequently causes a relatively large variability. Therefore, the medium cup should be replaced by a larger cup size. We also demonstrate that a logarithmic transformation of the water tensions is necessary to stabilize their variance and to allow pooling of the variances. The use of larger tensiometer cup sizes and the pooling of variances of log-transformed soil water tension measurements will lead to a lower number of tensiometers per irrigated field or environmental site.
Journal of Irrigation and Drainage Engineering-asce, 1990
Numerous methods for estimating effective rainfall have been proposed in the past, including: dir... more Numerous methods for estimating effective rainfall have been proposed in the past, including: direct measurement techniques; empirical methods; and soil water balance methods. The best estimates of effective rainfall can be obtained by conducting soil water balance computations. A soil water balance model (SWBM) for estimating effective rainfall was used to test the accuracy of the United States Department of Agriculture Soil Conservation Service (USDA-SCS) and the Hershfield effective rainfall estimation methods for a well-drained soil and for a poorly drained soil. Estimates of mean annual monthly effective rainfall by the USDA-SCS and estimates of mean annual growing season effective rainfall by the Hershfield method were found to compare closely with estimates from the SWBM for the well-drained soil but not for the poorly drained soil. Effective rainfall estimates by these two methods for either soil condition did not compare well with the SWBM estimates for annual events with return periods higher than the mean annual event. 182 ownloaded 13 Jan 2010 to 134.84.209.58. Redistribution subject to ASCE license or copyright; see http://pubs.asce.org/copyr 184 ownloaded 13 Jan
Journal of The American Water Resources Association, 2009
Abstract: The most widely used approach for evaluating the performance of stormwater best manage... more Abstract: The most widely used approach for evaluating the performance of stormwater best management practices (BMPs) such as rain gardens is monitoring, but this approach can involve a long time period to observe a sufficient number and variety of storm events, a high level of effort, and unavoidable uncertainty. In this paper, we describe the development and evaluation of three approaches for performance assessment of rain gardens: visual inspection, infiltration rate testing, and synthetic drawdown testing. Twelve rain gardens in Minnesota underwent visual inspection, with four determined to be nonfunctional based on one or more of the following criteria: (1) presence of ponded water, (2) presence of hydric soils, (3) presence of emergent (wetland) vegetation, and (4) failing vegetation. It is believed that these rain gardens failed due to a lack of maintenance. For the remaining eight rain gardens, an infiltrometer was used to determine the saturated hydraulic conductivity (Ksat) of the soil surface at several locations throughout each basin in what is termed infiltration rate testing. The median Ksat values for the rain gardens ranged from 3 to 72 cm/h. Synthetic drawdown testing was performed on three rain gardens by filling the basins with water to capacity where possible and recording water level over time. The observed drain times for two of those rain gardens were in good agreement with predictions based on the median of the infiltrometer measurements. The observed drain time for the third rain garden was much greater than predicted due to the presence of a restrictive soil layer beneath the topsoil. The assessment approaches developed in this research should prove useful for determining whether the construction of the rain garden was performed properly, a rain garden is functioning properly, and for developing maintenance tasks and schedules.
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