Ted Zobeck - Academia.edu (original) (raw)

Papers by Ted Zobeck

Soil Science Society of America Journal, 2011

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or b... more All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Soil Science Society of America eBooks, Nov 2, 2015

AGU Fall Meeting Abstracts, Dec 1, 2006

Shrub encroachment, a widespread phenomenon in arid landscapes, creates ''islands of fertility'' ... more Shrub encroachment, a widespread phenomenon in arid landscapes, creates ''islands of fertility'' in degraded systems as wind erosion removes nutrient-rich soil from intercanopy areas and deposits it in nearby shrub-vegetated patches. These islands of fertility generally are considered to be irreversible. Recently, fire has been observed to alter this pattern of resource heterogeneity through the redistribution of nutrients from the fertile islands of burnt shrubs to the surrounding bare soil areas. Despite the recognized relevance of both fires and wind erosion to the structure and function of arid ecosystems, the interactions between these two processes remains poorly understood. This study tests the hypothesis that fire-induced soil hydrophobicity developing in the soils beneath burned shrubs enhances soil erodibility by weakening the interparticle wet-bonding forces. To test this hypothesis, the effects of grass and shrub fires on changes in soil erodibility and on the intensity of fire-induced soil water repellency are compared at both the field and patch scales in heterogeneous arid landscapes. Higher water repellency was observed in conjunction with a stronger decrease in wind erosion threshold velocity around the shrubs than in grass-dominated patches affected by fire, while neither water repellency nor changes in threshold velocity was noticed in the bare soil interspaces. Thus, fires are found to induce soil hydrophobicity and to consequently enhance soil erodibility in shrub-vegetated islands of fertility. These processes create temporally dynamic islands of fertility and contribute to a decrease in resource heterogeneity in aridland ecosystems following fire.

Paper - American Society of Agricultural Engineers, 1990

Wind Tunnel Designs and Their Diverse Engineering Applications, 2013

AGU Fall Meeting Abstracts, Dec 1, 2012

SSSA special publication series, Oct 26, 2015

Transactions of the ASAE, 1985

S ATURATED hydraulic conductivity (K) was measured on two structured clay-rich soils in Ohio usin... more S ATURATED hydraulic conductivity (K) was measured on two structured clay-rich soils in Ohio using samples with three different cross-sectional areas, 625, 44 and 13 cm 2. One of the soils tested had approximately 35 macropores/m 2 and the other soil had few or no macropores. The mean vertical saturated hydraulic conductivity of the soil with macropores varied from 20 to over 100 times that of the soil without macropores. Vertical K values were ten times greater than horizontal K values in the soil with macropores. The method employing the largest cross-sectional area resulted in the smallest coefficient of variation in the soil with macropores and fewer large samples than samples with a smaller cross-sectional area were needed to estimate the mean K value within ±20%. The number of samples required depended on the mean K and coefficient of variation of the method tested. The soil with macropores had such a large mean K that, although it had greater absolute variability, fewer samples were required to achieve the same relative degree of precision compared to the soil without macropores. The standard deviations of K for the soil with macropores were 20 to over 70 times those of the soil without macropores.

Aeolian Research, Mar 1, 2016

Horizontal (saltation) mass flux is a key driver of aeolian dust emission. Estimates of the horiz... more Horizontal (saltation) mass flux is a key driver of aeolian dust emission. Estimates of the horizontal mass flux underpin assessments of the global dust budget and influence our understanding of the dust cycle and its interactions. Current equations for predicting horizontal mass flux are based on limited field data and are constrained to representing transport-limited equilibrium saltation, driven by the wind momentum flux in excess of an entrainment threshold. This can result in large overestimation of the sediment mass flux. Here we compare measurements of the soil entrainment threshold, horizontal mass flux, and their temporal variability for five undisturbed dryland soils to explore the role of threshold in controlling the magnitude of mass flux. Average and median entrainment threshold showed relatively small variability among sites and relatively small variability between seasons, despite significant differences in soil surface conditions. Physical and biological soil crusts had little effect on the threshold value, and threshold appeared to play a minor role in determining the magnitude of sediment transport. Our results suggest that horizontal mass flux was controlled more by the supply limitation and abrasion efficiency of saltators present as loose erodible material or originating from neighboring soil sources. The omission of sediment supply and explicit representation of saltation bombardment from horizontal flux equations is inconsistent with the process representation in dust emission schemes and contributes to uncertainty in model predictions. This uncertainty can be reduced by developing greater process fidelity in models to predict horizontal mass flux under both supply-and transport-limited conditions.

AGU Fall Meeting Abstracts, Dec 1, 2009

... Ed L. Frederickson, Eastern Kentucky University Teddy Zobeck Matthew Baddock Robert Van Pelt.... more ... Ed L. Frederickson, Eastern Kentucky University Teddy Zobeck Matthew Baddock Robert Van Pelt. Abstract. Dry playa lake beds can be a significant source of fine dust emissions during high wind events in arid and semiarid landscapes. ...

Elsevier eBooks, 2013

Anthropogenic aeolian environments are those where wind erosion – principally mineral dust produc... more Anthropogenic aeolian environments are those where wind erosion – principally mineral dust production – is related to human influences. This chapter begins by citing regions where anthropogenic factors have resulted in significant wind erosion. It details the major human-related controls on aeolian activity, such as changes to soil surface erodibility, especially by tillage, and alteration of vegetation characteristics. The environmental consequences of wind erosion are classifiable as on-site and off-site effects. Field techniques used for studying anthropogenic wind erosion are also discussed, as well as the models used to estimate soil loss and methods for the control of erosion on agricultural land.

Transactions of the ASABE, 2010

Wind erosion is a natural process that has formed landscapes but threatens sustainable agricultur... more Wind erosion is a natural process that has formed landscapes but threatens sustainable agriculture in many locations. Wind tunnels have been used for several decades to study wind erosion processes. Portable wind tunnels offer the advantage of testing natural surfaces in the field, but they must be carefully designed to ensure that a logarithmic boundary layer is formed and that wind erosion processes may develop without interference from the tunnel structures. Although large portable tunnels often meet the aerodynamic criteria, their size and transportation requirements often limit the locations where they may be employed. We designed and built a self-contained portable wind tunnel that is easily transported on a tandem-axle trailer and pulled with a pickup truck. The wind tunnel uses a centrifugal blower, a flow-conditioning section with optional abrader material feed, and a 1 m tall and 0.5 m wide working section that can vary in length from 2 m to 6 m. The maximum wind velocity attainable is 18.7 m s-1 although a mid-height centerline velocity of 12.6 m s-1 is normally used for field testing of natural surfaces. Based on measured wind velocity profiles in the tunnel working section, a conservative estimate of boundary layer depth within the working section is 0.5 m. Even though no wind tunnel can truly duplicate the scale and variability of the forces that drive wind erosion, tunnels such as this one with deeply developed boundary layers offer reasonable estimates of dust emissions and erodibilities of natural surfaces. This wind tunnel has been used to test rangeland and cropped surfaces in several locations and has provided reliable and useable soil erodibility and dust emission data.

Journal of Environmental Quality, Jul 1, 1984

Simulations of the global dust cycle and its interactions with a changing Earth system are hinder... more Simulations of the global dust cycle and its interactions with a changing Earth system are hindered by the empirical nature of dust emission parameterizations in climate models. Here we take a step towards improving global dust cycle simulations by presenting a physically-based dust emission model. The resulting dust flux parameterization depends only on the wind friction speed and the soil's threshold friction speed, and can therefore be readily implemented into climate models. We show that our parameterization's functional form is supported by a compilation of quality-controlled vertical dust flux measurements, and that it better reproduces these measurements than existing parameterizations. Both our theory and measurements indicate that many climate models underestimate the dust flux's sensitivity to soil erodibility. This finding can explain why dust cycle simulations in many models are improved by using an empirical preferential sources function that shifts dust emissions towards the most erodible regions. In fact, implementing our parameterization in a climate model produces even better agreement against aerosol optical depth measurements than simulations that use such a source function. These results indicate that the need to use a source function is at least partially eliminated by the additional physics accounted for by our parameterization. Since soil erodibility is affected by climate changes, our results further suggest that many models have underestimated the climate sensitivity of the global dust cycle.

Soil Science Society of America Journal, 2011

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or b... more All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Soil Science Society of America eBooks, Nov 2, 2015

AGU Fall Meeting Abstracts, Dec 1, 2006

Shrub encroachment, a widespread phenomenon in arid landscapes, creates ''islands of fertility'' ... more Shrub encroachment, a widespread phenomenon in arid landscapes, creates ''islands of fertility'' in degraded systems as wind erosion removes nutrient-rich soil from intercanopy areas and deposits it in nearby shrub-vegetated patches. These islands of fertility generally are considered to be irreversible. Recently, fire has been observed to alter this pattern of resource heterogeneity through the redistribution of nutrients from the fertile islands of burnt shrubs to the surrounding bare soil areas. Despite the recognized relevance of both fires and wind erosion to the structure and function of arid ecosystems, the interactions between these two processes remains poorly understood. This study tests the hypothesis that fire-induced soil hydrophobicity developing in the soils beneath burned shrubs enhances soil erodibility by weakening the interparticle wet-bonding forces. To test this hypothesis, the effects of grass and shrub fires on changes in soil erodibility and on the intensity of fire-induced soil water repellency are compared at both the field and patch scales in heterogeneous arid landscapes. Higher water repellency was observed in conjunction with a stronger decrease in wind erosion threshold velocity around the shrubs than in grass-dominated patches affected by fire, while neither water repellency nor changes in threshold velocity was noticed in the bare soil interspaces. Thus, fires are found to induce soil hydrophobicity and to consequently enhance soil erodibility in shrub-vegetated islands of fertility. These processes create temporally dynamic islands of fertility and contribute to a decrease in resource heterogeneity in aridland ecosystems following fire.

Paper - American Society of Agricultural Engineers, 1990

Wind Tunnel Designs and Their Diverse Engineering Applications, 2013

AGU Fall Meeting Abstracts, Dec 1, 2012

SSSA special publication series, Oct 26, 2015

Transactions of the ASAE, 1985

S ATURATED hydraulic conductivity (K) was measured on two structured clay-rich soils in Ohio usin... more S ATURATED hydraulic conductivity (K) was measured on two structured clay-rich soils in Ohio using samples with three different cross-sectional areas, 625, 44 and 13 cm 2. One of the soils tested had approximately 35 macropores/m 2 and the other soil had few or no macropores. The mean vertical saturated hydraulic conductivity of the soil with macropores varied from 20 to over 100 times that of the soil without macropores. Vertical K values were ten times greater than horizontal K values in the soil with macropores. The method employing the largest cross-sectional area resulted in the smallest coefficient of variation in the soil with macropores and fewer large samples than samples with a smaller cross-sectional area were needed to estimate the mean K value within ±20%. The number of samples required depended on the mean K and coefficient of variation of the method tested. The soil with macropores had such a large mean K that, although it had greater absolute variability, fewer samples were required to achieve the same relative degree of precision compared to the soil without macropores. The standard deviations of K for the soil with macropores were 20 to over 70 times those of the soil without macropores.

Aeolian Research, Mar 1, 2016

Horizontal (saltation) mass flux is a key driver of aeolian dust emission. Estimates of the horiz... more Horizontal (saltation) mass flux is a key driver of aeolian dust emission. Estimates of the horizontal mass flux underpin assessments of the global dust budget and influence our understanding of the dust cycle and its interactions. Current equations for predicting horizontal mass flux are based on limited field data and are constrained to representing transport-limited equilibrium saltation, driven by the wind momentum flux in excess of an entrainment threshold. This can result in large overestimation of the sediment mass flux. Here we compare measurements of the soil entrainment threshold, horizontal mass flux, and their temporal variability for five undisturbed dryland soils to explore the role of threshold in controlling the magnitude of mass flux. Average and median entrainment threshold showed relatively small variability among sites and relatively small variability between seasons, despite significant differences in soil surface conditions. Physical and biological soil crusts had little effect on the threshold value, and threshold appeared to play a minor role in determining the magnitude of sediment transport. Our results suggest that horizontal mass flux was controlled more by the supply limitation and abrasion efficiency of saltators present as loose erodible material or originating from neighboring soil sources. The omission of sediment supply and explicit representation of saltation bombardment from horizontal flux equations is inconsistent with the process representation in dust emission schemes and contributes to uncertainty in model predictions. This uncertainty can be reduced by developing greater process fidelity in models to predict horizontal mass flux under both supply-and transport-limited conditions.

AGU Fall Meeting Abstracts, Dec 1, 2009

... Ed L. Frederickson, Eastern Kentucky University Teddy Zobeck Matthew Baddock Robert Van Pelt.... more ... Ed L. Frederickson, Eastern Kentucky University Teddy Zobeck Matthew Baddock Robert Van Pelt. Abstract. Dry playa lake beds can be a significant source of fine dust emissions during high wind events in arid and semiarid landscapes. ...

Elsevier eBooks, 2013

Anthropogenic aeolian environments are those where wind erosion – principally mineral dust produc... more Anthropogenic aeolian environments are those where wind erosion – principally mineral dust production – is related to human influences. This chapter begins by citing regions where anthropogenic factors have resulted in significant wind erosion. It details the major human-related controls on aeolian activity, such as changes to soil surface erodibility, especially by tillage, and alteration of vegetation characteristics. The environmental consequences of wind erosion are classifiable as on-site and off-site effects. Field techniques used for studying anthropogenic wind erosion are also discussed, as well as the models used to estimate soil loss and methods for the control of erosion on agricultural land.

Transactions of the ASABE, 2010

Wind erosion is a natural process that has formed landscapes but threatens sustainable agricultur... more Wind erosion is a natural process that has formed landscapes but threatens sustainable agriculture in many locations. Wind tunnels have been used for several decades to study wind erosion processes. Portable wind tunnels offer the advantage of testing natural surfaces in the field, but they must be carefully designed to ensure that a logarithmic boundary layer is formed and that wind erosion processes may develop without interference from the tunnel structures. Although large portable tunnels often meet the aerodynamic criteria, their size and transportation requirements often limit the locations where they may be employed. We designed and built a self-contained portable wind tunnel that is easily transported on a tandem-axle trailer and pulled with a pickup truck. The wind tunnel uses a centrifugal blower, a flow-conditioning section with optional abrader material feed, and a 1 m tall and 0.5 m wide working section that can vary in length from 2 m to 6 m. The maximum wind velocity attainable is 18.7 m s-1 although a mid-height centerline velocity of 12.6 m s-1 is normally used for field testing of natural surfaces. Based on measured wind velocity profiles in the tunnel working section, a conservative estimate of boundary layer depth within the working section is 0.5 m. Even though no wind tunnel can truly duplicate the scale and variability of the forces that drive wind erosion, tunnels such as this one with deeply developed boundary layers offer reasonable estimates of dust emissions and erodibilities of natural surfaces. This wind tunnel has been used to test rangeland and cropped surfaces in several locations and has provided reliable and useable soil erodibility and dust emission data.

Journal of Environmental Quality, Jul 1, 1984

Simulations of the global dust cycle and its interactions with a changing Earth system are hinder... more Simulations of the global dust cycle and its interactions with a changing Earth system are hindered by the empirical nature of dust emission parameterizations in climate models. Here we take a step towards improving global dust cycle simulations by presenting a physically-based dust emission model. The resulting dust flux parameterization depends only on the wind friction speed and the soil's threshold friction speed, and can therefore be readily implemented into climate models. We show that our parameterization's functional form is supported by a compilation of quality-controlled vertical dust flux measurements, and that it better reproduces these measurements than existing parameterizations. Both our theory and measurements indicate that many climate models underestimate the dust flux's sensitivity to soil erodibility. This finding can explain why dust cycle simulations in many models are improved by using an empirical preferential sources function that shifts dust emissions towards the most erodible regions. In fact, implementing our parameterization in a climate model produces even better agreement against aerosol optical depth measurements than simulations that use such a source function. These results indicate that the need to use a source function is at least partially eliminated by the additional physics accounted for by our parameterization. Since soil erodibility is affected by climate changes, our results further suggest that many models have underestimated the climate sensitivity of the global dust cycle.