Graduate Theses and Dissertations Iowa State University Capstones, Theses andDissertations

2018

Surface energy balance partitioning in tilled andnon-tilled bare soilsOhene AkuokoIowa State University

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Surface energy balance partitioning in tilled and non-tilled bare soils

by

Ohene Akuoko

A thesis submitted to the graduate faculty

in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

Major: Environmental Science

Program of Study Committee:

Robert Horton, Major Professor

Thomas J. Sauer

Andrew Manu

The student author, whose presentation of the scholarship herein was approved by the

program of study committee, is solely responsible for the content of this thesis. The

Graduate College will ensure this thesis is globally accessible and will not permit

alterations after a degree is conferred.

Iowa State University

Ames, Iowa

2018

Copyright © Ohene Akuoko, 2018. All rights reserved.

ii

DEDICATION

To my family, my father Joseph, my three brothers; Nana, Barima, Kwadjo and

sister Imani, you all have always been there in every step along the way, showing much

support and love from day the beginning. But I especially want to dedicate this to my late

mother, Imani Akuoko, who until this day is the strongest person I have known and an

even better parent who always supported and believed in her children’s dreams.

iii

TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS ................................................................................................. iv

ABSTRACT ........................................................................................................................ v

CHAPTER 1. GENERAL INTRODUCTION ................................................................... 1

CHAPTER 2. MATERIALS AND METHODS ................................................................ 5 Field Description ........................................................................................................... 5 Surface Energy Balance Measurements ........................................................................ 6

Micro-lysimeter Measurements ..................................................................................... 9

CHAPTER 3. RESULTS AND DISCUSSION ................................................................ 11

Seasonal Soil Physical Properties ................................................................................ 11 Net Radiation and Soil Heat Flux ................................................................................ 13

Diurnal Latent and Sensible Heat Fluxes .................................................................... 16 Soil Water Evaporation Comparisons ......................................................................... 18 Partitioning of Energy Balance Components .............................................................. 21

CHAPTER 4. CONCLUSIONS ....................................................................................... 24 Future Work: ................................................................................................................ 24

REFERENCES ................................................................................................................. 26

iv

ACKNOWLEDGMENTS

I would first like to give great thanks and praise to my Lord, Jesus Christ, who has

been an ever present helper and guide throughout the entire process. To my major

professor, Dr. Horton, thank you for receiving me into your group, the countless

opportunities to grow as a scientist and most importantly as a person; professionally,

socially and spiritually have equipped me with tools to use for the rest of my life. Thank

you to my committee members Dr. Sauer and Dr. Manu for all of your steady support,

wisdom and always being available.

I would also like to say thank you to all of my Soil Physics groupmates; Dilia

Kool, Zhuangji Wang, Chenyi Luo, Ben Carr, Minmin Wen, Bing Tong and Junko

Nishiwaki, you have been great sources of knowledge, support and inspiration, as I have

been able to glean many worthwhile intangibles from you all. I thank Dedrik Davis, you

were an immense help and timely mentor my first field season during your sabbatical. To

Forrest Goodman and your team, for your availability and expertise in installing and

maintaining field and lab instrumentation. This thesis could not have been completed

without your assistance.

To all of my new found friends and colleagues, I have had the chance to meet and

grow with here in Ames. David Kwaw-Mensah, I appreciate your steady support and

consistent encouragement to succeed since I stepped foot on campus.

I thank all of my family members, whether directly or indirectly, you have all

been a driving force enabling me to push on to the completion of this thesis.

v

ABSTRACT

Knowledge of the partitioning of the surface energy balance (SEB) components is

essential in understanding heat and water budgets at the soil-plant-atmosphere interface.

Agriculturally, changes in soil structure due to soil tillage in the fall and spring affects the

magnitude of these components. SEB partitioning determined by modeled and measured

studies usually assumes a constant near surface bulk density values for extended periods

of time. The objectives of this study is to determine the effect of soil bulk density

changes on net radiation (Rn), soil heat flux (G), latent heat flux (LE) and sensible heat

flux (H) of a tilled and non-tilled bare soil with time. Micro-Bowen ratio (MBR) system

were set up for 91 days on a tilled (Till) soil and a non-tilled (NT) soil at the Iowa State

University Agronomy and Agricultural Engineering Research farm near Ames. MBR

systems measured water vapor and air temperature at 0.01 and 0.06 m heights above the

soil surface, to determine atmospheric gradients for LE and H calculations. Net radiation

was obtained by a net radiometer positioned at 1.25 m above the surface, while soil heat

flux measurements were obtained from soil heat flux plates at a 0.06 m depth and soil

heat storage calculations (0- 0.06 m). Evaporation rates were also determined by micro-

lysimeters. Two time periods, were selected early and late in the season (relative to

tillage), to determine the effect of changing bulk density with time. Bulk density showed

little to no change following tillage in Period 1 but increased by 0.11 g cm-3 following

substantial rainfall events at the Till plot during Period 2. In Period 1, Rn and G fluxes did

not differ between plots as bulk density and water contents were similar. The Till soil LE

flux was 12% higher than in the NT according to the MBR measurements and 15%

higher according to the ML measurements. In Period 2 (DOY 262-266), Rn and G fluxes

vi

showed relatively larger daytime difference between Till and NT. As bulk density and

water content increased during this period, G fluxes represented 7% more Rn at the Till

soil than in the NT soil. A subsequent 7% lower available energy was observed at the Till

plot leading to 10% lower LE values for Period 2. The results of this research highlight

the value of considering dynamic bulk density measurements with time when

determining the distribution of energy at the soil surface.

1

CHAPTER 1. GENERAL INTRODUCTION

Energy distribution at the Earth’s surface plays a significant role in global

meteorological and hydrologic cycles. Increasing global climate change and variability is

intricately tied to the distribution of radiant energy at the Earth’s surface affecting regional

and local climate processes and ecosystems functions (Chen et al., 2014; Huber et al., 2014;

Ogée et al., 2001). In agro-ecosystems, understanding how energy is allocated at the land-

atmosphere interface is vital to managing essential processes of seed germination, irrigation

scheduling, water use efficiency, and microbial activity (Hatfield et al. 2015; Gaudin et al.,

2015).

The modeling of surface energy balance fluxes is becoming increasingly prevalent, as

estimating energy partitioning at the land-atmosphere interface of various scales is relatively

inexpensive and non-labor intensive (Chen et al., 2014; Gentine et al., 2011; Huber et al.,

2014; Ogée et al., 2001). Many atmosphere and land properties are utilized in determining

these fluxes through measurements or models. An important soil physical property that is

often used in agroecosystems is bulk density. Bulk density has considerable implications in

the transfer of heat and water between the soil and the atmosphere, hence understanding of

bulk density dynamics is important in estimating surface energy balance partitioning

accurately (Evett et al., 2012; Ochsner et al., 2007). In agro-ecosystems, bulk density values

can be altered by plant roots, machine use, cropping management practices, wetting/drying

cycles and freeze/thaw cycles. The management practice of pre-seeding and post-harvest

tilling remains a common and preferred practice amongst many agricultural land users in the

Mid-Western United States. Pre-seeding soil tillage is primarily performed with the purpose

of modifying the soil surface environment to develop suitable conditions for seed

2

germination/emergence and crop growth (Hatfield and Prueger, 2015; Strudley et al., 2008).

This environment is usually obtained by disturbance of the tilled layer’s soil structure,

altering important thermal and hydraulic properties that influence the partitioning of energy

balance components.

The surface energy balance is predominately comprised of four components. The

surface net radiation (Rn) is usually partitioned into three major fluxes: soil heat flux (G),

latent heat flux (LE) and sensible heat flux (H). Net radiation is the energy at a surface, after

shortwave incoming solar radiation has either been reflected or absorbed and longwave

radiation has been either emitted or absorbed at the surface. Hence, solar radiation is a

primary factor influencing daytime surface net radiation values. The Soil heat flux includes

the energy transferred in the soil. Latent and Sensible heat fluxes are turbulent fluxes. LE

includes the energy used to convert liquid water into water vapor, and H is the convective

process of energy movement through air particles. In this thesis, Rn is considered positive

when the flux is towards the surface and G, LE, and H, are considered positive for fluxes

going away from the surface.

Net radiation at a bare soil surface is generally affected by changes in surface

roughness and water content, both indicators of albedo (Idso et al., 1975; Matthias et al.,

2000). Matthias et al. (2000) compared how multiple tillage practices and soil wetness

affected bare soil albedo, observing the more intensive tillage techniques to result in lower

albedo and therefore higher Rn due to higher surface roughness. Schwartz et al. (2010),

comparing strip till and no-till surfaces in Texas, observed 18% higher Rn at the tilled soil,

yet following intense precipitation events, the differences in Rn could no longer be

distinguished. The partitioning of Rn into G, H, and LE is largely dependent on surface

3

volumetric water content (VWC). VWC, a measurement of soil water availability, could be

influenced by the soil structure as soil water retention is determined by pore distribution.

Studies have reported that bulk density changes following the disturbance of the soil surface

by tilling, sometimes affects VWC dynamics. This happens because tillage destroys pore

structure and changes the pore size distribution affecting hydraulic conductivity (Logsdon et

al., 1999; Moret and Arrúe, 2007). Generally, VWC of a more disturbed soil due to tillage, is

initially higher, but decreases through evaporation and/or drainage at a much quicker rate as

non-disturbed non-tilled soils. As a result, authors have reported either no significant

differences or slightly larger VWC in the no-till for both point measurements and over longer

periods of time (Cellier et al., 1996; Potter et al., 1987). Differences in water content between

tilled and no-tilled sites typically impact soil thermal properties which either directly or

indirectly affect the partitioning of net radiation. Thermal conductivity (λ) of soil increases

with water content up until a point depending on soil texture and structure, but soil

volumetric heat capacity (C), a measurement of the ability of the soil to absorb energy,

increases linearly with water content and bulk density. Allmaras et al. (1977) studied VWC

in four different tillage practices, and reported water contents following a rainfall event could

differ by as much as 0.04 m3 m-3 in tilled treatments. He also observed that lower values of

water content in the surface soil layer led to an average 6% lower G flux.

Latent heat and sensible heat fluxes are generally affected by the amount of available

energy (the sum of Rn and G), surface soil water content, and soil surface/aerodynamic

resistances. Schwartz et al. (2010) observed in semi-arid loam that soil water evaporation

from a tilled soil was higher than at a non-tilled soil due to higher available energy. He

observed that on average 11% more evaporation occurred at the recently tilled soil, while

4

surface water contents were significantly lower. Later in the season, they did observe a larger

10-day cumulative evaporation at the no-till site, which was attributed to higher VWC.

Sensible heat fluxes are also affected by VWC. Surface temperature is usually dampened by

the presence of water during days immediately following rainfall, significantly reducing the

temperature gradients. Richard and Cellier (1998) reported a smaller temperature gradient in

a recently tilled soil compared to a soil tilled the previous year, resulting in a 20% lower H.

Several studies have illustrated how energy partitioning of bare tilled and no-till

surfaces effects surface energy balances components (Allmaras et al., 1977; Ashktorab et al.,

1989; Cellier et al., 1996b; Potter et al., 1987; Richard and Cellier, 1998). However, few

studies consider changes in soil bulk density following tillage throughout a season and the

effect on soil thermal properties (Schwartz et al., 2010). A full assessment of how all four

components of the energy balance behave with changes in bulk density following tillage will

allow for a better understanding of the effects of changing soil properties and associated field

hydrology and thermal conditions on energy balance partitioning.

In this study, our objective is to quantify the effect of changing bulk densities on the

partitioning of Rn, G, LE and H fluxes on a tilled and non-tilled bare soil surface with time

under rain-fed conditions. We do this through the use of both mass and energy balance

measurements with time, including the micro-Bowen ratio technique.

The thesis will include the materials and methods section in Chapter 2, the results and

discussion section in Chapter 3, and general conclusions in Chapter 4.

5

CHAPTER 2. MATERIALS AND METHODS

Field Description

Measurements were conducted in a field plot at the Iowa State University Agronomy

and Agricultural Engineering Research farm near Boone, Iowa (42.0173° N, -93.76161° W).

The study was conducted on a Clarion-loam soil (USDA-NRCS Web Soil Survey) in a

relatively level area. Particle size analysis indicated 40% sand, 39% silt and 21% clay. The

area receives 892 mm average precipitation per year. On July 18, 2017 (DOY 199) the

eastern half of the field site was tilled in a North-South orientation, to a depth of about 20 cm

using a rotary tiller. In order to maintain an effectively bare plot, herbicide was applied

intermittently throughout the measurement period. Continuous measurements of soil and

atmospheric conditions were conducted from July 18, 2017 to October 17, 2017 (DOY 199-

290). Sensors were installed on the non-tilled soil from July 18 (DOY 199), and at the tilled

soil on July 26 (DOY 207) shortly after the first rainfall event on July 20 (DOY 201) in order

to install instrumentation at a time when the soil was more stable. Throughout the rest of the

paper we identify the tilled soil as (Till) and the non-tilled soil as (NT).

Bulk density (𝜌𝑏) measurements were obtained following substantial rainfall events

(Precipitation > 10 mm d-1). Stainless steel cores, 0.05 m in height and 0.08 m in diameter,

were utilized. Each core had sharpened edges to decrease disturbance while sampling. Three

samples from the 0 – 0.05 m depth were taken during each sampling. Samples were then

brought to the lab, weighed, and put in an oven at 105 °C for 48 hours or more. After

sufficient drying, samples were re-weighed. The bulk density was then determined by dry

soil mass and soil core volume.

6

Surface Energy Balance Measurements

The surface energy balance can be described as,

Rn = G + LE + H (1)

where Rn is net radiation (W m-2), G is the soil heat flux (W m-2), LE is the latent heat flux

(W m-2), and H is the sensible heat flux (W m-2). Rn – G represents available energy

partitioned to turbulent fluxes LE and H.

Rn was measured above the Till and NT soils with net radiometers (NR Lite 2, Kipp

and Zonen, Delft, Netherlands) attached to metal rod at a height of 1.25 m above the surface.

The soil heat flux was computed as the sum of the soil heat flux measured using a soil heat

flux plate at a 0.06 m depth and the soil sensible heat storage in the 0 m - 0.06 m layer of soil

(Sauer and Horton, 2005). Soil sensible heat storage was determined for three 0.02 m

increments above the plates as,

ΔS = C * Δz (ΔT/Δt) (2)

where ∆S is the change in heat storage (W m-2) above the soil heat flux plate, C is the

volumetric heat capacity of the soil (J m-3 K-1), Δz is the thickness of the soil layer (0.02 m),

∆T is the change in temperature between the top and the bottom of the soil increment (°C), ∆t

is the time step (s). The volumetric heat capacity is determined by de Vries (1963) as,

C = Cm (1- Φf) + Cw 𝜃 (3)

where Cm is the volumetric heat capacity of the soil particles (2.35 MJ m-3 K-1), Φf is the

fraction of soil volume that is pore space (calculated from bulk density), Cw is the volumetric

heat capacity of water (4.18 MJ m-3 K-1), and 𝜃 is the volumetric water content (m3 m-3). We

assume that the volumetric heat capacity of the air and organic matter fractions within the

soil is negligible.

7

Soil heat flux plates (PHF-03, Prede Co, Tokyo, Japan) were installed at a depth of

0.06 m, in both the Till and NT soil plots. The temperature profile above each plate was

determined using type T thermocouples measuring soil temperature at the 0.02, 0.04, and

0.06 m depths near each soil heat flux plate. The surface temperature was measured using an

infrared thermometer (IRR, Apogee Inc, Logan, Utah) at 1.25 m height. Soil volumetric

water content used to calculate soil volumetric heat capacity (De Vries, 1963), was measured

using four CS655 sensors (Campbell Scientific Inc, Logan, Utah) positioned horizontally at

0.03 m depths near each soil heat flux plate.

The latent and sensible heat fluxes were determined using the Bowen ratio method

(Bowen, 1926), the Bowen ratio is the ratio of the sensible and the latent heat flux and is

obtained by measuring atmospheric temperature and water vapor concentration at two

different heights above a surface. The Bowen ratio (𝛽) is determined as,

P L U

L U

PC (Θ Θ )H

LE λε(e e )

(4)

where P is the atmospheric pressure (kPa), Cp is the specific heat capacity of air at a constant

pressure (1004.67 J kg-1 K-1), ϴL is the potential temperature at the lower height (L), ϴU is the

potential temperature at the upper height (U), λ is the latent heat of vaporization of water

(2.45 MJ kg-1), ε is the ratio of the molecular weights of air and water (0.622), and е is the

atmospheric vapor pressure.

The potential temperature is calculated as,

Θ = 𝑇(𝑃𝑜 𝑃⁄ )𝑅 𝐶𝑃⁄ (5)

where T is the air temperature in K, Po is the standard reference pressure (100 kPa), P is the

measured atmospheric pressure (kPa), R is the gas constant (8.314 J K-1 mol-1) and Cp is 29.1

J mol-1K-1.

8

Latent heat flux (W m−2) is then calculated as,

nR GLE

1

(6)

The sensible heat flux can then be calculated as,

H = Rn – G0 –LE (7)

A micro-Bowen ratio system was set up on each soil. The design of (Holland et al.,

2013) was modified by replacing the LI840 infrared gas analyzer (IRGA) with an LI7500

open path gas analyzer (LI-Cor Bioscience Inc. Lincoln, NE). Since the LI7500 is an open

path IRGA, the tube normally used for calibration was permanently fixed inside the analyzer.

Each system had two air intakes at 1 and 6 cm heights above the soil surface. The air intake

tube was connected to the calibration tube fixed inside the LI7500.Air was drawn into each

intake by a NMP 830 pump (KNF Neuberger, Inc., Trenton NJ) to the LI7500 where

atmospheric water vapor concentration (mmol m-3) was measured. All intakes were also

fitted with a mesh screen of glass wool meshed with two layers of nylon to prevent dust entry

into the IRGA. Thermistors were located directly behind the mesh air filter within each air-

intake tube, to measure air temperature (°C). Thermistors temperatures were scanned at 1-

second intervals then averaged and recorded every 5 minutes. A 30-minute average was then

calculated from the measured data at each height to determine ΔT.

To obtain vapor pressure gradients, a solenoid valve (L01, Pneumatic Store,

Clarkston, MI) switched every 5 minutes alternating the pumping of air flow from one in-

take height to the other height. In order to minimize measurement error, the data logger was

programmed to omit vapor concentrations measured during the first minute after solenoid

switching. Hence, the vapor concentrations were recorded as 4 minute periods, which were

later averaged into three 4 minute periods to provide 30 minute periods Δe. These values

9

were then used to calculate β. When -1.1 < β > -0.90, the values were discarded as these

values could lead to invalid LE and H estimates, especially during morning or evening hours

as shown by Perez et al. (1999). Data were not recorded during precipitation events, to avoid

condensation inside the sample lines, pump and IRGA. When the relative humidity exceeded

95% the data logger discontinued data collection. Both systems were located on the NT soil

for 8 days, DOY 199-206 (July 18-July 25) for inter calibration prior to sensor installation at

the Till soil.

Other meteorological data collected during the measurement period included

barometric pressure (SB-100, Apogee Inc, Logan, Utah), atmospheric temperature, and

relative humidity by a weather station (Temp/RH; Rotronic HC2S3, Hauppauge, NY)

attached to each MBR system. A rain gauge (TE525, Texas Instruments, Dallas, TX) was

attached at 1.25 m height. A data logger (CR1000, Campbell Scientific Inc., Logan, UT) was

programmed to read all measurements every 5 seconds, the measured data were then

averaged and recorded as 5-minute means. The 5-minute averages were then averaged to 30

minute increments to determine the energy balance fluxes.

Micro-lysimeter Measurements

In addition to the MBR measurements, evaporation was measured using a mass

balance approach. We utilized the micro-lysimetric method as described by Boast et al.

(1982). The micro-lysimeters (ML) were 10 cm long PVC pipes with an inner diameter of 10

cm and sharpened edges on one end. We observed ML evaporation during three

measurement periods; 7/27-7/28, 8/4-8/8, 9/19-9/22, immediately following rainfall events.

To begin each measurement, an undisturbed soil sample was obtained, sealed from the

bottom with a cap, weighed and immediately re-installed back into the soil. The mass of the

ML columns were determined using a balance (SPX2202, OHAUS corp., Parsippany, NJ)

10

with a repeatability of 0.02 g. This was done for three repetitions on each soil. After 12

hours, the MLs were re-weighed and placed back into the soil. Samples were utilized for a

maximum of 48 hours or 4 measurement periods, as measurement accuracy is known to

decrease after this period (Boast et al., 1982). Any loss in mass between weighing was

attributed to a loss in water content and since the sealed bottom precludes drainage, the water

loss equals soil water evaporation.

11

CHAPTER 3. RESULTS AND DISCUSSION

Seasonal Soil Physical Properties

A total of 254 mm of precipitation between DOY 195 and 290 fell at the experimental

site. Rainfall occurred regularly late in the experimental period. Bulk density values were

affected largely by precipitation and time following the tillage event on DOY 199 (Fig. 1A).

Initial bulk density values for the Till and NT were not noticeably different.

Although, in the latter portion of our season, we observed an increased bulk density in the

Till soil while NT bulk density values remained unchanged. On DOY 262, bulk density

values were 1.14 g cm-3 ± 0.03 and 1.04 g cm-3 ± 0.01 in the Till and NT soils, respectively.

The higher bulk density at the Till soil is likely due to substantial rainfall events occurring

near the middle of the season. Moret and Arrúe (2007), studying tillage’s effect of hydraulic

properties in a semi-arid environment, observed that overtime rainfall events lead to 0.07 g

cm-3 higher bulk density in a rotary tilled plot over an 8-month period. Schwartz et al. (2010)

using a strip till technique observed no initial effect of tillage but after nearly 100 mm of

rainfall over a 48 day period following tillage, a strip till soil had a significantly lower bulk

density than a no-till soil. We observed 102 mm of total rainfall between our early and late

season bulk density measurements, including an intense 16 mm event in 2 hours on DOY

235, which could have had the opposing effect, increasing the bulk density of our rotary

tilled soil as compared to the decreasing affect observed by Schwartz et al. (2010) in a strip

tilled soil.

12

Figure 1. Bulk density (ρb) , hourly rainfall, and volumetric water content (VWC) (0.03 m

depth) for the tilled (Till) and non-tilled (NT) soils throughout the 2017 experimental

season. Focus periods are marked by tan bars. Error bars indicate one standard error of the

mean bulk density values.

To contrast energy balance partitioning before and after significant change in bulk

density we selected two periods of focus. Period 1 (DOY 216-220) began 17 days after

tillage and Period 2 (DOY 262-266) began 63 days after tillage. Each period followed a

multi-day precipitation event (DOY 213-215 and DOY 258-261), where 36 mm and 21 mm

of rainfall were observed prior to Period 1 and Period 2, respectively. Increases in VWC

throughout the season, occurred following precipitation events, as expected. In Period 1, we

observed similar VWC values in Till and NT. In Period 2, we consistently observed higher

VWC at the Till soil throughout the period (Fig. 1B). VWC ranged from 0.08 to 0.13 m3 m-3

and 0.06 to 0.09 m3 m-3 in the Till and NT soils during Period 2, respectively. A larger bulk

0

10

20

30

40

50

0

0.05

0.1

0.15

0.2

0.25

0.3

190 205 220 235 250 265 280 295

Rain

fall (

mm

h-1

)

VW

C (

m3

m-3

)

Day of Year

Till 0.03 m NT 0.03 m RainfallB)

1.00

1.05

1.10

1.15

1.20

190 205 220 235 250 265 280 295

ρb

(g c

m-3

)

Till 0-5 cm NT 0- 0.05 m Focus PeriodA)

13

density in the top layer of the Till soil could lead to less drainage in the 0- 0.05 m layer,

increasing the water retention at the Till soil during Period 2. Studies have reported

differences in VWC with increased bulk density. Horton et al. (1994) and Moret and Arrúe

(2007) observed that more compaction, decreased macro-porosity, and therefore, unsaturated

hydraulic conductivity in soil, causing higher soil water contents. Conversely, similar to

observations in Period 1 for our study, Potter et al. (1985) observed that when bulk density

values are not significantly different, soil VWC was likewise insignificantly different despite

chisel tillage.

Net Radiation and Soil Heat Flux

Net Radiation was compared at the Till and NT soils, during Period 1 and Period 2

(Fig. 2A and 2B). Diurnal Rn curves at both soils were affected by cloud cover during each

period, except on DOY 220, when cloud cover was minimal. Rn peaked at 630 W m-2 and

505 W m-2 in Period 1 and Period 2, respectively. This difference in net radiation is due to

natural seasonal changes in solar radiation input. There was little difference in Rn between

tillage treatments for all days in Period 1. However, in Period 2, we observed slightly smaller

Rn values in the Till soil than the NT. Midday Till Rn peaked on average 38 W m-2 lower than

NT Rn. Authors have found that tillage usually increases the Rn, Matthias et al. (2000) found

that tillage increased Rn by increasing surface roughness, which decreased the surface albedo.

Schwartz et al. (2010) observed 18% lower daytime Rn values on a bare untilled soil

compared to a strip tilled soil, though later in the season, the difference was insignificant.

14

Figure 2. Diurnal net radiation (Rn), soil heat flux (G), surface temperature (Ts), soil

temperature at 0.04 m depth, soil volumetric heat capacity (C) comparisons at the Till and

NT soils, for Period 1 (DOY 216-220) and Period 2 (DOY 262-266).

The decrease in surface roughness was observed to correspond to intense

precipitation events between measurements. Other studies have supported the impact of

significant rainfall events as Allmaras et al. (1977) reported a major reduction in soil surface

0

10

20

30

40

50

262 263 264 265 266 267

Till_TsNT_TsTill_0.04NT_0.04

D)

0

10

20

30

40

50

216 217 218 219 220 221

Te

mp

era

ture

( C

)

C)

0.9

1.1

1.3

1.5

1.7

1.9

216 217 218 219 220 221

Vo

l He

at C

ap

ac

ity

(MJ

m-3

K-1

)

Day of Year

E)

0.9

1.1

1.3

1.5

1.7

1.9

262 263 264 265 266 267Day of Year

Till Soil_CNT Soil_C

F)

-100

0

100

200

300

400

500

600

216 217 218 219 220 221

En

erg

y F

lux

(W m

-2)

A)

-100

0

100

200

300

400

500

600

262 263 264 265 266 267

Rn NT G NTRn Till G Till

B)

Period 1 Period 2

15

roughness after 46 mm of rainfall, to within 10% of its pre-tillage state. Our Till soil surface

became smooth during the 1st period following a cumulative 73 mm of precipitation since

tillage, resulting in no visual difference when compared to the NT soil. The similarity

between Till and NT Rn values indicates that albedo between the soil was not very different.

Another contributing factor to Rn is longwave radiation, where a higher surface temperature

corresponds to higher outgoing longwave radiation and lower Rn. Richard and Cellier (1998)

observed a 3.4°C larger surface temperature at a spring tilled soil compared to an autumn

tilled soil, due to a larger surface roughness resulting in a 8% higher Rn at the spring tilled

soil. Differences in albedo lead to differences in Rn. Our albedo is assumed to be similar

during all periods, and surface temperature, an indicator of outgoing longwave radiation

component of net radiation, can impact Rn differently in Till and NT. During both

measurement periods, we see minimal differences in the diurnal surface temperature curves,

which mirror the minimal to no differences in Rn curves (Fig. 2C and 2D).

Soil heat fluxes were determined on the NT and Till soils during Period 1 and Period

2 (Fig. 2A and 2B). Tillage had minimal effect on bulk density and also little influence in

diurnal G values in Period 1. In Period 2, diurnal G was slightly higher in the Till soil

throughout the measurement period. We observed midday G values averaging 107 W m-2 and

71 W m-2 at the Till and NT, respectively. A prior study in central Iowa (Potter et al., 1987)

reported similar magnitudes in a chisel plow plot and a ridged no-till plot, which produced G

peak values of 102 W m-2 and 70 W m-2, respectively.

Soil temperature and volumetric heat capacity were also compared to determine

potential causes for differences in G (Fig. 2C-F). Gradients in soil temperature between the

surface and 0.04 m soil depth were similar in Till and NT early in Period 1, though we

16

observed a 1.5°C higher soil temperature gradient later in the period (DOY 220). In Period 2,

we observed similar magnitudes of soil temperature gradients from the surface to 0.04 m

depth, indifferent of bulk density values. However, surface temperatures and 0.04 m

temperatures in the Till soil were noticeably smaller than in the NT soil during Period 2. The

heat storage component (∆S) of the surface soil heat flux calculation is driven largely by soil

temperature and volumetric heat capacity fluctuations (Allmaras et al., 1977; Azooz et al.,

1997; Cellier et al., 1996; Richard and Cellier, 1998).

Soil volumetric heat capacity is primarily dependent on VWC and bulk density.

These variables of soil C varied with differences between Till and NT during both periods.

Similar bulk density and VWC values in Period 1, led to similar soil C and ultimately diurnal

G values. In Period 2 increased soil C due to higher VWC and bulk densities at the Till soil

increased G flux at the NT soil throughout the period.

Diurnal Latent and Sensible Heat Fluxes

MBR-based partitioning of diurnal turbulent fluxes was possible only for Period 1,

due to missing atmospheric water vapor measurements after DOY 221. MBR estimates of

latent heat fluxes are larger for the Till soil, while sensible heat is lower at the Till soil early

in Period 1 (Fig. 3C). Partitioning of energy to latent heat during the initial day of the period

(DOY 216) was observed to be larger in the Till than in the NT soil. Energy partitioned to

latent heat peaked at 500 W m-2 and 175 W m-2, at midday on the Till and NT soil,

respectively. In the immediate days following rainfall, no distinct differences in diurnal LE

were observed until DOY 219, where latent heat peaked at 275 W m-2 and 132 W m-2 in the

Till and NT soils, respectively. During the first day when LE was largest, β was 0.52 and

2.95 at the Till and NT respectively. β values near or less than 1 are associated with periods

of high evaporation as Sauer et al. (1998), utilizing a Bowen ratio system to measure the

17

energy balance at a surface covered by crop residues, observed a similar β of 0.59 the day

following a rainfall event. This is usually when the soil surface is most wet, increasing the ∆e

(eq. 4).

Figure 3. Diurnal available energy (Rn -G), sensible heat flux (H), latent heat flux (LE)

comparisons during Period 1 (DOY 216-220).

H estimates, during the same period were smaller in Till than NT on the initial day

following rainfall as H peaked near 200 W m-2 and 430 W m-2 in Till and NT, respectively

-100

0

100

200

300

400

500

600

216 217 218 219 220 221

En

erg

y F

lux (

W m

-2)

Rn-G TillRn- G NT

A)

-100

0

100

200

300

400

500

600

216 217 218 219 220 221En

erg

y F

lux (

W m

-2)

Day of Year

LE Till

LE_NT

C)

-100

0

100

200

300

400

500

600

216 217 218 219 220 221

En

erg

y F

lux (

W m

-2)

H Till

H NT

B)

Period 1

18

(Fig. 3B). During the first day, the LE was larger than available energy at the Till soil in the

morning until noon, indicating advection at the Till surface in the morning hours. This can

also be observed by the higher bulk density values with time. This advection effect was not

observed in the NT soil.

Soil Water Evaporation Comparisons

MBR and ML both show higher evaporation at the Till than the NT during Period 1

(Fig. 4A and 4B). In Period 1, MBR estimated were similar to those of the ML

measurements, as we observed a higher evaporation rate at the Till soil the first day

following precipitation. Evaporation rates in this initial day ranged from 1.5 - 2.0 mm d-1 to

0.4 - 1.2 mm d-1 at the Till and NT soils, respectively. Schwartz et al. (2010) similarly

observed higher evaporation at a Till soil the first day following rainfall, when a 3.5 mm and

2.2 mm was observed at a strip Till and NT soil, after a total of 48 mm of precipitation fell

since tillage. For the remainder of the period, ML daytime evaporation remained larger at the

Till soil with the exception of DOY 217 where we observed insignificant differences, as Rn

reached a period low 7.6 MJ m-2 d-1.

During Period 1, we presume that a significant crust had not yet developed at the Till

soil surface, assumingly lowering the soil’s resistance to evaporation. The disturbance of the

surface due to tillage has been shown to affect pore connectivity, pore size distribution and

decrease the surface resistance to surface water vapor transfer (Aase and Tanaka, 1987;

Moret and Arrúe, 2007; Wang et al., 2015). The altered soil surface structure at the recently

tilled soil allowed more evaporation to occur, due to lower resistance to water vapor flow

even though available energy was similar, 83% and 85% on average at the Till and NT soils,

respectively.

19

Figure 4. Daytime micro-Bowen ratio (MBR) and micro-lysimeter (ML) evaporation,

sensible heat flux (H) and above ground ΔT (0 – 0.06 m height temperatures) during Period

1 (A,C,E) and daytime micro-lysimeter (ML) evaporation, sensible heat flux (H) and above

ground ΔT during Period 2 (B,D,F). Period 2 values only represent days of micro-lysimeter

measurements. Error bars indicate one standard error of the mean micro-lysimeter values.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

262 263 264 265

Till NTB)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

216 217 218 219 220

Ev

ap

ora

tio

n (

mm

d-1

)Till_ML Till_MBR

NT_ML NT_MBR

A)

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

262 263 264 265

NT TillD)

Period 1 Period 2

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

216 217 218 219 220

Se

nsib

le H

eat F

lux (M

J m

-2d

-1)

Till_ML Till_MBR

NT_ML NT_MBR

C)

-5

0

5

10

15

20

262 263 264 265 266

Day of Year

Till 0 - 0.06 m

NT 0 - 0.06 m

F)

-5

0

5

10

15

20

216 217 218 219 220 221

Te

mp

era

ture

( C

)

Day of Year

E)

20

MBR and ML both show higher evaporation at the Till than the NT during Period 1

(Fig. 4A and 4B). In Period 1, MBR estimated were similar to those of the ML

measurements, as we observed a higher evaporation rate at the Till soil the first day

following precipitation. Evaporation rates in this initial day ranged from 1.5 - 2.0 mm d-1 to

0.4 - 1.2 mm d-1 at the Till and NT soils, respectively. Schwartz et al. (2010) similarly

observed higher evaporation at a Till soil the first day following rainfall, when a 3.5 mm and

2.2 mm was observed at a strip Till and NT soil, after a total of 48 mm of precipitation fell

since tillage. For the remainder of the period, ML daytime evaporation remained larger at the

Till soil with the exception of DOY 217 where we observed insignificant differences, as Rn

reached a period low 7.6 MJ m-2 d-1. During Period 1, we presume that a significant crusting

was not presumed to have developed at the surface decreasing the resistance to evaporation.

The disturbance of the surface due to tillage has been shown to affect pore connectivity, pore

size distribution and decrease the surface resistance to surface water vapor transfer (Aase and

Tanaka, 1987; Moret and Arrúe, 2007; Wang et al., 2015). The altered soil surface structure

at the recently tilled soil allowed more evaporation to occur, due to lower resistance to water

vapor flow even though available energy was similar, 83% and 85% on average at the Till

and NT soils, respectively.

In contrast to Period 1 evaporation rates, Period 2 daily evaporation rates were lower

at the Till soil on the initial days following precipitation (DOY 262 and DOY 263; Fig 4B).

In these two days, 0.7 mm (21%) and 0.3 mm (16%) less water evaporated from the Till soil

on DOY 262 and 263, respectively. Soil water evaporation of soil water at the surface is

subject to the availability of water, radiative energy and soil surface resistance ( Xiao et al.,

2011; Heitman et al 2010; Xiao, et al., 2010). Xiao et al. (2010) in a two year study observed

21

that, after rainfall, the energy-limiting stage of soil surface evaporation (Stage 1 evaporation)

could last for up to two days followed by the water-limiting stage (Stage 2 evaporation).

Although our measured VWC at a 0.03 m depth was larger at the Till than the NT soil

throughout Period 2, evaporation for the first two days was energy-limited. Therefore, when

considering the higher available energy at the NT soil, a higher evaporation at our NT than

our Till soil in the first two days of Period 2 is likely to occur.

Heitman et al. (2010b), using heat pulse probes to determine sub-surface evaporation

in central Iowa, observed soil water evaporation to occur mainly at the soil surface (< 0.03

m) when evaporation was energy-limited. Only after evaporation became water-limited, did

the authors see substantial evaporative losses at soil depths ≥ 0.03 m. Hence, a higher VWC

at the Till soil would not have a substantial impact in the initial days’ evaporation rate

comparisons in Period 2. This also explains an observed increase in evaporation at the Till

soil on DOY 265, because the soil is water limiting, a higher 0.03 m VWC would be

expected to result in higher evaporation.

Partitioning of Energy Balance Components

The fractions of Rn partitioned to G, LE and H fluxes are compared for the two soils

and two periods in figure 5 (A-D). The fractions are obtained from daytime cumulative

values (MJ m-2 d-1) for each component on days where micro-lysimeter measurements of

evaporation were observed.

We observed the lowest G and H fractions the first day of each measurement period

irrespective of tillage. This was consistent with days when the largest percentage of Rn was

partitioned to evaporation. On average in Period 1, minimal differences were found between

tillage practices as 17% and 16% of Rn was partitioned to G at the Till and NT, respectively.

In Period 2, the G/Rn fraction was 7% larger at the Till soil as we observed 21% and 14% at

22

the Till and NT, respectively. Differences between the Till and NT soils in Period 2

corresponded to larger bulk density and VWC measurements at the Till soil during the

period. The increase in bulk density along with a slightly higher VWC has been shown to

have a positive effect on the soil thermal properties of volumetric heat capacity and thermal

conductivity (Cellier et al., 1996; Ochsner et al., 2007; Ogee et al., 2001). Allmaras et al.

(1977) reported similar averages of soil G/Rn fractions of 14.5% and 19%, corresponding to

bulk density values at a moldboard plowed and a moldboard plowed then compacted soil.

Figure 5. Daytime percentage of net radiation (Rn) partitioned into soil heat flux (G), latent

heat flux (LE) and Sensible heat flux (H) for the Till (A and B) and NT (C and D) during

Period 1 (DOY 216-220) and Period 2 (DOY 262-266).

H partitioning trends were similar to G trends, as they were lowest the initial days

following the rainfall and increasing as the soil dried. In Period 1, Till H/Rn fractions

0%

20%

40%

60%

80%

100%

262 263 264 265H/Rn LE/Rn G/Rn

D)

0%

20%

40%

60%

80%

100%

216 217 218 219 220

C)

0%

20%

40%

60%

80%

100%

216 217 218 219 220

Ene

rgy

Pa

rtiti

oni

ng

Day of Year

0%

20%

40%

60%

80%

100%

262 263 264 265

B)A)

Period 1 Period 2

23

increased daily from 8% to 42% and 36% to 56% at the Till and NT respectively. In Period 2,

H/Rn daily increased was -8% to 68% and 1% to 59% for the period. A negative H flux at the

NT soil for the first day of the period (DOY 262) corresponded to an LE fraction larger than

the energy available at the NT soil, this indicates that horizontal wind contributing energy

affecting measurements on this day causing daytime advection. Wind speed data observed at

a meteorological station near our site, shows a period high average wind speed of 10 m s-1 on

DOY 262. Holland et al. (2013) and Kool et al. (2016) also saw negative H fluxes due to

mechanical wind energy partitioning transferring energy flux at the surface affecting

gradients of air temperature and atmospheric water vapor.

Latent heat fractions in Period 1, decreased daily throughout the period from 81% to

37% and 50% to 26% of total Rn on the Till and NT soils, respectively. These results are

similar to trends in ML measurements (Fig. 4), where evaporation rates were largest the first

day following precipitation and decreased with time as the soil dried. In Period 2, 4-day LE

partitioning decreased from 85% to 17% and 103% to 16% of total Rn at the Till and NT

soils, respectively. Partitioning of Rn in LE at bare soil surfaces have previously shown

similar values following rainfall, where the highest values ranged from 61% - 80% followed

rainfall while decreasing to 30% - 45% after three to four days of soil drying (Allmaras et al.,

1977; Richard and Cellier, 1998; Schwartz et al., 2010). Differences observed in the range of

LE fractions between Period 1 and 2 could be caused by a higher evaporative demand later in

the season as a cold front settled over our plot in the 2nd and 3rd day of Period 1, decreasing

the daytime net radiation, while increasing atmospheric humidity and therefore suppressing

evaporation in Period 1.

24

CHAPTER 4. CONCLUSIONS

Surface bulk density was not significantly different during Period 1 which was shortly

after tillage, thus, Rn and G flux were similar at the Till and NT soils. Although bulk density

changes were insignificant, LE, on average, was 15% larger and H was 17% smaller in Till

soil versus NT soil during a drying period soon after tillage. Disrupting the soil by tillage led

to increased fractions of Rn being partitioned to LE.

After a significant amount of rainfall, the Till soil bulk density increased by 0.11 g cm-3.

Associated with the increase in bulk density of the Till soil was a 7% larger G value in the

Till soil compared to the NT soil. The available energy at the Till soil surface, due to G, was

smaller than the NT available energy. This lower available energy resulted in a 10% lower

fraction of Rn being partitioned to LE for the Till soil than for the NT soil in Period 2, while

H, on average, was 13% larger at the NT than for the Till soil.

Future Work:

1. The thermo-TDR method to determine bulk density and thermal properties

continuously is a novel and relatively new method. This would be invaluable in tracking

continuous measurements of surface bulk density changes. It would also provide

measurements of soil volumetric heat capacity, thermal conductivity, G, LE and H fluxes,

therefore increasing confidence in surface energy partitioning (Heitman et al., 2010; Ochsner

et al., 2007; Xiao et al.,2011).

2. We were limited in the interpretation of Rn results, because surface properties such

as albedo, surface roughness, near-surface water content, and wind speed, were not

measured. Additional measurements would provide more information about Rn components

and the subsequent partitioning of surface energy fluxes.

25

3. The study was performed on one soil type following rotary tillage. The

implementation of more variables such as various climates, soil types, and tillage techniques,

would provide a greater range of data to understand the effects of bulk density on surface

energy balance partitioning.

26

REFERENCES

Aase, J. K., & Tanaka, D. L. (1987). Soil Water Evaporation Comparisons Among Tillage

Practices in the Northern Great Plains 1. Soil Science Society of America Journal, 51(2),

436-440.

Allmaras, R.R., Hallauer, E.A., Nelson, W.W., Evans, C.D., 1977. Surface energy balance

and soil thermal property modifications by tillage-induced soil structure. Minnesota

Agric. Exp. Stn. Tech. Bull. 306, 1-40.

Ashktorab, H., Pruitt, W.O., Paw, K.T., George, W. V, 1989. Energy balance determinations

close to the soil surface using a micro-Bowen ratio system. Elsevier Sci. Publ. B.V 46, 259–

274.

Azooz, R. H., Lowery, B., Daniel, T. C., & Arshad, M. A. (1997). Impact of tillage and

residue management on soil heat flux. Agricultural and forest meteorology, 84(3-4), 207-222.

Boast, C. W., & Robertson, T. M. (1982). A “Micro-Lysimeter” Method for Determining

Evaporation from Bare Soil: Description and Laboratory Evaluation 1. Soil Science Society

of America Journal, 46(4), 689-696.

Cellier, P., Richard, G., Robin, P., 1996. Partition of sensible heat fluxes into bare soil and

the atmosphere. Agric. For. Meteorol. 82, 245–265. https://doi.org/10.1016/0168-

1923(95)02328-3

Chen, S., Su, H., & Zhan, J. (2014). Estimating the impact of land use change on surface

energy partition based on the Noah model. Frontiers of Earth Science, 8(1), 18-31.

Evett, S.R., Agam, N., Kustas, W.P., Colaizzi, P.D., Schwartz, R.C., 2012. Soil profile

method for soil thermal diffusivity, conductivity and heat flux: Comparison to soil heat flux

plates. Adv. Water Resour. 50, 41–54. https://doi.org/10.1016/j.advwatres.2012.04.012

Gaudin, A. C., Tolhurst, T. N., Ker, A. P., Janovicek, K., Tortora, C., Martin, R. C., & Deen,

W. (2015). Increasing crop diversity mitigates weather variations and improves yield

stability. PLoS One, 10(2), e0113261.

Gentine, P., Entekhabi, D., & Polcher, J. (2011). The diurnal behavior of evaporative fraction

in the soil–vegetation–atmospheric boundary layer continuum. Journal of Hydrometeorology,

12(6), 1530-1546.

Hatfield, J.L., Prueger, J.H., 2015. Temperature extremes: Effect on plant growth and

development. Weather Clim. Extrem. 10, WACED1400046.

https://doi.org/10.1016/j.wace.2015.08.001

Heitman, J.L., Horton, R., Sauer, T.J., Ren, T.S., Xiao, X., 2010. Latent heat in soil heat flux

measurements. Agric. For. Meteorol. 150, 1147–1153.

https://doi.org/10.1016/j.agrformet.2010.04.017

27

Heitman, J. L., Xiao, X., Horton, R., & Sauer, T. J. (2008). Sensible heat measurements

indicating depth and magnitude of subsurface soil water evaporation. Water Resources

Research, 44(4), WOOD05.

Holland, S., Heitman, J.L., Howard, A., Sauer, T.J., Giese, W., Ben-Gal, A., Agam, N., Kool,

D., Havlin, J., 2013. Micro-Bowen ratio system for measuring evapotranspiration in a

vineyard interrow. Agric. For. Meteorol. 177, 93–100.

https://doi.org/10.1016/j.agrformet.2013.04.009

Horton, R., Ankeny, M. D., & Allmaras, R. R. (1994). Effects of compaction on soil

hydraulic properties. In Developments in agricultural engineering (Vol. 11, pp. 141-165).

Elsevier.

Huber, D., Mechem, D., Brunsell, N., 2014. The effects of Great Plains irrigation on the

surface energy balance, regional circulation, and precipitation. Climate 2, 103–128.

https://doi.org/10.3390/cli2020103

Idso, S. B., Jackson, R. D., Reginato, R. J., Kimball, B. A., & Nakayama, F. S. (1975). The

dependence of bare soil albedo on soil water content. Journal of Applied Meteorology, 14(1),

109-113.

Kool, D., Kustas, W.P., Ben-Gal, A., Lazarovitch, N., Heitman, J.L., Sauer, T.J., Agam, N.,

2016. Energy and evapotranspiration partitioning in a desert vineyard. Agric. For. Meteorol.

218–219. https://doi.org/10.1016/j.agrformet.2016.01.002

Logsdon, S. D., Kaspar, T. C., & Cambardella, C. A. (1999). Depth-incremental soil

properties under no-till or chisel management. Soil Science Society of America Journal,

63(1), 197-200.Matthias, A.D., Fimbres, A., Sano, E.E., Post, D.F., Accioly, L., Batchily,

A.K., Ferreira, L.G. 2000. Surface roughness effects on soil albedo. Soil Sci. Soc. Am. J.

64:1035–1041.

Moret, D., & Arrúe, J. L. (2007). Dynamics of soil hydraulic properties during fallow as

affected by tillage. Soil and Tillage Research, 96(1-2), 103-113.

Ochsner, T. E., Sauer, T. J., & Horton, R. (2007). Soil heat storage measurements in energy

balance studies. Agronomy journal, 99(1), 311-319.

Ogée, J., Lamaud, E., Brunet, Y., Berbigier, P., Bonnefond, J.., 2001. A long-term study of

soil heat flux under a forest canopy. Agric. For. Meteorol. 106, 173–186.

https://doi.org/10.1016/S0168-1923(00)00214-8

Potter, K. N., R. M. Cruse, and R. Horton. 1985. Tillage effects on soil thermal properties.

Soil Sci. Soc. Am. J. 49:968-973.

Potter, K. N., R. Horton, and R. M. Cruse. 1987. Soil surface roughness effects on radiation

reflectance and soil heat flux. Soil Sci. Soc. Am. J. 51:855-860.

28

Richard, G., & Cellier, P. (1998). Effect of tillage on bare soil energy balance and thermal

regime: an experimental study. Agronomie, 18(3), 163-181.

Sauer, T. J., Hatfield, J. L., Prueger, J. H., & Norman, J. M. (1998). Surface energy balance

of a corn residue-covered field. Agricultural and Forest Meteorology, 89(3-4), 155-168.

Sauer, T.J., and R. Horton. 2005. Soil heat flux. p. 131–154. In Micrometeorology in

Agricultural Systems. Madison, USA.

Schwartz, R.C., Baumhardt, R.L., Evett, S.R., 2010. Tillage effects on soil water

redistribution and bare soil evaporation throughout a season. Soil Tillage Res. 110, 221–229.

https://doi.org/10.1016/j.still.2010.07.015

Strudley, M.W., Green, T.R., Ascough, J.C., 2008. Tillage effects on soil hydraulic

properties in space and time: State of the science. Soil Tillage Res. 99, 4–48.

https://doi.org/10.1016/j.still.2008.01.007

Wang, Z.-H., Bou-Zeid, E., 2012. A novel approach for the estimation of soil ground heat

flux. Agric. For. Meteorol. 154, 214–221. https://doi.org/10.1016/j.agrformet.2011.12.001

Xiao, X., Horton, R., Sauer, T.J., Heitman, J.L., Ren, T., 2011. Cumulative soil water

evaporation as a function of depth and time. Vadose Zo. J. 10, 1016–1022.

https://doi.org/10.2136/vzj2010.0070

Zeggaf, A.T., Takeuchi, S., Dehghanisanij, H., Anyoji, H., Yano, T., 2008. A Bowen ratio

technique for partitioning energy fluxes between maize transpiration and soil surface

evaporation. Agron. J. 100, 988–996. https://doi.org/10.2134/agronj2007.0201