156 M.D. Novak et al. Agricultural and Forest Meteorology 102 2000 155–171
was considerably improved by increasing the residue area, mostly in the lower part of the mulch, which is consistent with the irrigation causing brittle pieces of mulch to break off and fall towards the soil surface. © 2000 Elsevier Science B.V. All
rights reserved.
Keywords: Straw mulch; Temperature; Humidity; Energy balance; Turbulent diffusivity
1. Introduction
This is the second in a series of two papers describ- ing turbulent exchange processes in a barley-straw
mulch. Mulching is practiced widely in agriculture, forestry, and horticulture because of its many advan-
tages, including erosion control, water conservation, soil temperature amelioration, and soil structure en-
hancement. Mathematical modelling of the physical effects of mulches is largely limited by our under-
standing of turbulent exchange processes within the mulch. For this reason we carried out a detailed mi-
crometeorological study of mulching in a series of ex- periments at the University of British Columbia Plant
Science Research Station in Vancouver, Canada, that occurred in July–October in each of three consecutive
years 1992–1994.
In the first paper Novak et al., 2000a, hereafter referred to as I measurements of wind and turbu-
lence regimes made within and just above a 10 t ha
− 1
barley-straw mulch in 1994 are reported. This paper presents the associated thermal and moisture regimes,
including sensible and latent heat flux densities, and the complete energy balance within and above this
mulch in both normal and artificially wetted states. Turbulent water-vapour diffusivities within the mulch
and the overall mulch water-vapour conductance, measured when the underlying soil surface was the
only source of moisture, i.e., when only the ‘far-field’ contributed to the resulting profile of vapour pres-
sure Raupach, 1989, are reported. The results allow a test of our recently developed air renewal model
for sensible heat flux Chen et al., 1997, hereafter referred to as CNBL97, which we showed works
well above the mulch, to heights within the mulch. The overall objective is to describe the temporal and
spatial variations of the thermal and moisture regimes of the barley-straw mulch, emphasizing the role of
turbulence in the vertical transfer of sensible and la- tent heat. Other descriptive studies of mulches have
been presented in the literature e.g., Kohnke and Werkhoven, 1963; Radke, 1982; Bristow, 1988; Bris-
tow and Abrecht, 1989; Steiner, 1989; Wagner-Riddle et al., 1996 but our study is unique for its accuracy,
breadth, and completeness.
During the late summer and early fall conditions studied, the mulch was dry for much of the daytime,
especially in the afternoon, but was quite moist dur- ing nighttime and early morning because of conden-
sation of dewfall from the atmosphere and moisture that evaporated from the underlying soil Chen and
Novak, 1997. To simulate the effects of rainfall, the mulch was thoroughly wetted by sprinkler irrigation
and the drying-out phase that followed in the next few days was studied intensively. This also has application
to understanding the processes involved in hay drying Tuzet et al., 1993; Barr and Brown, 1995.
2. Field experiments
2.1. Plot characteristics Barley straw was manually applied at different rates
successively on a single circular plot 14 m in diam- eter in 1993 and 1994 in the southeast corner of a
25 m×40 m area. The remaining 25 m×25 m area ad- jacent to and upwind of the mulch was maintained
bare as a reference. In some of the years we attempted to keep the bare area wet enough to maintain poten-
tial evaporation using sprinkler irrigation but this ef- fort had been abandoned by the end of August in 1993
and 1994, which are the periods considered here. The sandy loam soil coupled with high potential evapora-
tion rates eventually made attaining this objective im- practical. The mulch and bare plots were instrumented
similarly, with all sensors installed near their centres within 3 m of each other. We report measurements
made on selected days in 1993 and 1994 with the 10 t ha
− 1
mulch, which was the second heaviest rate used; results with the other rates are qualitatively sim-
ilar. The height, h, of the 10 t ha
− 1
mulch was 6.6 cm.
M.D. Novak et al. Agricultural and Forest Meteorology 102 2000 155–171 157
Unless indicated otherwise, all data were sampled and averaged every 10 s and 5 min, respectively, in 1993
and every 20 s and 10 min, respectively, in 1994 using a programmable data logger model CR7, Campbell
Scientific Inc., Logan, UT, USA.
2.2. Mulch thermal regimes Mean air temperature, T
a
, was measured simul- taneously at various heights above the soil sur-
face, z, within and above the mulch using fine-wire chromelconstantan thermocouples 25 mm above the
mulch and 76 mm diameter within the mulch, for additional robustness mounted on a 50 cm high,
1.3 cm diameter rod installed vertically in the mulch plot. The thermocouples were placed about 20 cm
away from the rod facing into the direction of the prevailing wind west–northwest. Mean straw ele-
ment temperature, T
e
, was measured simultaneously at six z within the mulch using chromelconstantan
thermocouples 250 mm in diameter. These thermo- couples were mounted on a shorter vertical rod as for
the T
a
sensors and each was attached to a selected straw element surface using transparent tape, which
not only ensured good contact but also protected the thermocouple from damage. The instrumented mulch
elements used were about 3 cm long, 0.5 cm wide, and 0.1 cm thick, and oriented flat in the horizontal plane,
typical of the majority of elements in the mulch. The thermocouples were installed on the lower element
surfaces in 1994 and on the upper surfaces in 1993. It was thought that this would make little difference
as it was expected that the elements would be nearly isothermal, which is usually assumed in mulch mod-
els e.g., Bristow et al., 1986; Bussière and Cellier, 1994. As will be seen later, the lower-surface and
upper-surface temperatures did differ greatly, with important consequences for sensible heat transfer
from the mulch. Note that daytime upper surface temperatures were likely underestimated using this
measurement method as the thermocouple protrudes slightly into lower temperature air and has a higher
albedo than the mulch element.
Sensible heat flux density, H, at various z within and above the 10 t ha
− 1
mulch was determined using a renewal model based on the cubic structure function of
measured high-frequency air temperature fluctuations, −
1T
a 3
, at each z CNBL97, with an added term to account for molecular diffusion, as follows:
H z = ρc
p
−
αβ
23
γ 1T
a 3
1t
13
u
∗
h
23
z −
D
′ hm
dT
a
dz ,
for 0.2h z ≤ 3h − 2d, ρc
p
−
αβ
23
γ 1T
a 3
1t
13
u
∗
z − d
′ 23
z −
D
′ hm
dT
a
dz ,
for z ≤ 0.2h or z 3h − 2d, 1
where ρ=1.2 kg m
− 3
and c
p
= 1020 J kg
− 1
K
− 1
are the density and specific heat of air, αβ
23
γ is a
combined coefficient 0.397 for all z within and above the 10 t ha
− 1
mulch canopy, 1T
a 3
is sam- pled at a frequency 11t=10 Hz, very near the op-
timum frequency, using 25 mm diameter fine-wire chromelconstantan thermocouples, u
∗
is the friction velocity determined from the measured mean ‘cup’
wind speed at z=9.6 cm using Monin–Obukhov simi- larity theory fully described in I; for z0.2h though,
u
∗
= σ
w
1.25, where σ
w
is calculated from u
∗
above the mulch using the regression equation presented in
I, d
′
is an adjusted displacement height d
′
= 2h−d
for z3h−2d and d
′
= 0 for z0.2h, d=5.7 cm is the
displacement height, D
′ hm
is the molecular diffusivity for heat within the mulch, and dT
a
dz is the measured vertical mean air temperature gradient at z. D
′ hm
is determined as Hares and Novak, 1992a
D
′ hm
= D
hm
f
e
− θ f
t
2 where D
hm
= 2.2×10
− 5
m
2
s
− 1
is the molecular diffu- sivity for heat in air, f
e
= 0.95 is the ‘effective’ mulch
porosity, i.e., that between the mulch elements, θ is the average volumetric water content of the mulch
negligible, and f
t
= 0.9 is the tortuosity of the mulch
which was roughly estimated. The f
e
was determined by quickly immersing a known mass of mulch into
a known volume of water and measuring the volume change, which yielded the volume of the mulch ele-
ments from which the volume between elements was calculated for the 10 t ha
− 1
mulch. The wind speed at
158 M.D. Novak et al. Agricultural and Forest Meteorology 102 2000 155–171
z= 9.6 cm was measured with a custom-made hot-wire
anemometer I. The αβ
23
γ = 0.397 was determined
by calibrating H calculated with Eq. 1 for four z above the mulch against H determined by the en-
ergy balance method of the whole mulch CNBL97, i.e., H=R
n
− G
− S
H,6
− λ
E , where R
n
is the net ra- diation flux density, G
is the soil surface heat flux density, S
H,6
is the sensible heat stored in all the mulch elements, and λE is the latent heat flux den-
sity, all measured as described further on. The cali- bration was done by a best fit for all half hours in
a single day, 16 August 1994, with the resulting fit shown in Fig. 4 of CNBL97 note that the date indi-
cated in this figure is in error; also the bare soil data in this figure uses DST, 1 h later, instead of PST as
indicated.
2.3. Mulch moisture regimes Mean water-vapour pressure in the air, e
a
, was mea- sured at various z within and above the mulch us-
ing capacitance-type relative humidity sensors model HMM-20D, Vaisala Inc., Helsinki, Finland. Each sen-
sor had a 75 mm diameter chromelconstantan ther- mocouple installed within its protective cap near it to
convert relative humidity to vapour pressure. To im- prove the response time of the sensor, which is about
15 s 90 response when the protective cap 6 cm long and 1.2 cm in diameter and its membrane filter
are installed, and prevent abnormal readings when the filter is wet from condensation within the mulch, we
removed the filter and cut away the bottom half of the cap. Slots in the remaining half of the cap were filled
in and the outside of the cap was painted white. The net effect was that the relative humidity sensor was
better ventilated while still being shielded from so- lar irradiance at least at high solar elevation angles
and protected from rainfall and dewfall. In addition, its response time was shortened to below 5 s. Because
the size of the probe was similar to the mulch ele- ments we felt that for the within-mulch measurements
there was minimal disturbance of wind and radiation regimes. A 1 mm diameter wire was wrapped around
each probe and pushed into the ground to place the sensor at the desired z. The humidity probes were cal-
ibrated against a dew-point generator model LI-610, LI-COR Inc., Lincoln, NE, USA in the laboratory to
within 2 for relative humidity in the 15–95 range. Values of relative humidity below 15 were rare in
our experiments but values above 95 were frequent. Apparent relative humidity above 100 up to 108
was sometimes measured at night when water, prob- ably from condensation, existed on the sensor. In all
such cases it was assumed that the relative humidity was 100.
The λE at the soil surface under the mulch, λE ,
was measured using our custom-made tension-plate system Chen and Novak, 1997. This system mea-
sures first-stage evaporation or condensation to within 5 W m
− 2
. Soil moisture contents were high enough that evaporation proceeded at the potential rate under
the mulch throughout the experiments. Evaporation from, or condensation onto, the ith of six horizontal
layers within the mulch numbered from the bottom layer up was determined from manual measurements
of the weight, W
i
, of straw from each layer. The layers were extracted completely from a cylindri-
cal area of mulch of horizontal cross-sectional area A
w
= 0.0139 m
2
. The straw from each of the six layers was placed in a 1.1 cm high acrylic cylindrical con-
tainer with netting on its bottom to allow vapour ex- change in the vertical direction. The containers were
then stacked vertically in the location from which the straw was extracted so that the straw within them
blended in with the surrounding undisturbed mulch in a realistic manner. The containers were removed and
weighed every 1 or 2 hours during daytime on se- lected days in the study. Nighttime values on selected
days are based on differences between late evening and early morning weighings, i.e., they are averages
for the nighttime period. The vertical distribution of mulch mass was assumed to be uniform so that each
layer contained the same amount of mulch, m
i
, nomi- nally equal to 0.0139×10006=2.3 g for the 10 t ha
− 1
mulch. The λE from the ith mulch layer was then calculated as
λE
i
= λ
A
w
1W
i
1t ,
3 where λ=2.47×10
6
J kg
− 1
is the latent heat of vapour- ization of water, t is the time, and 1W
i
is the change in W
i
in the time interval 1t. Then, λE at any z within and above the mulch is given by
M.D. Novak et al. Agricultural and Forest Meteorology 102 2000 155–171 159
λEz = λE +
i
z
X
i= 1
λE
i
, 4
where i
z
is the layer just below and up to z. The water content of each mulch layer on a mass basis is
w
i
= W
i
− m
i
m
i
, where m
i
was measured precisely by oven drying the mulch samples in the containers
at 105
◦
C at the end of the experiments. 2.4. Net radiation, soil thermal regimes, and mulch
heat storage R
n
above the mulch was measured using a net ra- diometer model S-1, Swissteco Instruments, Oberriet,
Switzerland mounted facing south at about z=50 cm near the centre of the mulch plot. The radiometer may
have overestimated R
n
at night because of dewfall ac- cumulating on its upper dome. Field calibration for
this and the miniature radiometer described further on was done in 1993 by shading out the beam compo-
nent of solar irradiance, which was measured inde- pendently by similarly shading a solarimeter model
CM5, Kipp Zonen, Delft, Holland which had been calibrated at the National Atmospheric Radiation Cen-
tre, Downsview, Ont., Canada. R
n
z within the mulch was simulated using a mulch canopy radiation model
Novak et al., 2000b. This model divides the mulch into elemental layers and accounts for first-order re-
flections between these. It also accounts for the effects of ‘clumping’ departures from a random distribution
of mulch elements as inferred from measurements of transmissivity made in an earlier study Hares and
Novak, 1992b, hereafter referred to as HN92b. The model was tested by comparing predicted and mea-
sured R
n
above the mulch and predicted downcom- ing total radiation under the mulch with measurements
made by a miniature net radiometer model minor MK II S-14, Swissteco Instruments, Oberriet, Switzerland
installed under the mulch in 1993 with its bottom clear dome replaced by a black-body cavity of known tem-
perature. Inputs to the model include solar radiation, upper-surface and lower-surface T
e
, and T
a
and e
a
at screen height z≈1.5 m. The latter three variables were measured every 30 min at an auto-climate station
located about 100 m from the mulch plot. G
under the mulch was measured using a custom-made thermopile-type heat flux plate installed
at a nominal depth of 1 cm with storage changes above the plate calculated from soil temperature, T
s
, measured at a nominal depth of 0.5 cm T
s,0.5
, i.e., G
= G
1
+ C
s
d
s
1T
s,0.5
1t ,
5 where G
1
is the value from the heat flux plate, C
s
= 2.1×10
6
J m
− 3◦
C
− 1
is the soil volumetric heat capacity HN92b, and d
s
= 0.01 m. T
s
at nominal depths of 2 and 5 cm were also measured. Each T
s
was measured using a 250 mm chromelconstantan thermocouple fixed with epoxy resin within a stainless
steel tube, 10 cm long and 2 mm in outside diameter. Soil surface temperature, T
s,0
, was calculated using Fourier’s law applied across the 0–0.5 cm layer as
follows: T
s,0
= d
s
2k
s
0.75G +
0.25G
1
+ T
s,0.5
, 6
where k
s
= 1.0 W m
− 1◦
C
− 1
is the soil thermal conduc- tivity HN92b.
The rate of change of sensible heat storage, S
H,i
, in the elements of the ith of the six mulch layers used to
measure W
i
was determined using S
H,i
= m
i
A
w
c
e
+ c
w
w
i
1T
e,i
1t ,
7 where c
e
= 1920 J kg
− 1
K
− 1
and c
w
= 4200 J kg
− 1
K
− 1
are the specific heats of the mulch elements and wa- ter, respectively, and 1T
e,i
is the change in T
e
of the i
th mulch layer in the time interval 1t. This neglects sensible heat stored in the mulch air despite the fact
that it is the largest fraction by volume. The total porosity of the mulch is almost 0.99, including spaces
within the mulch elements, for a bulk density of about 10.066=15 kg m
− 3
and an assumed particle density of 1300 kg m
− 3
. Latent heat flux storage in the air was similarly neglected in Eq. 3.
3. Results and discussion
