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
3.1. Temperature and sensible heat flux — non-wetted mulch
Profiles of mean T
a
and T
s
measured on 20 Au- gust 1994, a typical mostly clear day, are shown in
160 M.D. Novak et al. Agricultural and Forest Meteorology 102 2000 155–171
Fig. 1. Vertical profiles of half-hour average air and soil tempera- tures for the non-wetted 10 t ha
− 1
straw mulch during the indicated time periods on 20 August 1994.
Fig. 1. The profiles show that a strong positive lapse rate exists above the mulch during daytime and an
inversion during nighttime. Stability conditions for the most part within the mulch though are the re-
verse of those mentioned earlier. The inversion in the mulch during daytime is much stronger than that ob-
served typically in plant canopies Monteith, 1976, although in I we showed that this inversion has lit-
tle effect on mean wind speed within the mulch. The daytime profile of T
a
reaches a maximum at about z
h=0.67, which is qualitatively similar to that found in other plant canopies Monteith, 1976; Denmead and
Bradley, 1985. According to renewal model concepts CNBL97, T
a
is determined by a combination of heat- ing from nearby mulch elements during ‘quiescent’
periods and cooling during gusts which replace mulch air with cooler air from above it. The latter presum-
ably is increasingly dominant as z increases in the up- per third of the mulch. The distinct above-mulch and
within-mulch thermal regimes at night are in contrast with the findings in relatively open plant canopies for
which the nighttime inversion continues down to the soil surface, although one exception to this is the maize
canopy of Jacobs et al. 1992 which apparently be- haved much like the mulch. Presumably leaf area den-
sity strongly governs this behaviour, and the mulch is probably the densest of possible canopies because of
its small h and considerable leaf area index 5.6 for the 10 t ha
− 1
mulch. Diurnal variations of H calculated using the re-
newal model for 20 August 1994, are shown in Fig. 2. Evidently, H varies strongly with t above and at the
top of the canopy and nighttime values are compar- atively small for all z. The 15 difference between
H
at z=6.6 and 9.6 cm expressed as a percentage of H
at z=9.6 cm is largely ascribed to uncertainties in the renewal model. Fig. 2 of CNBL97 shows that the
best-fitted α and therefore also αβ
23
g for 16 August
1994, decreased with z above the mulch. We chose the mean of four values between z=7.6 and 12.6 cm to
use in the model, and this mean was very close to the value for z=9.6 cm. Extrapolating the trend in that fig-
ure to z=6.6 cm shows that about a 10 difference is expected. The remaining 5 difference is almost too
small to comment on but we note that it could be due to an error in the height of the z=6.6 cm sensor. Because
H
increases strongly with z at the mulch top see Fig. 9 of CNBL97, which presents a more detailed profile
of cumulative H determined with the renewal model positioning the sensor a bit too low would cause an
underestimate of H. Also, the height of the mulch is somewhat variable, so that even with a sensor posi-
tioned perfectly at z=h some sources of sensible heat still exist above z=h, which would result in an increase
of H with z just above the mulch. We discount advec- tive effects in the explanation though, since H for the
surrounding bare soil was less than H above the mulch
Fig. 2. Diurnal variations of half-hour average sensible heat flux density at the 1.1, 3.3, 6.6, and 9.6 cm heights within and above
the non-wetted 10 t ha
− 1
straw mulch on 20 August 1994.
M.D. Novak et al. Agricultural and Forest Meteorology 102 2000 155–171 161
and the fetch at z=9.6 cm was better than 200:1. For z≥h
, generally H0 at night and H0 during daytime, which correspond to the inversion and positive lapse
rate profiles, respectively, shown in Fig. 1. This occurs typically above any canopy and the mulch behaves
similarly.
H varies strongly with z in the mulch, especially in
the daytime during which H at z=3.3 cm is about 7 of that at z=6.6 cm and 6 of that at z=9.6 cm. In
contrast to what occurs in many plant canopies Rau- pach, 1989, the major source of H is very near the
top of the mulch, which we ascribe to the uniform dis- tribution of mulch element area with z and the high
density of the mulch, so that most shortwave radiation is absorbed in the upper 20 Novak et al., 2000b.
At z=1.1 cm, H0 during daytime which differs from what occurs in most plant canopies Denmead and
Bradley, 1985; Raupach, 1989. Because the gradient of T
a
is positive in the canopy up to about z=4.4 cm during the daytime, H is transported vertically counter
the local gradient at z=3.3 cm but not at z=1.1 cm. From the profiles of mean T
a
, this constrains the eddy size at z=3.3 cm to at least 2 cm, because according
to the renewal model of turbulent transfer, for vertical transfer to occur the air at any level must be replaced
by air from above that is cooler on average than at that level the reverse is true when H changes sign
at night. Similarly at z=1.1 cm the eddies must be smaller than 3.3 cm. The H0 at z=1.1 cm is not be-
cause the contribution from molecular diffusion, H
m
, assumed in Eq. 1 to be added in parallel to the re-
newal contribution, H
r
, is large enough to overcome H
r
. Both components contribute to the negative flux although the contribution from molecular diffusion is
about double that from the turbulence Fig. 3. This suggests that larger eddies generated by shear near the
canopy top, which are apparently effective at transport- ing H at z=3.3 cm, either do not penetrate to z=1.1 cm
or do not contribute much to H, possibly because of the nearness of the soil surface. At z=3.3 cm molecu-
lar and renewal components oppose each other during daytime with the magnitude of the molecular compo-
nent being only about a quarter as great as the renewal component. Nighttime H is mostly positive at both
z=
1.1 and 3.3 cm, with H
r
small and highly variable and H
m
the major term. Since within-canopy convec- tion likely dominates at night I, this may be because
the renewal model is not valid under these conditions.
Fig. 3. Diurnal variations of the renewal H
r
and molecular H
m
contributions to the total H half-hour average sensible heat flux density at the 1.1 and 3.3 cm heights within the non-wetted
10 t ha
− 1
straw mulch on 20 August 1994.
Lower-surface T
e
on 20 August 1994, is very sim- ilar to T
a
for all z within the mulch throughout the day not shown. Diurnal variations of upper-surface
T
e
, T
eu
, measured on 30 August 1993, another mostly clear day, are shown in Fig. 4. In contrast with T
a
, T
eu
increases with z up to z=h during daytime and de- creases with z to z=h at night. This is similar to the
measurements of HN92b made in a 20 t ha
− 1
straw
Fig. 4. Diurnal variations of half-hour average upper-surface mulch element temperature at the 0, 1.0, 2.0, 3.0, 4.4, 5.5, and 6.6 cm
heights within the non-wetted 10 t ha
− 1
straw mulch on 30 August 1993.
162 M.D. Novak et al. Agricultural and Forest Meteorology 102 2000 155–171
mulch. They inserted thermocouples previously used to measure T
s
, and so encased in the stainless steel tubes described earlier, into the mulch at various z and
also measured surface temperature with a bolometer. Because the size of the steel tubes is similar to the
mulch elements their temperatures were apparently more typical of T
eu
rather than T
a
. The increase of upper-surface T
e
with z during daytime is not uniform. The largest increases occur at the top of the mulch
and apparently near the bottom between z=1.0 and 2.0 cm. At the top, this is ascribed to the strong source
strength for H there. At the bottom, it may be related to the lower turbulent transport diffusivity reported
later although the relatively small increase between z=
0 and 1.0 cm is then hard to explain. Diurnal variations of the difference between T
eu
and T
a
for 30 August 1993, are shown in Fig. 5. Large pos- itive differences occur near the top of the mulch dur-
ing daytime, which corresponds to the location where most of the solar radiation is absorbed Novak et al.,
2000b and of the main source density for H see Fig. 9 of CNBL97. At night, the differences near the top of
the mulch are reversed, in agreement with the change of sign of H above the mulch Fig. 2. These large
differences show that the assumptions usually made that T
a
and T
e
are equal at any z within the mulch and that implicitly the mulch elements are isother-
mal Bristow et al., 1986; Bussière and Cellier, 1994
Fig. 5. Diurnal variations of the difference between half-hour av- erage upper-surface mulch element temperature and air tempera-
ture at the 1.0, 2.0, 3.0, 4.4, 5.5, and 6.6 cm heights within the non-wetted 10 t ha
− 1
straw mulch on 30 August 1993.
are in error. They also imply that the time constant for cooling of the mulch elements must be consider-
ably longer than the duration between gusts ≈1 s, as described in I that bring cooler air down from above
the mulch. Otherwise, T
eu
would be similar to T
a
. Ac- cording to the z=1.0 cm data, T
eu
− T
a
is negative dur- ing daytime, which would be reasonable if warmer
air was penetrating down to this level from higher in the mulch, and positive at night. This appears to hold
also at z=3.0 cm but not at z=2.0 cm, although the latter data is somewhat suspect the nighttime trend
for T
eu
− T
a
does not appear to be reasonable and the T
a
sensor was completely off-scale for a brief period early in the morning for unknown reasons; the erro-
neous data were interpolated for the graph. But these trends do not hold on all days, and considering lateral
variability in the mulch and the fact that the T
e
profile is determined in a separate location from T
a
, our only firm conclusion from this figure is that T
eu
and T
a
are nearly equal in the bottom two-thirds of the mulch.
3.2. Latent heat flux, vapour pressure, and mulch water content — non-wetted mulch
The λE and e
a
measured within and just above the mulch on 20 August 1994, are shown in Fig. 6. The
Fig. 6. Diurnal variations of hourly average latent heat flux density at the 0, 1.1, 3.3, 5.5, and 6.6 cm heights and water-vapour pressure
at the 0, 1.5, 3.5, 6.6, and 9.6 cm heights within the non-wetted 10 t ha
− 1
straw mulch on 20 August 1994. The λE at the 2.2 and 4.4 cm heights were omitted for clarity.
M.D. Novak et al. Agricultural and Forest Meteorology 102 2000 155–171 163
value of e
a
at z=0 is calculated as eT
s,0
, where e is the saturated water-vapour pressure function. Be-
cause the W
i
’s were not measured on this day, the λ
E
i
’s were approximated for each half hour by us- ing averages for corresponding times from the four
days 11, 12, 17, and 23 August when such measure- ments were made for the non-wetted mulch in 1994.
The times when the earliest and latest W
i
measure- ments were made on each of these days were 8:00 and
18:00 hours PST, respectively. Values of λE
i
for the 6:00–8:00 and 18:00–21:00 hours PST periods were
extrapolated according to W
i
measurements made in 1993 when the times for the earliest and latest mea-
surements were 6:00 and 21:00 hours PST. Since the W
i
’s were not measured during 18:00–8:00 hours PST, it was assumed that the λE
i
’s for each half hour were equal to the average value estimated from the total
amount of water accumulated on each layer overnight. For much of the day, the mulch is a significant
source or sink of water vapour, which is evident by the variation of λE with z, except for 14:00–16:00 hours
PST during which λE varies little with z and the flux is from the soil surface only. The λE above and be-
low the mulch differ greatly and are governed by dif- ferent processes. At night, λE is negative above the
mulch and positive under the mulch. The former is because of dewfall while the latter is attributed to tur-
bulent mixing associated with free convection of the unstable air within the mulch Fig. 1 and I. During
early morning, λE increases strongly above the mulch and decreases almost to zero under the mulch on
some days it actually becomes negative, e.g., on 25 August 1994, as shown in Chen and Novak, 1997.
This is attributed to the increase of solar irradiance which warms the wet mulch, ultimately tending to
drive moisture both upwards above the mulch and downwards within the mulch. By early afternoon,
λ
E under the mulch increases to its maximum value
while above the mulch it continues to fall from its early morning maximum, eventually becoming nega-
tive again at around 21:00 hours PST. The temporal variation of λE above the mulch qual-
itatively resembles that of a drying bare soil Idso et al., 1974 but the processes are different in detail.
Roughly half of the nighttime re-wetting of the mulch is from upward flow from the soil, which within the
mulch occurs by a process of distillation. The other half of the re-wetting is from dewfall. For a bare soil,
most of the nighttime re-wetting near the soil surface is by liquid water flow from the wetter soil below.
Because turbulent transfer in the upper part of the mulch is dominated by large-scale eddies I, it is not
expected that λE should be related to vertical gradi- ents of e
a
in the mulch in a consistent manner Rau- pach, 1989. As we have seen for H, we measured
counter-gradient flow within the mulch during the day. Nevertheless, λE and gradients of e
a
appear to be qual- itatively consistent with flux-gradient theory in the
mulch, as counter-gradient flow at least is not in evi- dence. For example, the early morning decline of λE
near the bottom of the mulch corresponds very well to a decrease in vertical gradients of e
a
at this time, and the increase of λE at z=0 in the early afternoon
corresponds to an increase in the gradient across the z=
0–1.1 cm layer note that λE, e
a
, and T
s,0
were all measured independently and in different locations in
the mulch. The average daytime variations of w
i
for five of the layers on the three sampling days in 1994 are shown
in Fig. 7. From the initial values, it is seen that dur- ing the night the bottom and top layers of the mulch
re-wet the most on average and that the drying out during the day occurs most rapidly at the top of the
mulch and progressively later deeper in the mulch. By t=
15:00 hours PST at the top by 16:00 hours all lay-
Fig. 7. Daytime variations of water content mass basis hourly of the 0–1.1, 2.2–3.3, 4.4–5.5, and 5.5–6.6 cm mulch layers within
the non-wetted 10 t ha
− 1
straw mulch. The curves are averages of measurements made on 11, 12, 17 and 23 August 1994. The w
i
of the 3.3–4.4 cm layer has been omitted for clarity.
164 M.D. Novak et al. Agricultural and Forest Meteorology 102 2000 155–171
ers in the mulch begin to re-wet, a process that con- tinues all night.
3.3. Diffusivity profile and mulch conductance for water vapour
Raupach 1989 showed that within a canopy con- centration profiles may be considered as the sum of
contributions from sources in the canopy that are near and far away mostly in the upwind direction. For a
source at the soil surface, the concentration profile de- pends on the far-field only. The far-field contribution
can be predicted using the standard diffusion equation with a well-defined diffusivity, equal to σ
2 w
t
L
, where σ
w
is the standard deviation of vertical fluctuations in wind speed and t
L
is the Lagrangian time scale. Fig. 7 shows that the mulch is very nearly neither a
source nor sink of moisture in the late afternoon, i.e., for t=14:00–16:00 hours PST, during which evapora-
tion from the mulch contributes less than 10 to the total evaporation from the system. By assuming that
the mulch is in steady-state on a half-hour basis dur- ing this time, we determined the within-mulch profile
of the far-field turbulent diffusivity for water vapour, D
v
, from the basic flux-gradient relation the analogue of Fick’s law:
D
v
z = − R
v
T
Ka
λE λ
de
a
dz ,
8 where R
v
= 461 J kg
− 1
K
− 1
is the gas constant for water vapour, T
Ka
is the absolute air temperature at z
, and a finite-difference approximation to de
a
dz is calculated from the measured profiles, with z at the
mid-point between any two measurement heights. Fig. 8 shows the vertical profile of D
v
averaged over all available half-hour measurement periods during
during 28 August–1 September 1993, and 10–25 Au- gust 1994. The average horizontal cup wind speed at
z= 57 cm for all these periods is s
57
= 1.66 m s
− 1
, and D
v
for each half hour was normalized by multiplying it by s
57
s
57
, where s
57
is the average value for each half hour, because of some dependence of D
v
on wind speed Fig. 9. Also shown for comparison in Fig. 8
is the molecular diffusivity for water vapour in air, D
vm
= 2.4×10
− 5
m
2
s
− 1
. The profiles of D
v
are very similar for 1993 and 1994 and D
v
increases approximately exponentially
Fig. 8. Vertical profile of the far-field turbulent diffusivity for water vapour within the non-wetted 10 t ha
− 1
straw mulch for all available half-hour periods between 14:00 and 16:00 hours PST
during 28 August–1 September 1993, and 10–25 August 1994. Error bars indicate one standard deviation. Also indicated is the
molecular diffusivity for air dashed line. The solid line is an exponential fit to the data for both years Eq. 9. Error bars show
the ranges of variation.
Fig. 9. The ratio of half-hour mulch conductance for water vapour to that in still air vs half-hour average cup wind speed at the 57 cm
height for the non-wetted 10 t ha
− 1
straw mulch. The data is for all available half-hour periods between 14:00 and 16:00 hours PST
during 28 August–1 September 1993, and 10–25 August 1994. The line is a linear regression y vs x through the data Eq. 12.
M.D. Novak et al. Agricultural and Forest Meteorology 102 2000 155–171 165
with depth below z=h within the mulch, with the best fit to the data from both years given by
D
v
= 0.00049 exp [2.6 z h − 1] m
2
s
− 1
, r
2
= 0.93, n = 7.
9 According to this equation, D
v
= 0.00049 m
2
s
− 1
at z=h
, which is about 5 less than the diffusivity for momentum at the same height under neutral condi-
tions D
m
h=0.4u
∗
h−d=0.00052 m
2
s
− 1
, where u
∗
= 0.15 m s
− 1
is the average value for all the half hours used. This is somewhat unexpected, as Raupach
1979 and Raupach et al. 1996 show that the turbu- lent diffusivity for scalars such as water vapour should
be about twice that for momentum just above the top of a canopy. It may be that this is because of extrap-
olating within-canopy values to the mulch top we have not investigated diffusivities above the canopy
yet, or perhaps because the water-vapour source is at z=0 most field measurements are for sources near
the top of the canopy. The attenuation coefficient of 2.6 lies within the general range of 2.5–4 reported
for plant canopies Lemon, 1965. To date, direct measurements of D
v
within a mulch canopy have not been reported. Instead, D
v
within mulches has been estimated using exponential functions as given
by Eq. 9, but with different attenuation coefficients. Thompson 1981 and Tuzet et al. 1993 used an
attenuation coefficient of 2.5, whereas Stigter et al. 1984 and Bussière and Cellier 1994 used a value
of about 8. Therefore, the attenuation coefficient re- ported here resolves some of these uncertainties, at
least for mulches similar to barley straw.
In I it was reported that σ
w
u
∗
= 0.026 exp2.9 zh,
based on measurements with a tri-axial hot-film probe, describes the variation of σ
w
within the mulch. Com- bining this expression with Eq. 9 allows calculation
of the profile for t
L
= D
v
σ
2 w
within the mulch. This profile decreases monotonically with z from a value
of 5.4 hu
∗
at z=0 to 0.22 hu
∗
at z=h, but because of the low wind speeds in the mulch we are not confident
about the values below z=5 cm. In Lagrangian mod- elling it is usually assumed that t
L
is constant within a canopy at the value determined for z=h, except very
near the soil surface, where it declines to zero. The value at z=h is often obtained by extrapolating down
from values measured in the inertial layer. A value of 0.22 hu
∗
is quite close to the generally recom- mended value of 0.3 hu
∗
and for such a dense canopy somewhat lower values are expected Raupach et al.,
1996. Furthermore, this t
L
provides an estimate of L
w
, the vertical single-point Eulerian integral length scale, i.e., L
w
≈ σ
w
t
L
= 0.10h at z=h. This is about three
times lower than the h3 generally accepted for most canopies.
For the same periods, mulch conductance, k
v
, was calculated from the integrated version of Eq. 8, i.e.,
k
v
= R
v
T
Ka
λ λE
e
a,0
− e
a,h
, 10
where T
Ka
is the average absolute air temperature across the mulch and e
a,0
and e
a,h
refer to the values of e
a
at z=0 and z=h, respectively. To see how much turbulence enhances vertical transfer in the mulch, k
v
is compared to the still air mulch conductance, k
vm
, given by
k
vm
= D
vm
f
e
− θ f
t
h .
11 Fig. 9 plots measured conductance ratio, k
v
k
vm
, ver- sus s
57
. Also shown is the fitted linear regression given by
k
v
k
vm
= 1 + 1.86s
57
, r
2
= 0.29, n = 52,
12 which is a standard representation used in the lit-
erature. For example, Kimball and Lemon 1971 calculated the conductance ratio for heptane flux
transferred through a 2 cm thick wheat straw mulch to be 1+0.58s
1
, where s
1
is the wind speed at z=1 m. Tanner and Shen 1990 found that the coefficient
factoring the wind speed changes to 0.83 for a 1.1 cm thick flail-chopped corn residue while Heilman et al.
1992 found a value of about 17 for a 25 cm high herbicide-killed winter wheat mulch. The low r
2
coef- ficient in Eq. 12 indicates a weak dependence of the
conductance ratio on wind speed. The variability of the observed ratios is probably due to a combination of
measurement error and other factors, especially those related to within-mulch turbulence regimes. Mea-
sured ratios are in the range 2–6 for s
57
in the range 1–2.5 m s
− 1
. Note that computing k
v
= 1
R
h
D
− 1
v
dz, with D
v
given by Eq. 9, yields k
v
k
vm
= 4.7, which is
near the midpoint of the 2–6 range, as expected. Sim- ilar results were found for other mulch application
166 M.D. Novak et al. Agricultural and Forest Meteorology 102 2000 155–171
rates Chen et al., 1994. Converting our wind speed measurements at z=57 cm to z=100 cm using the
logarithmic profile determined in I yields 1.63 for the coefficient factoring the wind speed, which indicates
that this coefficient is sensitive to mulch attributes.
The conductance ratios are in good agreement with the value of 4.5 found by HN92b by fitting daytime
λ E
measured with micro-lysimeters on one day un- der a similar barley-straw mulch applied at 20 t ha
− 1
to predictions from a physically-based numerical model of the soil-mulch-air system. The height of
their 20 t ha
− 1
mulch was 6 cm, which is attributed to settling and decomposition because the mulch was
applied in the fall and the λE measurements were
made the following spring. Agreement in a 10 t ha
− 1
mulch was similarly good with this conductance ratio Hares, 1988. However, this good agreement per se
is actually coincidental. The numerical model over- estimated daytime T
s,0
under the 20 t ha
− 1
mulch by about 2
◦
C. When this effect is accounted for, a conductance ratio of about 8 is then required to
correctly predict daytime λE under the mulch. But,
Chen and Novak 1997 showed that micro-lysimeter measurements of λE
under such thick mulches are overestimated by 56 for a 10 t ha
− 1
mulch, and likely greater for a 20 t ha
− 1
mulch, because of water loss during the however brief weighing period and
disturbances to the mulch when removing and replac- ing the lysimeter. Correcting for this then reduces
the conductance ratio back down to 4–5, in good agreement with those shown Fig. 9.
3.4. Energy balance closure and components — non-wetted mulch
R
n
calculated above and within the mulch with the radiation model is compared to the sum
G +
S
H,i
+ λ
E+H at the corresponding z, with each
component measured as described earlier, for 20 Au- gust 1994 in Fig. 10. H at z=0 cm was assumed to be
the same as at z=1.1 cm and the z=9.6 cm refers to H
being measured at that z. Agreement is quite good at all z during the daytime. Perhaps this is not that
surprising at z=9.6 cm since the renewal model for H was calibrated to fit well above the mulch on a sim-
ilar day. The poor agreement for the evening data at z=
9.6 cm is probably due to a change in cloud cover,
Fig. 10. Diurnal variations of hourly average net radiation either calculated with the mulch radiation model lines or determined
as the sum of soil, storage, latent, and sensible heat flux densi- ties symbols at the 0, 1.1, 3.3, and 9.6 cm heights within the
non-wetted 10 t ha
− 1
straw mulch on 20 August 1994.
as the radiation model assumed an average cloudiness for the day. The good agreement within the mulch
during the daytime is a vindication of both the radia- tion model and the independent measurements of G
, S
H,i
, λE, and H. Because H is small even compared to G
+ S
H,i
and λE at and below z=3.3 cm Fig. 2 this is not a sensitive test of whether the coefficient α
is independent of z within the mulch, but assuming it to be so is consistent with the energy balance. Similar
measurements not shown made at midday on 23 August 1994, at ten z within and above the mulch
demonstrate that the renewal model as described in Eq. 1 did well in the upper half of the mulch; again
in the lower half the values are too small for a sensi- tive test. At night, the radiation model predicts small
negative values near the bottom of the mulch but the sum of the components is actually positive. This may
be another symptom of using a daily average cloudi- ness in the radiation model. Since as we have seen,
H
is likely positive and underestimated by the re- newal model at night within the mulch, an improved
determination of H would worsen the disagreement. The stronger re-wetting of the 0–1.1 cm mulch layer
at night Fig. 7 suggests that perhaps some of water loss measured by the tension-plate apparatus might
be in the form of liquid flow from plate to mulch pieces in direct contact with it. However, the total
M.D. Novak et al. Agricultural and Forest Meteorology 102 2000 155–171 167
Fig. 11. Diurnal variations of hourly average energy balance com- ponents net radiation, soil, latent, and sensible heat flux densities
for above 9.6 cm height and below 0 cm height the non-wetted 10 t ha
− 1
straw mulch on 20 August 1994. The net radiation has been calculated as the sum of the soil, latent, and sensible heat
components shown.
change in storage for the bottom layer only accounts for 20–30 of measured λE
, which is not enough to change the sign of the sum of the components.
Energy balance components above and below the mulch on 20 August 1994, are shown in Fig. 11. R
n
shown is calculated as G +
S
H,i
+ λ
E+H , i.e., perfect
closure. Above the mulch, H is the dominant compo- nent in the energy balance throughout the day except
for a brief period in the early morning when the mulch evaporates at nearly the potential rate because it is
wet with dew. Under the mulch, both G
and λE are the most important components, with H being small
throughout the day. Qualitatively, the energy balance under the mulch resembles that for a wet bare soil
Novak and Black, 1985, except that the magnitude of the fluxes is much smaller.
3.5. Thermal, moisture, and radiation regimes — artificially wetted mulch
The 10 t ha
− 1
straw mulch was thoroughly wetted by a combination of sprinkler irrigation mostly at
night and rain during t=17:00 hours PST on 27 Au- gust to t=9:00 hours PST on 29 August 1994. Intensive
measurements then were made from t=11:00 hours PST on 29 August about 2 h after irrigation ceased
to t=18:00 hours PST on 1 September 1994. Fig. 12
Fig. 12. Daytime variations of water content mass basis hourly of the 0–1.1, 1.1–2.2, 2.2–3.3, 4.4–5.5, and 5.5–6.6 cm mulch layers
within the wetted 10 t ha
− 1
straw mulch during 11:00 hours PST on 29 August to 18:00 hours PST on 1 September 1994. The w
i
of the 3.3–4.4 cm layer has been omitted for clarity.
shows the variations of w
i
for five of the mulch lay- ers during this period. The initial w
i
’s in the range 3.7–4.0 kg kg
− 1
for all layers below the top layer ex- ceed the ‘saturated’ value, 3.2 kg kg
− 1
, measured in the laboratory by immersing the mulch elements in
water until constant weight was achieved and then de- termining the water content mass basis gravimetri-
cally. This suggests that droplets of water still existed on the mulch elements at this time. The mulch dries
steadily during the 4 days and approaches the water content of the non-wetted mulch by 1 September ex-
cept for the z=0–1.1 cm layer which is still wetter than for the non-wetted mulch Fig. 7. The z=5.5–6.6 cm
layer dries the most rapidly and reaches values typi- cal of the non-wetted mulch as early as the afternoon
of 29 August. Nighttime re-wetting of the layers ap- pears to be slight but some of this might have been
missed because weighing did not begin until late in the morning.
Mulch thermal and moisture regimes for 29 August–1 September are shown in Figs. 13 and 14, re-
spectively. One striking feature is the rapidity within 1 day with which H at z=9.6 cm again became the
dominant component in the energy balance above the mulch. The large difference between H at z=6.6 and
9.6 cm developed after the irrigation and we suspect that the z=6.6 cm thermocouple was pushed down
168 M.D. Novak et al. Agricultural and Forest Meteorology 102 2000 155–171
Fig. 13. Diurnal variations of hourly average sensible heat flux density at the 1.1, 3.3, 6.6, and 9.6 cm heights and half-hour
average air temperature at the 1.1, 2.2, 3.3, 4.4, 5.5, and 6.6 cm heights for the wetted 10 t ha
− 1
straw mulch during 11:00 hours PST on 29 August to 18:00 hours PST on 1 September 1994.
into the mulch during this process, but unfortunately this was not checked at the end of the experiment. As
shown in CNBL97, the renewal model H at z=9.6 cm was in quite good agreement with H determined
Fig. 14. Diurnal variations of hourly average latent heat flux density at the 0, 1.1, 3.3, and 6.6 cm heights and half-hour average
water-vapour pressure at the 0, 1.5, 3.5, 6.6, and 9.6 cm heights for the wetted 10 t ha
− 1
straw mulch during 11:00 hours PST on 29 August to 18:00 hours PST on 1 September, 1994. The first
three hourly λE at z=6.6 cm are off-scale, the second and highest of them being 215 W m
− 2
at t=12:30 hours PST.
from the energy balance, i.e., H=R
n
− G
− S
H,6
− λ
E ,
where all components are measured independently on a half-hour basis as described earlier. The renewal
model correctly simulated more than two-thirds of the increase in H about 150 W m
− 2
that occurred as the mulch dried. As for the non-wetted mulch, H in
the bottom half of the wet mulch is very small. An- other feature of the wet mulch is that the z at which
T
a
reaches a maximum decreases as the mulch dries. On 29 August the maximum occurs at z=h but by 1
September, the maximum occurs near z=4.4 cm as observed for the non-wetted mulch. On all days, T
a
at the top of the mulch is much greater than at the bottom, the reverse being true at night.
During daytime, condensation occurs onto the soil surface under the mulch λE0 at z=0. On 29 August
some of this might be due to the dripping of excess water in the mulch onto the tension plate, as suggested
by the initially high values of w
i
exceeding mulch saturation. The condensation measured was confirmed
by comparison with micro-lysimeters installed under the mulch Chen and Novak, 1997. The λE varies
strongly with z in the mulch, as expected given the changes in the w
i
’s Fig. 12. Missing values were interpolated as based on our previous results. Fig. 12
shows that changes between the late afternoon and early morning are here clearly less when the mulch is
wet if compared with Fig. 7 for the dry mulch. The vertical gradient of e
a
between z=0 and z=1.5 cm is negative during daytime which corresponds closely to
the period of condensation at z=0. R
n
calculated with the radiation model is compared to the sum G
+ S
H,i
+ λ
E+H , with each component
measured as described earlier, at various z within the mulch for 29 August–1 September in Fig. 15. Agree-
ment is excellent at z=3.3 and 9.6 cm but is much poorer than for the non-wetted mulch at z=0 and
1.1 cm Fig. 10. R
n
is overestimated by the radiation model at these z. HN92b reported that the transmis-
sivities of fresh barley-straw mulches were about dou- ble those for mulches that had been in the field on
plots for nearly 1 year. Wagner-Riddle et al. 1996 found that a rye mulch doubled its residue area index
over a period of 2 months because of decomposition combined with disturbances such as wind and rain.
Since their experiment mimicked ours more closely, and because our mulch was in the field for about 40
days, we recalculated R
n
z with the radiation model
M.D. Novak et al. Agricultural and Forest Meteorology 102 2000 155–171 169
Fig. 15. Diurnal variations of hourly average net radiation either calculated with the original mulch radiation model lines or de-
termined as the sum of soil, storage, latent, and sensible heat flux densities symbols at the 0, 1.1, 3.3, and 9.6 cm heights within
the wetted 10 t ha
− 1
straw mulch during 11:00 hours PST on 29 August to 18:00 hours PST on 1 September, 1994.
but with the residue-area index increased by 75. The increase was distributed linearly with z, with the max-
imum increase at z=0 and no increase at z=h, because when small pieces break off they tend to fall down
onto lower layers, which probably was accelerated by the intensive irrigation. With this change, agreement
is greatly improved at z=0 and 1.1 cm, although some discrepencies remain Fig. 16. The poor agreement at
the beginning of the measurement period is most likely caused by the dripping of water onto the soil surface
from the mulch elements, which led to an overestimate of the condensation rate.
Above the artificially wetted mulch canopy, H is still the largest fraction of R
n
among the energy com- ponents, despite the high water content of the mulch
elements. H is 53 of R
n
on the first day and grad- ually increases to 76 on the fourth day. In contrast,
H accounted for only 27 of R
n
for a stubble win- ter wheat mulch after it was wetted Heilman et al.,
1992 with the remaining 62 and 11 of R
n
attributed to λE and G
. For our wetted mulch, λE accounts for only 40 on the first day, and decreases to 13 by
the fourth day. G varies within 7–10 of R
n
during the 4 days, which is lower than the G
for the win- ter wheat stubble. This suggests that a mulch of hor-
izontally distributed elements is more effective than
Fig. 16. Same as Fig. 15 except that in the radiation model the residue-area index is increased by 75, with the increase
distributed linearly with height in the mulch, the maximum being at the soil surface and there being no increase at the mulch top.
a stubble mulch in terms of reducing water loss by evaporation and lowering soil temperature.
4. Summary and conclusions
