| Summary: | Mulching can be both beneficial, promoting soil and water conservation, and
detrimental, slowing soil warming in spring and delaying seed germination. However, the
processes which govern the exchanges of energy and mass within the soil-mulch-atmosphere
system are not well understood. While researchers have successfully
measured and modeled the exchange processes within the soil and atmosphere, progress
within mulch canopies has been limited by technical difficulties in measuring turbulence
and radiative fluxes with current micrometeorological techniques, such as eddy
correlation. In searching for alternatives, we developed and tested an improved tensionplate
system for measuring first-stage evaporation or condensation rates under a mulch to
an accuracy of 5 W m⁻² (Chapter 2), a renewal model which calculates sensible heat flux
within and above canopies from the statistics of measured temperature fluctuations and the
friction velocity (Chapter 3 and 4), and a radiation distribution model that determines
short and long-wave radiation components at all levels (Chapter 5). Chapter 6 and 7
report turbulence statistics, turbulent fluxes of sensible and latent heat, and energy
balance within 2, 5, 10, and 15 t ha⁻¹ straw mulches.
The renewal model is calibrated and tested with data measured above and within a
Douglas-fir forest, and above a straw mulch and bare soil. We show that the renewal
model describes half-hour variations of sensible heat flux very well both within and above
the canopy for stable and unstable atmospheric conditions. The radiation model, which
uses a measured clumping index and temperature differences between upper and lower
surface of the mulch elements, agrees well with measurements for net radiation at the top
of and total downwelling radiation beneath 2, 5, 10, and 15 t ha⁻¹ mulches.
The air flow measured within mulch canopies was highly turbulent, with its
longitudinal turbulence intensity varying from 0.64 to 0.91. At an occurrence frequency of
about 1 Hz (determined from analyzing air temperature and wind speed time series), large scale
coherent eddies dominated the canopy flow. The dominance of these coherent eddy
structures was also evidenced by the large values of skewness and kurtosis of the velocity
components. The strong day-time thermal stratification within the mulch had no effect on
the canopy flow, but under nocturnal low-wind conditions the thermal instability may have
caused the observed nearly steady within-canopy convective air flow.
Evaporation from non-wetted mulch elements was relatively small except during
early morning hours when the elements were wet from dew fall and soil evaporation at
night. At night, soil water evaporation under the mulch remained considerable and nearly
constant. It decreased to near or below zero in the early morning, and increased to a peak
in the afternoon. Condensation was observed throughout the daytime under a mulch
wetted completely by sprinkle irrigation.
In the daytime, the net radiation flux attenuated quickly through the top of the
mulch canopy, and then decreased slowly with the height in the middle and bottom layers
of the mulch. The strong clumping of elements in the middle and lower layers resulted in a
relatively large fraction of net radiation arriving at the soil surface. A similar pattern was
observed for sensible heat flux using the renewal model, i.e., its main source strength was
near the mulch top. Counter-gradient sensible heat flux was measured during daytime at
the middle of the canopy, while air temperature reached a maximum at about 2/3 of the
mulch canopy height. Energy budget closure was reasonably good at all heights within
and above the mulch.
Turbulence enhanced the water vapour conductance within straw mulches by
typically 2 to 7 times the molecular value for wind speeds in the range 0.5-3 m s⁻¹ ,
computed from the measured soil evaporation rate and the vapour pressure profile when
the water content of the mulch canopy was steady. === Land and Food Systems, Faculty of === Graduate
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