Turbulent transfer and radiation distribution within and above a straw mulch
Creators: name: Chen, Wenjun
Publisher: name: University of British Columbia
Year: 2009

description: 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.