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
