Agricultural and Forest Meteorology 102 2000 173–186
Simulating the radiation distribution within a barley-straw mulch
Michael D. Novak
a,∗
, Wenjun Chen
b
, Mohammad A. Hares
c
a
Faculty of Agricultural Sciences, University of British Columbia, 266B-2357 Main Mall, Vancouver, BC, Canada V6T 1Z4
b
Environmental Monitoring Section, Canada Centre for Remote Sensing, 588 Booth St., Ottawa, Ont., Canada K1A 0Y7
c
60 Provender Avenue, Ottawa, Ont., Canada K1K 4N4 Received 26 July 1999; received in revised form 23 November 1999; accepted 22 December 1999
Abstract
Mulching is a technique widely used to moderate soil microclimate. Modelling radiation transfer within mulches is a critical step in the development of comprehensive microclimate models of the soil–mulch–atmosphere system and similar hay-drying
systems. The objective of this research was to develop a relatively simple physically-based model that predicts profiles of shortwave and longwave radiation flux densities within barley-straw mulches and to test it against hourly radiation flux
measurements made above and below mulches applied at rates of 2, 5, 10, and 15 t ha
− 1
in successive field-plot experiments. Unique features of the model include accounting for upper-surface and lower-surface mulch element temperatures using
effective view factors and neglecting all shortwave reflections beyond secondary ones, so that calculation of the fluxes above any layer is explicit and does not require matrix inversion. Model input parameters were for the most part measured completely
independently of the field tests. Measured transmissivities demonstrated that mulch elements were uniformly distributed, as expected, for low mulch application rates but were clumped for higher rates, which was attributed to the greater effort then
needed to separate the elements of baled straw. Sensitivity tests showed that solar irradiance, atmospheric emissivity, and mulch element reflectivity are important input parameters to the model and that measuring the difference between upper-surface
and lower-surface mulch element temperatures is not as critical. Modelled and measured net radiation flux density above the mulch and total downward radiation flux density near the bottom of the mulch were generally in excellent agreement, with
some exceptions. These were attributed mainly to measurement error condensation on the upper dome of the net radiometer above the mulch before and just after sunrise and inadequate spatial averaging under the thin mulches. Modelled profiles
of daytime and nighttime radiation fluxes in a 10 t ha
− 1
mulch and simulations of the effects of uniformity, randomness, and clumping of mulch elements are reported. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Straw mulch; Radiation; Model; Clumping; Field plots
1. Introduction
Crop residues conserve soil and modify soil micro- climate when left on the surface as a mulch and are
valuable animal foods, fuels, and manufactured ma-
∗
Corresponding author. Tel.: +1-604-822-2875; fax: +1-604-822-8639.
E-mail address: novkinterchange.ubc.ca M.D. Novak
terials when harvested Unger, 1994. The effects of mulching on soil microclimate are largely controlled
by both radiation regimes and turbulent transfer within the mulch. Similarly, the quality of hay, which mainly
depends on how fast it dries after cutting, is largely controlled by the radiation distribution and turbulent
transfer within the hay. Therefore, we carried out a detailed series of micrometeorological field studies of
barley straw applied at various rates. Two other papers
0168-192300 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 0 0 0 0 0 9 6 - 4
174 M.D. Novak et al. Agricultural and Forest Meteorology 102 2000 173–186
Novak et al., 2000a, b describe the wind and turbu- lence regimes and the transfer of heat and moisture
within and above a 10 t ha
− 1
mulch. This paper deals with the radiation distribution within the mulch for all
the application rates 2, 5, 10, and 15 t ha
− 1
that we studied in our field plots.
To simulate the radiation distribution, some re- searchers have treated the mulch as a single bulked
layer Ross et al., 1985; Chung and Horton, 1987; Hares and Novak, 1992. This approach, however,
does not provide details of the distribution within the residue layers, which is particularly critical for, say,
hay drying. Others have divided the mulch or hay into many layers Bristow et al., 1986; Tuzet et al., 1993;
Bussière and Cellier, 1994 and used standard radi- ation modelling techniques Kreith, 1973; Norman,
1979 to simulate the vertical profiles of radiation flux densities. Residue elements were usually assumed to
be randomly distributed although within-mulch ra- diation flux densities were not measured to test the
validity of this assumption. In some cases there is an indirect indication that the radiation simulation is in
error. For example, the model of Bussière and Cel- lier 1994 predicted small daytime evaporation rates
under a sugar-cane mulch but their measured values were quite large 150–200 W m
− 2
. One possible rea- son for this error is the underestimation of downward
solar radiation flux at the underlying soil surface. To improve the simulation of radiation distribution
in a mulch, features specific to mulches must be con- sidered Tanner and Shen, 1990, hereafter referred to
as TS90. An important attribute is whether the mulch elements are uniformly or randomly distributed, or
clumped, as found also in plant canopies Chen and Black, 1991. TS90 found that the residue elements
were nearly uniformly distributed in a flail-chopped corn mulch but Wagner-Riddle et al. 1996 found
a near random distribution in a rye mulch. The uniformity, randomness, or clumping of mulch ele-
ments probably depends strongly on the method of spreading and therefore can be altered. Dead residue
elements have much smaller transmissivities than live leaves, e.g., flail-chopped corn residue elements
have transmissivities of 0.005 and 0.02 in the visible and near-infrared bands, respectively, compared to
0.07 and 0.36, respectively, for senesced corn leaves TS90. There can be a large difference between the
temperatures of the upper and lower surfaces of a sunlit mulch element, in part because of the lack
of transpiration which is an important component in the temperature regulation of leaves Norman, 1979.
This difference can be accentuated by the low wind speeds within the mulch and the low thermal conduc-
tivity of dry residue elements.
The objective of this research was therefore to develop a relatively simple but physically-based radi-
ation model of a barley-straw mulch that incorporates these features. We present such a model in this paper
and test it against measurements of radiation fluxes made both above and below the mulches applied at
the various rates in our field plots. We then assess the sensitivity of the model to its input parameters, use
the model to determine the profiles of shortwave and longwave components within the mulch under field
conditions, and simulate the effects of uniformity, randomness, and clumping of mulch elements on the
shortwave radiation distribution.
2. Theory
