Simulating the radiation distribution within a barley-straw mulch

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.