Mulch resistance to water vapor transport

Mulches augment soil moisture availability to plants by restraining direct evaporation of soil water. Yet, in field conditions wind decreases their resistance to water vapor transport, diminishing their efficiency as a water conservation measure. The relation between vapor transport resistance and wind speed was investigated in a wind tunnel where air flow was turbulent. The mulch material was chopped straw with bulk densities of 31 and 37 kg m(-3), and chemically stabilized aggregates segregated in diameter classes 1-2, 2-4, 4-8, 8-11.2 mm, in layers 10-100 mm thick. The resistance decreased exponentially with increasing wind speed from the molecular diffusion value at zero wind speed, suggesting that turbulence penetrates the pores of the mulch and drives convective water vapor transport. Resistance rose exponentially with increasing layer thickness, a mirror reflection of the turbulence decay profile. Higher bulk density of the straw and finer aggregates augmented the resistance. The convective component of the vapor transport resistance was related to mulch area index, defined as the surface area of the solid elements of mulch per unit covered ground area. This procedure merged the effect of layer thickness and that of straw bulk density or aggregates size into a single function, indicating that friction forces proportional to internal area of the solid fabric restrain the penetration of momentum in the porous medium. Two-layered mulches combining straw and aggregates have a higher resistance than the sum of the resistances of the individual components as is expected from the attenuation of convection in the top layer. The functions derived in this study can serve as input for models evaluating the impact of mulches on soil water balance. (C) 2011 Elsevier B.V. All rights reserved.