Turbulent exchange processes within and above a straw mulch.: Part II: Thermal and moisture regimes

Abstract

This paper, the second of a two-part series, reports on measurements of thermal and moisture regimes, including sensible and latent heat flux densities, and the complete energy balance made above and within a 10 t ha −1 (6.6 cm high) barley-straw mulch in both normal and artificially wetted states. Soil, mulch, and air temperatures were measured with fine-wire thermocouples, the sensible heat flux was determined with an air renewal model from the cubic structure function of measured air temperature fluctuations, water-vapour pressures were measured within and above the mulch with capacitance sensors, the (energy-limited) latent heat flux below the mulch was measured with a custom-made tension-plate system and latent heat fluxes at heights above the soil surface were determined from additional measurements of weight loss or gain of the mulch elements, net radiation flux was measured above the mulch and downcoming radiation measured below the mulch with thermopile radiometers, soil heat flux was determined with a custom-made thermopile plate installed at the 1 cm depth and corrected for heat storage in the soil above the plate, and heat storage within the mulch was determined from measured mulch element temperatures. During daytime on fine summer days, the source of the most of the sensible heat flux is in the upper third of the mulch, which corresponds to where the upper-surface mulch element temperature exceeds air temperature. Sensible heat is transferred counter to the local vertical gradient near the middle of the mulch, which puts a lower limit of about 2 cm on the size of the eddies responsible for most of the transfer. Near the bottom of the mulch, the sensible heat flux is small and directed downward, in accordance with the strong air temperature inversion within the mulch. The largest source of latent heat during daytime is from the underlying soil surface except early in the morning when evaporation of dew near the top of the mulch dominates. Turbulence within the mulch enhances latent heat transfer above that due to molecular diffusion by 2–6 times, with mild dependence on wind speed. During nighttime, sensible heat flux within the mulch was small but latent heat flux was a large component in the energy balance, which was attributed in part to the unstable conditions that then existed within the mulch. The sum of sensible, latent, soil, and storage heat fluxes (all measured independently) at all heights within and above the mulch are in good agreement with net radiation flux determined using a radiation transfer model developed for the mulch under normal (non-wetted) conditions. The largest effect of artificially wetting the mulch was on latent heat flux below the mulch which became negative, indicating condensation, during daytime. For the data after the irrigation, net radiation flux from the radiation transfer model is no longer in good agreement with the sum of sensible, latent, soil, and storage heat fluxes near the bottom of the mulch. Agreement 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.