Turbulent exchange processes within and above a straw mulch.: Part I: Mean wind speed and turbulent statistics

Mulching is a technique widely used to conserve soil and moderate its microclimate. Modelling transfer processes in mulches is limited by our lack of understanding of turbulent exchange within the mulch. This paper, the first of a two-part series, reports on measurements of wind and turbulence made above and within a 10 t ha−1 barley-straw mulch using custom-made hot-wires and a tri-axial hot-film probe. Wind regimes within the mulch during daytime (relatively high wind) and nighttime (low wind) differ greatly. During daytime, 10 min average horizontal wind speeds at all levels in the mulch (where they vary nearly exponentially with height) correlate well with (near-logarithmic profile) wind speeds above the mulch and are not affected by the strong temperature inversion existing in the mulch. During nighttime, 10 min average horizontal wind speeds within the mulch are decoupled from (poorly correlated with) wind speeds in the generally stable air above the mulch. Unstable conditions in the mulch at night lead to free convection, which explains the good correlation of 10 min average wind speeds at all heights within the mulch and the high evaporation rates we measured below the mulch. Under high wind conditions most of the drag occurs very near the top of the mulch which behaves as an aerodynamically smooth surface similar to a bare soil. Turbulence within the mulch is of high intensity and is dominated by intermittant gusts, with the extreme values described by a Gumbel distribution. The frequency of the gusts agrees reasonably well with that found for laboratory mixing layers. The wind and turbulence regimes in the mulch resemble in many ways those in plant canopies much larger in height and lower in leaf area density.