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 micro climate. 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-wir es and a tri-axial hot-film probe. Wind regimes within the mulch during day time (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 (nea r-logarithmic profile) wind speeds above the mulch and are not affected by the strong temperature inversion existing in the mulch. During nighttime, 1 0 min average horizontal wind speeds within the mulch are decoupled from (p oorly correlated with) wind speeds in the generally stable air above the mu lch. Unstable conditions in the mulch at night lead to free convection, whi ch explains the good correlation of 10 min average wind speeds at all heigh ts within the mulch and the high evaporation rates we measured below the mu lch. Under high wind conditions most of the drag occurs very near the top o f the mulch which behaves as an aerodynamically smooth surface similar to a bare soil. Turbulence within the mulch is of high intensity and is dominat ed by intermittant gusts, with the extreme values described by a Gumbel dis tribution. The frequency of the gusts agrees reasonably well with that foun d for laboratory mixing layers. The wind and turbulence regimes in the mulc h resemble in many ways those in plant canopies much larger in height and l ower in leaf area density. (C) 2000 Elsevier Science B.V. All rights reserv ed.