# Informations et ressources scientifiques sur le développement des zones arides et semi-arides

Accueil du site Doctorat Canada Impact d’un brise-vent vivant sur le microclimat, l’eau et la production de fourrages irrigués en Tunisie

## Impact d’un brise-vent vivant sur le microclimat, l’eau et la production de fourrages irrigués en Tunisie

Titre : Impact d’un brise-vent vivant sur le microclimat, l’eau et la production de fourrages irrigués en Tunisie

The effects of a Casuarina glauca windbreak (1) on environmental variables as a function of distance from the windbreak, (2) on the response of an irrigated and protected lucerne (Medicago sativa) or, in winter, bersim (Trifolium alexandrinum) culture, and (3) on the water use efficiency (WUE) of the culture is studied in Tunisia under semi-arid conditions (Fahs) from August 1991 to July 1992. Wind speed (u) is reduced at 3H (a horizontal distance equivalent to three times the height of the windbreak) from 52% under weak winds and 58-60% under moderate to strong winds when the incidence angle ($\alpha$) is 90$\sp\circ$. Wind speed is reduced up to 62% when $\alpha$ equal 45$\sp\circ$. Under wet soil conditions, the deviation of maximum air temperature (Tm) at 3H relative to the zone outside the influence of the windbreak ranges $\pm 2\sp\circ$C, the negative values being observed when vegetation is very active. The zone where thermal effects occur coincides with a 40% reduction of u obtained at 7H or 11H for $\alpha$ equal to 45$\sp\circ$ or 90$\sp\circ$. The vapour pressure (ea) is increased even more in protected when the air is warm and dry ; a maximum deviation of 7.5 mb is observed at 3H. Minimum temperatures are reduced mainly during warm nights where an extreme deviation compared to control of $-3\sp\circ$C is observed at 3H. Potential evaporation (Ep) is reduced to 26% when u is decreased by 55%. The response of the crop to these conditions is varied over the season depending on environmental conditions during the crop period. Under humid soil conditions, it is largely influenced by temperature. A thermal use efficiency index (EUT, g/m$\sp2$/degree-days) is developed to explain this temperature effect and is considered in relation to mean thermal conditions of the culture (Ta) and their position relative to optimal (TOC), maximum (TMC), minimum (TNC) and lethal (TLC) temperatures of the crop. Under favourable thermal conditions at 3H, lucerne production is 333 g/m$\sp2$ (versus 225 when unprotected) in April and 582 g/m$\sp2$ (versus 379 when unprotected) in June ; EUT is 27 and 39 g/m$\sp2$/dd. But a gain of 7.3 dd also favorizes parasite activity and no significant difference in yield is observed. In summer, when Ta $>$ TMC, there is no longer a positive effect of the windbreak on the crop. In winter, more intense cold nights extending through the early morning and a decrease of temperature at 3H on the NW side of the hedge gives a bersim production of 137 g/m$\sp2$ (versus 191 when unprotected) and provokes a 4.5 g/m$\sp2$/dd reduction of EUT. Under favourable thermal and hydric conditions at 3H, real evapotranspiration (ETR) is not significantly modified because leaf production increase (+28%). WUE however is increased by 7.1 g/mm. Under the same thermal conditions but with strong water constraints, there is a 2.9 g/mm decrease in WUE along with +41% increase in ETR. Extreme temperatures inhibit the effect of the windbreak on crop production, and has a variable effect on ETR ($-$25% to +37%) and WUE according to the degree of thermal stress water availability.