Effects of a shading screen on microclimate and crop water requirements

Despite the steadily increasing area under protected agriculture there is a current lack of knowledge about the effects of the 30% black shading screen on microclimate and crop water requirements. Meteorological and lysimeter measurements inside a screenhouse planted with sweet pepper were compared to external reference data. Irrigation water use efficiency (IWUE) was calculated from yield records Y and water meter readings I-applied. Shading reduced mean global radiation RG by more than 40%, and the screen transmissivity tau(screen) was shown to vary with solar elevation angle beta. Wind speed inside the screenhouse U-in was reduced by more than 50%. Crop water requirements ETc were 38% lower than estimates for an open field crop, suggesting a significant water saving potential when using screenhouses. However, the screen did not significantly modify maximum temperature T-max and daily vapor pressure deficit. The FAO-Penman-Monteith approach based on meteorological measurements in the screenhouse accurately predicted daily crop evapotranspiration, and was in close agreement with lysimeter measurements. IWUE was relatively high (10.7 kg m(-3) in 2004 and 13.5 kg m(-3) in 2005), but additional research is required to quantify the effect of shading on yield as well as to determine the water saving potential of other commonly used screens.