Do windbreaks reduce the water consumption of a crop field?

Two ratios, Ac=[(q*c−qa)|u=αu0(q*c−qa)|u=u0]/[(rc+ravc)|u=αu0(rc+ravc)|u=u0] for the canopy layer and equivalent ratio Ag for the soil layer, were proposed for use to assess if soil evaporation (Eg) and canopy transpiration (Ec) decrease when wind speeds are reduced by windbreaks by a fraction of α, with qa being the specific humidity of the air, q*c the saturated specific humidity of the canopy layer, rc the canopy resistance, and ravc the aerodynamic resistance for moisture transfer. These ratios can be organized to form criteria, ΔEc < 0 (Ac < 1) and ΔEg < 0 (Ag < 1). Thus ΔE < 0 if Ac < 1 and Ag < 1. If only one of the ratios is smaller than unity, the sign of ΔE depends on that of ΔEg + ΔEc. The criteria were examined by a dual-source crop community model to simulate energy and water balances of a crop field with data obtained in the Nile Delta. It was found that both ΔE ≥ 0 and ΔE < 0 were possible and ΔE was mainly determined by ΔEg during the fallow and early stages of the cropping seasons and by ΔEc in the late cropping period. Overall, the scale of the roughness elements hc and soil moisture θ were found to be the major factors to determine ΔEc, ΔEg, and ΔE. A larger hc tends to produce ΔE ≥ 0; and ΔEc and ΔEg decrease as θ increases.