干旱区生态环境知识资源中心

In some and land, the irrigated fields are not contiguous and are surrounded by large patches of bare land. During the summer time and rainless season, the solar radiation flux is high and the surface temperature during daylight in the dry bare areas, is much higher than that of the air. The sensible heat generated over these areas may be advected to the irrigated fields. The crops are usually planted in rows and the irrigation systems used (trickle) do not wet the whole surface, the dry bare soil between the rows may develop high soil surface temperatures and lead to convective activity inside the canopy above the bare soil. Advection from the surrounding fields and convective activity inside the canopy affect the layer above the crop. We studied the surface layer above an irrigated tomato field planted in Israel’s Negev desert. The crop was planted in rows, trickle irrigated and the distance between the outer edges of two adjacent rows was 0.36 m at the time of measurement. The gradients in temperature and water vapor pressure were obtained at various heights above the canopy using a Bowen ratio machine. The residual in the energy balance equation was used as a criterion to determine the equilibrium layer. During the morning, unstable conditions prevail, and the equilibrium layer was between Z/h similar to 1.9 and 2.4. In some particular circumstances, in the late morning, the bare soil between the rows reached extremely high temperatures and during conditions with low wind speeds free convection was identified. During these hours the "residuals" of the energy budget to the heights Z/h=1.5 and 2.4 were significantly different from zero and an extremely large variability was evident for the Z/h=3.2 layer. Local advection took place during the afternoon resulting in an increase in the stability of the uppermost measured layer and propagated slowly downwards. The equilibrium layer was between Z/h similar to 1.5 to 2.4. The residuals were significantly different from zero for the uppermost layers Z/h=2.7 and 3.2 during these periods. Our findings suggest that the depth and location of the internal equilibrium layer above trickle irrigated row crop fields surrounded by dry bare areas, vary in response to wind speed and the temperature of the soil in between the rows of the crop. For some time intervals, the computation of fluxes using the conventional flux-gradient approach measurements was not possible.