Shelter-effects on microclimate, growth and water use by irrigated sugar beets in the great plains

Abstract

Two rows of corn windbreaks spaced at 15 m were used to protect an irrigated sugar beet crop at Scottsbluff, Nebraska. Yield data for wind protected sugar beet crops from three consecutive years indicate that yield increases of as much as 25% are possible during years when yields are low. During years of high yield, however, the windbreaks did not increase sugar beet yield further. Except in the immediate vicinity of the windbreak where shading occurred, no differences were found in net radiation between the sheltered and open plots. The daytime air temperature averaged 1.8°C greater and the vapor pressure 4.0 mbar greater in the sheltered plot than in the open. Both air temperature and the water vapor pressure varied as a function of distance from the windbreaks. An area of maximum influence was often observed near the center of the protected plot. Differences between shelter and open were decreased during periods of strong wind. Differences in temperature and vapor pressure between shelter and open were greater during the day than at night. The wind speed at 25 cm above the crop was 40% of that in the open field whenever wind speed in the open was more than 120 cm/sec. The daytime concentration of CO 2 was an average of 1 p.p.m. lower in shelter than in the open. Nocturnal concentration was an average of 3.5 p.p.m. greater in shelter. The latter differences were inversely related to wind speed and were greatest when wind direction was perpendicular to the windbreak. Vertical mixing in the atmosphere is, apparently, vigorous enough to prevent significant depletion of CO 2 in a windbreak protected field. The 1 p.p.m. lower concentration observed over the sheltered crop should have only a negligible influence on photosynthetic rate. The increase in vapor pressure and the slight decrease in [CO 2] during the day suggest a decrease in the intensity of turbulent exchange in shelter. The concentration of water vapor exceeds that of CO 2 by several orders of magnitude. Decreased turbulent exchange in shelter has, therefore, a far greater influence on the change in concentration of water vapor than of CO 2.