Evapotranspiration reduction by field geometry effects in peanuts and grain sorghum

Abstract Scope and methods of study: The objectives of this study were to identify the factors causing Spanish peanuts ( Arachis hypogaea , L.) planted in narrow north-south rows to consume less water than in other spacings and orientations, and to look for the effect of reduced water consumption with narrow north-south rows of grain sorghum ( Sorghum bicolor , (L.) Moench). Both crops were studied for two growing seasons. With peanuts, two row-spacings, 30 and 90 cm, combined with two orientations, north-south and east-west, were studied for the principal objective. Grain sorghum was studied in 45 and 142 cm row spacings for the second objective. Supplemental water was provided in both studies. All plots were irrigated at intervals to keep the soil moisture base of all plots as uniform as possible. Water content of the soil profile was determined by the neutron method, and the deep drainage component from the profile was monitored with tensiometers. Evapotranspiration (Et.) over periods of several days in length was calculated. Meteorological conditions were monitored to correlate with the Et. loss during a period. Energy budget measurements were obtained during parts of the second season to characterize solar radiation and treatment differences related to net radiation, advective energy and latent heat of evaporation by the Bowen ratio. Wind profile measurements were obtained during part of the second season to characterize aerodynamic roughness length parameters. Findings and conclusions. In two years of study, peanut planting treatments of 30 cm rows of north-south orientation and sorghum treatments of 45 cm rows had the lowest Et. in 5 of 7 periods of measurement. These treatments were consistently the lowest in Et. during periods of moderate to high evaporative demand (daily maximum temperature greater than 32°C, clear skies, and moderate to high wind velocity). The two periods where other treatments had the lowest Et. were of low evaporative demand conditions (daily maximum temperature less then 29°C, clear to partly cloudy skies, and low wind velocity). During periods of moderate to high evaporative demand, Et. from the narrow-row treatments was heavily influenced by advective energy exchange and some apparently physiological effects relating to stomatal action which caused a depression in Et. during the day. Net radiation was lower for the narrow-row treatments in relation to the other plots during periods of high evaporative demand. Treatment Et. rates were most clearly defined in periods of constant meteorological conditions. North-south rows showed signs of generating greater turbulent transfer of heat from the plots on windy, high evaporative demand days. East-west rows showed lesser effect of advection on these days. Thus, the following scenario is suggested. It appears that on low evaporative demand days, there is no narrow, north-south effect. Et. is proportional to net radiation. On high evaporative demand days there is narrow, north-south effect tending to conserve water: aerodynamic roughness length seems greater in the north-south rows (to a south wind) and net radiation is lesser. Et. is not proportional to net radiation possibly because of stomatal closure effects. Stomates close more readily in the north-south rows and the excess energy is evidently advected away through the effect of the increased turbulence.