Fertility investigations on the black earth wheatlands of the Darling Downs, Queensland. II. Moisture and evapotranspiration in relation to the wheat crop

The effects on wheat yields of available soil moisture at planting, rainfall during the growing period, and evapo-transpiration were examined for the years 1951-1954. The relation between free water surface evaporation, available soil moisture, and evapo-transpiration was examined for the years 1951 and 1952. Wheat yields were found to be correlated with available soil moisture at planting for the four years combined, the two drier years combined (1951 and l953), and the two wetter years combined (1952 and 1954). The relationship for the four years combined was sufficiently strong to suggest that available moisture at planting could be successfully used for yield prediction. Yield was correlated with evapo-transpiration only for the dry year 1953. The ratio of yield to evapo-transpiration was found to be lower for the years of high rainfall during the growing period, the mean values for the wet and dry years being 2.1 and 3.5 bus./in. respectively. Growing-perlod rainfall was correlated with yield only for the month of August for the two dry years of 1951 and 1953 combined. Evapotranspiration (Etr) was found to be correlated with free water surface evaporation (Ew) and Ew0.75 in the monthly data, the value for C in the relationship Etr = CEw0.75 being calculated as 0.53. Available soil moisture was correlated with evapo-transpiration using data from the beginning and end of the season in 1953, but was not correlated using monthly data. The method of calculating monthly available soil moisture is shown to be important in this correlation. A method of calculating available soil moisture over a monthly period, based on a mean daily figure for the month, is described.