Responses of grain sorghum to varying irrigation frequency in the Ord irrigation area. II. Evapotranspiration, water use efficiency and root distribution of different cultivars

Abstract

Four grain sorghum cultivars (Quicksilver, Texas 610SR, E57 and Q7844), representing early-to-late maturity types, were grown under different irrigation frequencies (7-day, 14-day, 28-day or 42-day intervals or once-only-at-sowing) in the dry season in the Ord Irrigation Area. Soil water distribution, root distribution and evapotranspiration were determined periodically. Frequent irrigation resulted in shallow root development and most water use was from the surface 40 cm of soil. Depth of water extraction increased with plant age and with less frequent irrigation. The depth of root penetration determined by direct measurement was similar to that inferred from soil water profiles. Water uptake per unit length of root was related to soil water content only in the 0-40 cm soil layers. Ea/Ep (actual evapotranspiration/pan evaporation) was classified into three stages according to the degree of soil drying after irrigation. During the first stage, immediately after irrigation, Ea/Ep was generally close to 0.75 irrespective of cultivar, leaf area index (LAI) or irrigation interval. During the second stage, linear functions adequately described Ea/Ep as a function of LA1 for LA1 less than 5. During the third stage, which commenced when total soil water to 1.8 m declined to 545 mm, poor relationships between Ea/Ep. Following an analysis of the soil water use data, it is believed that maximum yields of sorghum may be attainable with irrigation at sowing followed by three carefully timed irrigations.