“…The calibration curves can be separated into three groups: (1) below the ASTM calibration curve (Leong, He, and Rahardjo 2002;Elgabu 2013) ; (2) at the ASTM D5298-16 calibration curve (Fawcett and Collis-George 1967;Greacen, Walker, and Cook 1987;Deka et al 1995;Marinho and Oliveira 2006;Kim, Prezzi, and Salgado 2017); and (3) above the ASTM D5298-16 calibration curve (Hamblin 1981;Chandler, Crilly, and Montgomery-Smith 1992;Power, Vanapalli, and Garga 2008). A closer examination shows that the calibration curve of Power, Vanapalli, and Garga (2008) agrees with the calibration curves of Leong, He, and Rahardjo (2002) and Elgabu (2013) within the suction range from 10 to 100 kPa, and the calibration curve of Hamblin (1981) agrees with the calibration curves of ASTM D5298-16; Fawcett and Collis-George (1967); Greacen, Walker, and Cook (1987); Deka et al (1995); Marinho and Oliveira (2006); and Kim, Prezzi, and Salgado (2017) within the suction range from 1 to 10 kPa. The calibration curve of Hamblin (1981) is based on their data and the averaged data from Fawcett and Collis-George (1967).…”