To assess the ability of endogeic earthworms to tolerate variations in soil water potential, groups of these worms were subjected to different, constant levels of soil water suction (PSI) over a period of 17 days. At water suctions varying from 0.3 kPa (pF 0.5) to 1990 kPa (pF 4.3), the earthworms showed no physiological ability to a maintain constant internal water content as assessed by the fresh weight. The relationship between weight loss and the increase in water, suction was modelled and the following critical thresholds were identified: PSI < 60 kPa did not affect earthworm weight; PSI > 620 kPa led to diapause; between these two values, earthworm weight was closely governed by the variation in PSI. At the intermediate suction of 167 kPa, the exchange of water between the earthworms and the soil was at a maximum, and this value is therefore proposed as the level at which the diapause is induced. These modelled values were compared with those obtained under field conditions.
