Effects of residual water saturation
Residual water saturation Swr or water contentƟr is a hydraulic parameter defining SWCC. It is dependent on various factors such as soil organic carbon and clay content as well as other soil physical properties (e.g., Qin, 2003; Poeplau et al., 2015). Soil with different Swr is expected to have different pattern of SWC. Besides C0, additional two simulations with differing designation of Swrwere run to investigate this effect. The differences in the profiles of VWC due to differing Swr are clearly seen in Fig. 12. The curve for C0 is in the between of the other two lines at the four times, because of its Swr has a value between the other both. Higher Swr allows more water to be retained in the pores and cannot be driven out by soil gases. This explains why higher Swr leads to higher VWC especially for the times free of irrigation (Fig. 12c, d). Enhancement of one-fold magnitude of Swr causes 6.12% and 20.4% higher VWC for the loamy sand, but 7.57% and 5.71% lower VWC for the underlying sandy loam, at the 160 hr and 720 hr respectively. Given a certain irrigation water amount, fast downward migration of the wet front and higher SWC in deep horizons are implications of low irrigation water use efficiency. High residual water saturation in the root zone benefits crop growth during rainless periods. Speaking in this sense, it is encouraged to take practical measures to alter the soil structure and physical properties (e.g. the organic content) to ensure a higher Swr in the root zone of the crops.
Figure 12. Profiles of VWC at various times for differing residual water saturation settings for the loamy sand (C0: Swr=0.135; C30: Swr=0.067; C31: Swr=0.270).