Effect of Grass Components on the Temporal Response of Surface Soil Water to Precipitation and Air Temperature in the Loess Plateau

The Loess Plateau of China is facing increasingly serious challenges related to water shortage issues despite the successful vegetation restoration. Surface soil water (0–40 cm) is crucial to water transfer and discharge in the soil-plant-atmosphere continuum, which is controlled by precipitation (P), air temperature (T_a), and plant components. This study selected one typical grass species and set different treatments of natural condition (NC), removing litter (CR), and only roots (OR), to analyze the responses of soil water storage (W) and its change (∆W) to P and T_a at different time scales (including daily, weekly and monthly scales). The results showed that P had significantly positive effects on W and ∆W, while T_a negatively influenced W and ∆W. Meanwhile, T_a also controlled the significance of P affecting W. P contributed more (53–63%) to ∆W variation at the daily scale, but T_a showed more effect (73–80%) at the weekly scale, and both of them contributed more than 77% at the monthly scale. P and T_a existed a threshold phenomenon for increasing soil water, where P thresholds were higher than 0.93, 4.98, and 28.17 mm and Ta threshold were lower than 28.69, 28.89, and 29.96°C at the daily, weekly and monthly scales, respectively. Moreover, grass components also affected the responses of ∆W to P and T_a, and the OR treatment displayed the lowest sensitivity and had the lowest loss rate of soil water. The total infiltration, total loss amount, and rainwater retention rate exhibited: NC > CR > OR. Overall, grass canopy and litter improved soil water condition, and the roots most strongly protected soil water after infiltration. This study can provide a better understanding of surface soil water responding to climate change under drought stress with water shortage implications, and may strength the management of grassland in semi-arid region.