Effect of Shrub Components on Soil Water and Its Response to Precipitation at Different Time Scales in the Loess Plateau

Abstract

Water shortages have become the major limiting factor for ecological protection and sustainable development in the Loess Plateau. Few studies have focused on the effects of different plant components on soil water and its response to precipitation at different time scales. This study conducted an observation of shrub plants with three treatments (natural condition (NC), canopy + roots after removing the litter (CR), and only roots (OR)) to monitor the dynamics of soil water during the rainy season of an extreme drought year in 2015. The results showed that the soil moisture content (SMC) and soil water storage (W) had a trend of OR > CR > NC. The response of the SMC to precipitation was gradually decreased and delayed for longer with increasing soil depth. Daily precipitation >10 mm was the threshold to trigger an SMC response below 20 cm of depth. The thresholds of precipitation to increase W were 2.09–2.54 mm at the daily scale and 29.40–32.56 mm at the monthly scale. The effect of precipitation on W and its change (∆W) also depended on the time scales. At the daily scale, precipitation only explained 1.6%, 0.9%, and 2.4% of the W variation in NC, CR, and OR, respectively. However, precipitation was more important for ∆W, making a contribution of 57.6%, 46.2%, and 56.6%, respectively, and the positive ∆W induced by precipitation happened more easily and frequently at deeper depths in OR. At the monthly scale, the contribution of precipitation to ∆W increased to 75.0%, 85.0%, and 86%, respectively. The ∆W of the whole rainy season was OR > NC > CR. Precipitation of the monthly scale displayed higher contributions to soil water than that of the daily scale. Plant components had different influences on soil water and its response to precipitation, which was strengthened by the roots, weakened by the canopy, and neutralized by the litter. Regular cutting of the canopy at the single-shrub scale may help increase water storage, which is useful for vegetation management and hydrologic regulation.

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

“…For some days with low daily P and high daily T a , the water storage instead decreased (∆W < 0) on rainy days. The threshold effect of P and T a on soil water has been proven by previous studies (Dai and Wang, 2021;Ge et al, 2022;Jin et al, 2018;Liu et al, 2023). 5 to 14 mm of rainfall events was required to trigger soil water increase at 10 cm soil depth in grassland and forestland (Ge et al, 2022;Jin et al, 2018).…”

“…The fitting lines of ∆W with P and T a were similar and close to coincide between the NC and CR treatments across the time scales, indicating that the grass litter had few effects on triggering the increase or decrease of soil water by P and T a . The slope values of fitting lines in the OR treatment were always lower than other treatments, leading to the lowest relative increase of soil water to increase P or decrease T a , which illustrated that the response of soil water to climate change was strengthened by the grass canopy and weakened by the roots, which had different performances in shrubland (Liu et al, 2023). The last stage of the growing season was the most effective time to increase soil water reserves, which reduced water consumption due to the weak transpiration and water evaporation activity, especially with large rainfall events.…”

“…Different plant components have been proven to significantly affect soil water (Liu et al, 2023). In this study, we found that soil water storage had a trend of OR > CR > NC.…”

“…For example, a low amount of daily P possibly happened in the days with the high W, and a high amount of daily P happened in the days with the low W. Besides, the high proportion of no-rain days (Figure 2) and the delayed effect of P on W (Figure 4) also disturbed the P-W relationship at the daily scale. Moreover, P always could replenish soil water through rainfall infiltration processes, which would bring rapid responses and differences of ∆W at daily scale (Liu et al, 2023).…”

“…In most cases, the maximum infiltration depth of soil during a rainfall event was 40 cm (Chen et al, 2020;Du et al, 2019;Jin et al, 2018;Wang et al, 2013), emphasizing the importance of the surface W to respond to P. Generally, when P dropped, the W would increase due to the replenishment effects. However, soil water may not increase or even the ∆W could be negative, when the P amounts were lower than 2 to 7 mm (Dai & Wang, 2021;Ge et al, 2022;Liu et al, 2023), due to the interception from vegetation and consumption of ET. Moreover, the delayed effect of rainfall infiltration on soil water was difficult to Effect of Grass Components on the Temporal Response of Surface Soil Water to Precipitation and Air Temperature in the Loess Plateau 2 Air, Soil and Water Research determine at shorter time scales (Dai & Wang, 2021), which could be considered by the cumulative effects of infiltration processes at the longer time scale.…”