Vegetation and terrain drivers of infiltration depth along a semiarid hillslope

Estimation of spatial distribution of soil moisture on steep hillslopes by state-space approach (SSA)

Soil moisture is a critical variable that quantifies precipitation, floods, droughts, irrigation, and other factors with regard to decision-making and risk evaluation. An accurate prediction of soil moisture dynamics is important for soil and environmental management. However, the complex topographic condition and land use in hilly and mountainous areas make it a challenge to monitor and predict soil moisture dynamics in these areas. In this study, the determinants of soil moisture variability were determined by structural equation modeling, and then an attempt was made to estimate the spatial distribution of soil moisture content on steep hillslope using the state-space method. Herein, soil moisture at different depths (0-10, 10-20, and 20-30 cm) was monitored by portable time-domain reflectometer (TDR) along this hillslope (100 m × 180 m). It showed that the spatial variability of soil moisture decreased with increasing soil wetness, primarily in the topsoil (0-10 cm). Soil moisture was correlated with elevation (r = 0.28, 0.50, and 0.28), capillary porosity (r = 0.06, 0.37, and 0.28), soil texture (r for Clay: 0.20, 0.24, and 0.16; r for Sand: -0.25, -0.18, and -0.28), organic carbon (r = -0.31, -0.08, and 0.10) and land use (r = -0.01, 0.28, and 0.24) under different conditions (dry, moderate, and wet). Among these determinants, elevation made direct contributions to soil moisture variation, especially under moderate conditions, while land use made its impacts by altering soil texture. It is encouraging that the state-space approach yielded precise and cost-effective predictions of soil moisture dynamics along this steep hillslope since it gives the minimum root-mean-square error (RMSE) and Akaike information criterion (AIC). Moreover, soil organic carbon (AIC = -4.497, RMSE = 0.104, R2 = 0.899), rock fragment contents (AIC = -4.366, RMSE = 0.111, R2 = 0.878), and elevation (AIC = -3.693, RMSE = 0.156, R2 = 0.629) effectively anticipated the spatial distribution of soil moisture under dry, moderate, and wet conditions, respectively. This study confirms the efficacy of the state-space approach as a valuable tool for soil moisture prediction in areas characterized by complex and spatially heterogeneous conditions.

Contrasting effects of native and exotic vegetation on soil infiltrability in the Sonoran Desert

Invasion by exotic grasses is transforming drylands across the planet, but the ecohydrological feedbacks of such invasions are not fully understood. For example, in the Sonoran Desert, previous studies have shown that buffelgrass (Cenchrus ciliaris) alters the spatial patterns of soil moisture, leading researchers to hypothesize that such alterations are related to the plants' effects on soil infiltrability. To evaluate this hypothesis, we compared field-saturated hydraulic conductivity (K_fs) in a native shrubland with that in a neighboring savanna extensively dominated by exotic buffelgrass. We measured K_fs during the dormant and growing seasons in both canopy and intercanopy zones. We found that K_fs was generally lower during the dormant season than during the growing season. There were no significant differences between sites during the dormant season, and at both sites, K_fs was 6-7 times higher under shrubs than in the intercanopies. During the growing season, K_fs for the exotic intercanopy was comparable to that for shrub cluster edges (140 mm h^-1) and was more than twice that for the native intercanopy. Both shrubs and buffelgrass improved K_fs by reducing soil bulk density (thus increasing porosity). Additionally, surface roughness in the exotic intercanopy was nearly 3 times higher than in the native intercanopy. The combination of greater surface roughness and higher infiltration rates during the growing season most likely alters hydrological connectivity in savannas invaded by exotic grasses such as buffelgrass. By capturing portions of the runoff generated in the intercanopy, these grasses reduce runon into shrub patches, with potentially substantial impacts on native vegetation dynamics and stability.

Experimental Design of Nature-Based-Solution Considering the Interactions between Submerged Vegetation and Pile Group on the Structure of the River Flow on Sand Beds

Designing correct engineering infrastructures to reduce land degradation processes and considering natural elements to achieve this goal are key to correctly managing potential natural hazards affecting human activities and natural ecosystems. This research investigated the scour depth and velocity vectors around bridge piles with and without upstream vegetation protection. A Doppler velocity meter was used to measure velocity components in a channel 90 cm wide, 16 m long, and 60 cm high. Variable parameters were the number of bridge piles, the height, density, and width of vegetation upstream, as well as the distance between bridge piles. Using a triple pile group with a distance between piles of 10 cm and overall vegetation across the channel, the depth of the scour hole upstream of the first pile decreased by 40% compared to the single pile with no vegetation. This result shows the significant impact of using vegetation and pile groups to reduce scour around piles. Lower vertical velocity gradients, more consistent velocity vectors, reducing the downstream flow range, and restraining horseshoe vortexes and wake vortices were observed in utilizing vegetation. We confirmed that vegetation is an essential factor in changing the flow, transportation of sediment, and conserving ecological services in rivers.

Effects of hillslope position on soil water infiltration and preferential flow in tropical forest in southwest China

The hillslope is an essential natural spatial gradient that influences hydrological processes by affecting water distribution, surface flow, soil erosion, and groundwater recharge. To date, few studies have addressed only the hydrological processes of tropical forest hillslopes. To reveal the effect of hillslope on soil hydrological functioning-including water distribution and exchange, infiltration capacity, and flow behaviour-we conducted 36 field infiltration and nine dye-tracer investigations of different hillslope locations in the natural rainforest of Xishuangbanna, southwest China. The soil physical properties-including soil noncapillary and total porosity, saturated water capacity, and field water capacity-decreased with decreasing elevation from hilltop to middle slope and the valley bottom. The water infiltration capacity-including the initial infiltration rate, saturated soil hydraulic conductivity, and average infiltration rate-decreased from the hilltop to the valley bottom. Preferential flow dominated soil water movement more in the upper locations than in the valley bottom. The infiltration capacity parameters and preferential flow were significantly correlated with soil water content, noncapillary and total porosity, root biomass, and termite holes. These results indicated that along with the soil physical properties, root systems, animal activity, cracks, and stones affected the soil infiltration capacity and preferential flow. Differences in the hydraulic processes of each hillslope position contributed to the redistribution, transportation, and storage of surface and belowground water, resulting in differing availabilities of soil water resources and utilisation by plants. The findings of this study can help understand eco-hydrological processes in the context of water resources management in tropical mountain ecosystems.

Stable isotopes of deep soil water retain long-term evaporation loss on China's Loess Plateau

Evaporation from the land surface enriches heavy isotope ratios (²H/¹H and ¹⁸O/¹⁶ O) in shallow soils, and downward water movement will carry the fractionation signal to deep soils. However, how to acquire the evaporation from water stable isotopes in deep soils remains untested. Here, we measured water stable isotope composition in the deep soils (2-10 m) across 20 sites on China's Loess Plateau. Our results show that the line-conditioned excess (lc-excess) in deep soils of these sites was invariable with depth at each site, but ranged between -14.0‰ and - 4.1‰ among these sites, indicating differing degree of enrichment in heavy water isotopes between sites. Moreover, the mean lc-excess in deep soils water was significantly correlated to mean annual precipitation (R² = 0.57), potential evapotranspiration (R² = 0.25), and the Budyko dryness (R² = 0.68), indicating that deep soil water lc-excess reflects land surface climate conditions. Furthermore, the deep soils correspond to a timescale of approximately 100 years at one site and more than 27 years at the remaining sites. These results together indicate that stable isotopes of deep soil water retained long-term land surface evaporation effects. Further, by implementing the steady-state isotope mass balance model into the lc-excess framework, we derived a new method to estimate evaporation loss fraction (f). Our f estimates at these sites varied between 5% and 15%, which may represent the lower bound of the actual evaporation to precipitation ratio. Nevertheless, our work suggests that in these and the other similar regions, deep soil is a novel archive for long-term soil evaporation loss, and f may be estimated through a snapshot field campaign of stable isotope measurements.

Effects of the “Grain for Green” Program on Soil Water Dynamics in the Semi-Arid Grassland of Inner Mongolia, China

The Grain for Green Program (GGP) initiated by Chinese government significantly impacts mitigating environmental degradation. Soil water resources probably constrain large-scale vegetation restoration projects in arid and semi-arid regions. Characterizing soil water dynamics after the GGP’s implementation is essential in assessing whether vegetation restoration can be sustained as part of ecological restoration. In this study, four sites were selected for field investigation: original natural grassland (NG) and grassland that was reconverted from cropland 12 years (12-year site), 8 years (8-year site), and 6 years (6-year site) before. Soil water at five depths was measured continuously at 10 min intervals at four sites. The findings showed that less rainfall infiltrated a deeper soil layer as the time after restoration augmented, and the 12-year site had the shallowest infiltration depth and soil water storage. Younger restored grassland (8-year and 6-year sites) had a higher soil water content than older restored grassland (12-year site) and NG. The soil water content decreased steadily with restoration age after an immediate initial rise, and the highest soil moisture was in the 8-year site. The results suggest that soil water dynamics varied with GGP and a soil water deficit could be formed after the GGP’s implementation for 12 years in semi-arid grassland.

Effect of vegetation patchiness on the subsurface water distribution in abandoned farmland of the Loess Plateau, China

Patchiness of grassland results in important effects on ecohydrological processes in arid and semiarid areas; however, the influences on subsurface water flow and soil water distribution remain poorly understood, particularly during vegetation succession on slopes. This study examined these effects by comparing the water flow behaviors and preferential infiltration between vegetation patches (VP) and interspace patches (IP) in three sites at different states of vegetation succession (grass, subshrub and shrub) in abandoned farmland of the Loess Plateau, China. Dye tracer infiltration showed that patchiness of vegetation increased spatial variations of soil water and preferential infiltration by increasing the densities of fine root length and fine root volume in the soil profile. Moreover, the more abundant and intricate roots following a lateral direction beneath VP likely contributed to lateral flow and infiltration variability. However, differences between VP and IP were not significant because considerable living fine roots and decayed roots of IP also provided preferential flow pathways. Our finding indicated that IP could compete with VP for access to soil water resources, which potentially increased hillslope-scale infiltration and reduced surface runoff and erosion risk. Under the different states of vegetation succession, subshrub patches showed significantly greater preferential infiltration volume (28.53 mm) and contribution of preferential infiltration to total infiltration (60.58%) than grass and shrub patches. Vegetation patch size made positive effects on improving preferential flow and water movement. Greater preferential flow in subshrub patches played a positive role in soil water storage and replenishment. Therefore, natural restoration of a slope area with small heterogeneity in preferential flow can be successfully applied in the Loess Plateau, particularly during the subshrub succession state.

Assessing land surface drying and wetting trends with a normalized soil water index on the Loess Plateau in 2001-2016

Long-term drought may cause severe damage to ecosystems. To assess drought intensity, we introduced a normalized soil water index (NSWI) of land surface, on the basis of rainfall and actual evapotranspiration. Areas undergoing land surface drying on the Loess Plateau in 2001-2016 were assessed on the basis of the values of NSWI combined with rainfall and land surface water storage (LSWS). The extent of soil drying and wetting at depths of 0-0.01 m, 0-0.1 m and 0-2 m was also quantified. Results showed that up to 7.16% of the Loess Plateau was subjected to decreasing LSWS. On an inter-annual time scale, land surface drying intensified starting in 2003, and this pattern was chiefly evident at the soil depth of 2 m. The approach proposed in this study could also be used to identify temporary dry soil layers (DSLs) in arid ecosystems.

Effects and mechanisms of erosion control techniques on stairstep cut-slopes

Many erosion control techniques, such as stone pitching, concrete revetment, and geotextile covering, have been effective at protecting cut slopes along roads or railways. However, these methods are expensive and hard to operate for high stairstep cut-slopes. To investigate the efficiency of several easily implemented and low-cost techniques, five plots with different treatments were built on stairstep cut-slopes. The five treatments consisted of vegetation coverage on platforms, counter-slope on platforms, upslope drainage, a control check (CK), and a comprehensive treatment of the first three techniques. During nineteen recorded rainfall events from June 2015 to August 2016, the runoff and sediment amount of each plot was measured. Soil water content and shear strength of the 2-m depth profile at each plot after the occurrence of rainfall and evapotranspiration were also investigated. The results indicated that runoff and the sediment amount from the five plots increased with an increase in rainfall amount with a threshold of 5 mm rainfall to produce sediment loss. Compared with the CK, the comprehensive and upslope drainage treatments had a larger reduction in runoff and sediment amount than those of the vegetation and counter-slope treatments. After either rainfall or evapotranspiration, the vegetation and comprehensive treatments had the highest soil water content, and the upslope drainage treatment had the lowest soil water content. The infiltration capacity followed the order of upslope drainage < CK < counter-slope < vegetation < comprehensive treatment. The shear strength of soil logarithmically decreased with soil water content in the five plots, and a critical water content of 12% determined the rate of change for shear strength. Finally, the upslope drainage technique was determined as the preferred recommendation to protect high stairstep cut-slopes.