Natural rainfall is the primary water source for rain-fed agriculture. However, natural rainfall is scarce in semi-arid and arid regions and rainwater evaporation occurs more intensely, which are problems that require a prompt solution to ensure maintenance of agricultural productivity [1-2]. Suppressing evaporation and increasing water availability is critical for natural vegetation restoration . In this regard, mulching can prohibit the loss of soil water by wind, decrease soil evaporation, and improve the soil hydrothermal status and ecological activity; moreover, it is an important technique for sustaining soil water storage [4-5]. Based on the mulching effect and cost, the materials generally involve plastic film synthesis, organic materials derived from agricultural and wood waste, and some specific materials, mainly gravel, sand, rock fragments, and zeolites [6-9].
Gravel mulching is an indigenous approach employed in agricultural production, and it has been used in the arid regions of many countries [10-11]. Gravel mulching has also been conducted for at least 300 years in the arid region of northwestern China . Several field trials have shown that gravel mulching can intercept and store rainfall [13-15], increase evaporation resistance and reduce evaporation from the soil surface [16-17], improve the soil water retention capacity , decrease the accumulation of surface salt , influence the microbial community composition and function [20-21], increase dry matter production [22-23], and improve energy-use efficiency and economic benefits . However, the mulching efficiency varies widely, depending on the characteristics of the mulch, including particle size , gravel texture , percent mulch coverage , and gravel mulch color . Yuan et al.  found that the evaporation inhibition of gravel mulch was inversely related to the grain size, while Ma and Li  found that greater mulch thicknesses had better effects on maintaining soil water. However, determining the optimal thickness of the gravel mulch requires further testing. In addition, it is well known that evaporation and infiltration are the dynamic interaction processes between the surrounding microclimate and water at the surface and within the soil . Water infiltration plays a key role in watershed hydrology. Cerdà  reported variations in infiltration at different rock fragment cover ratios. Infiltration and runoff on fallow land slopes with different gravel sizes and coverages were determined by Guo et al. . Further, Dang et al.  studied the influence of different thicknesses and positions of coal gangue on the soil water infiltration. Studies on water infiltration under different mulching conditions are few compared to studies that have investigated evaporation inhibition.
The northwest region is characterized by rich coal, low rainwater, and high evaporation . Coal gangue, as a type of solid waste discharged during coal mining and coal washing, occupies a large amount of land resources owing to accumulated gangue, which eventually results in ecological fragility, various environmental problems, and potential health risks . To date, coal-gangue reclamation has mainly recovered vegetation on covered soils. The greatest challenges for the ecological reconstruction of mine lands are derived from scarce fertile soil [34-35]. The reconstructed soil, which mainly comprises raw soil and is similar to the soil parent material used in the reclamation of coal gangue dump land, was characterized by a lack of organic matter, weak soil structure, and an elevated salt content [34, 36-37]. Mixing coal gangue with soil to reconstruct degraded soil can influence soil infiltration rates and saturated hydraulic conductivity [38-39]. Further, a physical crust is present at the soil surface because of raindrops splashing and the dispersion of aggregates, which hampers water movement and thus the hydrological cycle . Overall, lower soil quality aggravates low water retention.
Given this background, a method to utilize this material can effectively solve the problem of coal gangue accumulation [41-42]. Previous research has demonstrated that coal gangue is a material that can be used in fertilizer production. Furthermore, as the concentration levels of trace elements are below permissible limits, coal gangue can also be used as a substitute for soil replacement because of its low transportation cost [36, 43-44]. More critically, granular coal gangue has a morphology similar to that of the gravel currently used and contains mineral elements similar to those in soil. As a result, coal gangue has been considered as a mulching material for increasing water efficiency in arid regions. However, the influence of mulching on water infiltration and evaporation characteristics has not been significantly investigated. In this study, we designed an indoor simulated soil column experiment. Our objective was to test the influence of different thicknesses and particle sizes of coal gangue mulch on soil water infiltration and evaporation and to determine an optimized design for coal gangue mulch.