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Gullies form easily on unpaved road surfaces during heavy rainstorms on China's Loess Plateau. The integrated effect of rainfall, topography, vegetation, land use, and other factors determines where and when gully erosion occurs; however, the mechanisms driving gully erosion on unpaved road surfaces need to be further understood. Repeated gully erosion on some roads during the storm season provides a good opportunity to better understand the mechanisms behind gully erosion. This article aimed to quantify the integrated threshold conditions required for gully initiation in terms of topography, event rainfall, and upslope land use, and to propose an event‐based model to predict the position and magnitude of road gully erosion. Rill and gully erosion on unpaved roads were investigated after an extreme rainstorm of 212.2 mm in 2017 and a regular rainstorm of 83.8 mm in 2018. A digital surface model (DSM) derived from unmanned aerial vehicle (UAV) images was used to analyze the road gradient (S) and upslope area (A). The runoff (QRS) of each road segment (RS) was estimated by the runoff curve number method. The results showed the following: (1) The mean and total gully volumes under the regular rainstorm were only 26.3% and 8.1% of those under the extreme rainstorm, respectively; (2) Gully formation on the surveyed roads under both the regular and the extreme rainstorm could be explained by the threshold relationship (S − 0.056)QRS3.363 ≥ 72.444; and (3) A non‐linear relationship between gully erosion in road segments and event runoff (QRS) and road gradient (S) was found, and was subsequently used to predict road gully erosion on an event basis.

A. fruticosa (Amorpha fruticosa L.) is widely used for revegetation in semiarid lands that undergo secondary salinization. Understanding A. fruticosa plants response to soil water and salt stress is essential for water irrigation management and proper revegetation practices. In this study, we measured sap flow, stomatal conductance, meteorological and soil characteristics in an A. fruticosa community that recently experienced secondary salinization in northwestern China. Results of our study showed that daytime and nocturnal sap flows averaged 804.37 g•cm −2 •day −1 and 46.06 g•cm −2 •day −1 , respectively, during the growing season. Within individual days, the highest sap flow appeared around noon local time and followed a similar pattern of photosynthetically active radiation (PAR). Despite the significant effect of meteorological factors on the characteristics of sap flow, our study highlighted that the sap flow of A. fruticosa is strongly regulated by the availability of soil relative extractable water (REW). The daytime sap flow, which is predominant compared to nocturnal sap flow, was strongly affected by PAR, air temperature and vapor-pressure deficit. With water stress in the top 40 cm of the soil (REW 0-40 cm < 0.4), daytime sap flow displayed a strong relationship with soil water content (SWC) (positive) and soil electrical conductivity (EC) (negative) in the relatively shallow soil profile (up to 40 cm). For the nocturnal sap flow, our results suggest that in the absence of soil water stress (REW 0-40 cm > 0.4), the nocturnal sap flow is mainly used to replenish the stem water content and sustain nocturnal transpiration. Under soil water stress, nocturnal sap flow is mainly used to replenish stem water content. The results of our study indicate that it is necessary to shorten the irrigation cycle during the primary growing period (May-July) of A. fruticosa. Moreover, in the absence of soil water stress (REW 0-40 cm > 0.4), A. fruticosa can survive well in an saline environment with soil EC < 5 mS•cm −1. Secondary salinization is the presence of high salts in soil, which has occurred widely in the semi-arid regions of northwestern China due to the poor irrigation management 1. To manage both problems of secondary salinization and land degradation, a series of ecological revegetation projects have been implemented in this region in the past few decades. The shrub Amorpha fruticosa L. (A. fruticosa) is an important species for the revegetation projects because of its drought and salt tolerance 2. Understanding plant responses to water and salt stress is essential for water irrigation management and proper revegetation practices 3. However, information about the water use characteristics of A. fruticosa remains limited. The responses of plants to water and salt stress can be evaluated by sap flow, as this is the mechanism of water movement in soil-plant-atmosphere continuum 4. For many trees and shrubs, daytime sap flow is the most important part of sap flow, but nocturnal sap flow may account for up to 4-...

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