**1. Introduction**

Gully erosion is a global problem, particularly in the southeastern United States, where erodible soils, high relief, and climatic and meteorological factors encourage soil erosion. Gully erosion is one of the most dangerous forms of soil degradation, which is caused by natural and anthropogenic activities. Gullies are composed of several continuous or discontinuous channels and rills with varying slopes, which may later develop into deep trenches, inhibiting effective remediation by tillage. Gully erosion can initiate from anthropogenic factors like farming or grazing on susceptible soils, increased runoff from land-use changes due to logging or construction, and poor vegetative cover from wildfire or high soil salinity. Additionally, natural drivers for soil erosion are meteorological variables, topography, and soil type and texture [1,2].

Changes in land use can increase soil erosion. Vast regions of the United States experienced soil erosion when forested lands were converted to croplands in the late 19th century and the early 20th century [3]. Estimates of the volume of soil erosion in the United States caused by both sheet and rill erosion combined is 6.7 Mg/ha/y in cultivated cropland, 0.90 Mg/ha/y on federal lands, and 1.55 Mg/ha/y in pasture lands [3]. Considerable land area in the southeastern US was converted from forest to agriculture to support cotton farming in the 1800s and pasture for animal grazing [4,5]. Land cover change due to logging and conversion of forest to crop and pasture was linked to nineteenth-century European settlement in the southern Blue Ridge Mountains and Appalachian hillslopes [6–9]. Harvesting on the steep Appalachian hillslopes has been identified as one potential cause of soil erosion [10]. After recognizing the problem as early as 1933, soil conservation programs were implemented in the United States. As part of present soil conservation efforts, afforestation on the reclaimed land has partially halted erosion, but severe erosional areas from the past cotton farming era are still prominent [4]. Some researchers have described a multi-stage formation of severe soil erosion [11,12], i.e., a process for gully development in the Appalachian Piedmont: (i) rills and gullies are initiated along existing paths, tracks, ditches, or animal burrows, where runoff is concentrated due to reduced infiltration; (ii) head scarp erosion begins as runoff gains energy and is concentrated in steeply sloped land; (iii) gully downcutting eventually stabilizes when weathered bedrock and the shallow groundwater zone are encountered; (iv) erosion continues laterally along channel sidewalls and headwalls by slumping and under caving, inhibiting effective control by tillage. Hence, reclamation can be expensive.

Sidewalls (or midslopes), gully channels (or valleys), and interfluves (or gully divides) are major topographical factors that influence soil erosion [7,8,13]. Soil erosion increases with slope steepness, which is more relevant to gully sidewall erosion and less relevant to interfluves. Gully channels are dynamic and can serve as intermittent sediment sinks and sources, transporting sediment to the gully outlet [14].

In addition to land cover change and topographic variation, water-induced soil erosion from severe precipitation events erodes fertile soil, mainly in areas with poor agricultural management, land degradation from mining, road construction, or wild fires [2]. Unique climatic conditions in the humid subtropical climate (Köppen Cfa) of the southeastern United States are a major contributing factor in gully erosion [15]. Cold periods in the south are short and winters are mild, inhibiting deeper ground freezing. The thin surface layer (5–10 cm) of frost-heaved soil becomes loose after a few freeze-thaw cycles, and can erode easily from subsequent heavy rain or snow-melt runoff [13]. During warm periods, intensive rainfall that falls on steep, sparsely vegetated slopes contributes to erosion. General precipitation trends in the Appalachian hillslopes indicate that high-intensity events occur more during summer months, while higher accumulation low-intensity storms are more prevalent in winter months. Seasonal variability in precipitation characteristics impacts erosion, but the extent and nature of this relationship are not well understood in this region.

A short-term study of hillslope erosion in the Appalachians found that duration and accumulation of precipitation were more important than storm intensity as drivers for gully erosion [13]. The same study also found antecedent precipitation is a stronger predictor of erosion and discrete precipitation events alone may not result in measurable erosion. Antecedent precipitation along with successive precipitation events can saturate the soil, reduce shear strength, and cause erosion. To examine inter-annual variability and longer-term effects from antecedent precipitation, as well as the influence of seasonal events on soil erosion, a more extensive time series of precipitation and corresponding erosion data is necessary [16], however, it will be important to retain a high temporal resolution in the data to assess seasonal scale patterns.

In this context, the Appalachian hillslopes in the southern US are representative of a region of historic and modern land degradation from unique meteorological conditions, variable topography, and land use/land cover change. Therefore, the objective of the present study is to examine the effect of meteorological parameters, specifically precipitation, on soil erosion through long term high-resolution monitoring. This paper summarizes six years of comprehensive weekly monitoring of precipitation events and soil erosion in an Appalachian hillslope paying particular attention to seasonal effect. An understanding of the seasonal pattern of soil erosion with respect to precipitation-related drivers of erosion will improve the potential to achieve conservation measures.
