Badlands are commonly defined as ‘intensely dissected natural landscapes where vegetation is sparse or absent and useless for agriculture (Bryan and Yair, 1982). Due to their high erosion rates, badlands are also viewed as degraded areas worldwide (Brandolini et al., 2017; Molina et al., 2009; Peng et al., 2015). Soft bedrock and a lack of vegetation both promote accelerated erosion in these landscapes that produce steep, highly dissected topography and the formation of gully networks and badlands (Fairbridge, 1968; Bocco, 1991). As a result, Badland areas have some of the highest erosion rates globally (Clarke and Rendell, 2010). Consequently, they are considered as ‘ideal field laboratories’ for studying landscape evolution, acting as miniature fluvial systems in which it is possible to directly observe hill slope-scale processes, interconnections, and resulting landforms (Bryan and Yair, 1982; Parsons and Abrahams, 1994; Campbell, 1997; Alexander et al., 2008; Dickie and Parsons, 2012; Yair et al., 2013).

Several factors and processes affect the development and dynamics of badlands, most notably lithology (Kasanin-Grubin and Bryan, 2007; Morenode las Heras and Gallart, 2016), climate (Bryan and Yair, 1982), and slope aspect (Nadal-Romero et al., 2007). Marked differences in badland slope morphology (Churchill, 1981) and species richness (Nadal-Romero et al., 2014) have been recognized between aspects, and slope aspect can also be one of the most critical factors determining the intensity of physical weathering processes (Regüés et al., 2000). For a specific badland site, lithology and slope aspects are fixed over long temporal scales relative to climate. Thus, an important aspect of forecasting erosion rates in these landscapes is determining the influence of individual climatic factors.

The rate at which badland landforms develop is the subject of continued speculation (Clarke and Rendell, 2006). For example, erosion rates between 32 and 77 Mg ha yr⁻¹ in non-vegetated badlands have been recorded in the Bardenas Reales, southeast Spain (Desir and Marin, 2009), compared with rates of between16 and 63 Mg ha yr⁻¹ in vegetated badlands in the Penedès region of Spain (Martínezcasasnovas et al., 2010). In the Guadalajara badlands of Spain, erosion rates of up to 114 Mg ha yr⁻¹ have been reported (Martín-Moreno et al., 2014).

Rainfall is an important driver of high erosion rates in badlands. For example, statistically significant relationships between sediment yield and the number of rainfall events have been reported (Cantón et al., 2001), which implies that wetting-drying cycles might have a more significant influence on weathering compared to the individual effects of heating and cooling cycles caused by temperature fluctuations (Cantón et al., 2001). Exceptions to this include badlands in Saudi Arabia, Kuwait, Qatar, and Yemen, where desert-like conditions prevail with substantial differences between day and night temperatures (Erguler and Shakoor, 2009). Erosion rates are assumed to be closely related to average annual precipitation (Bull and Kirkby, 2001), with the relationship between cumulative rainfall and cumulative erosion being particularly strong (Clarke and Rendell, 2006; Schumm, 1964). In addition, rain splash and creep caused by rainfall events can contribute to the detachment and transport of particles from slope face (Cantón et al., 2001). Thus, we hypothesized that in humid subtropical areas, erosion rates would be more strongly correlated with precipitation than temperature.

Although the role of temperature in the rapid weathering of badland materials has been studied (Yan et al., 2019), the impact of temperature on the erosion processes remains relatively understudied. Many existing studies on erosion rates in badlands have been undertaken on a yearly or decadal scale (Clarke and Rendell, 2006; Clarke and Rendell, 2010) and, as a result, the temperature is often averaged across an entire year. About methods, most previous studies have deployed erosion pins, which are considered an inaccurate means of monitoring short-term erosion rates and, therefore, have limited value in assessing the impacts of environmental variables. Thus, fine-scale erosion rate measurements are required to study the possible influences of temperature and predict future erosion rates to inform erosion-control measures (Cantón et al., 2001). Therefore, this study aimed to determine the relative contributions of temperature and rainfall to the erosion of badland slopes.

The Nanxiong Basin (24°33′–25°24′ N, 113°52′–114°45′ E), an ancient, Cretaceous faulted rift basin in the fold belt of the Nanling Mountains, is located in north Guangdong Province, southeast China. The basin has an elevation range of 48 to 1,421 m above sea level (Dogan and Aslan, 2017) and covers an area of 2,214 km² (Figure 1). The basin has a subtropical monsoon climate with long hot summers and short winters. According to the Nanxiong meteorological station (1956–2010), the mean annual temperature, precipitation, and potential evaporation rate are 19.6°C, 1,555.1 mm, and 1,678.7 mm, respectively (Yan et al., 2017). In addition, continuous successions of red fluvial-lacustrine clastics with a maximum thickness of more than 7 km are preserved in the basin (Ma et al., 2018).

Visually spectacular badland landscapes are well developed over a homogeneous and monotonous series of calcareous silty mudstones and siltstones in the Nanxiong Basin (Figure 2). These are incised into continental purple mudstones from the Nongshan Formation (En) in Dahangkeng Village, the Shanghu Formation (Esh) in Huangtian Village, and the Zhutian formation (Kzt) in Jiangtian Village. Most studies conducted in this area have focused on the nature of the purple soils rather than erosion of the badland landscapes themselves (Jiang et al., 2015).

In some areas, frost action, snowmelt, and freeze-thaw processes are known to have important impacts on the weathering and erosion of badland surfaces (Regüés et al., 2000; Schumm, 1964; Regüés et al., 1995). In the Nanxiong Basin, with its subtropical climate, freezing occurs only a few times a year and it is not, therefore, considered to be a significant geomorphic factor. Excluding freezing processes, badland erosion occurs in response to individual rain events via rain-splash, concentrated and unconcentrated surface flow, creep, mass movement, and piping (Bryan and Yair, 1982). Rain-splash and creep in particular shape the characteristic convexity of the upper slopes of badland landscapes (Parsons and Abrahams, 1994; Schumm, 1956; Carson and Kirkby, 1972). Surface runoff in the form of thin films is likely to be important for transporting material that has been detached and mobilized by splash and raindrop impact (Parsons and Abrahams, 1992; Abrahams et al., 1994). The effectiveness of creep and mudflow becomes apparent at high slope angles, where the thin weathered surface, once wetted, can begin to sag downslope under the influence of gravity (Clarke and Rendell, 2006). The impact of rain and mudflow was also notable at our study site, with mudflows appearing to transport most of the fragments from the slope face (Figure 6). A similar phenomenon is also recorded in Basilicata, Italy, where the effect of creep appears to be limited to a 10–40 mm thick weathered layer, where the sagging effect is visible on steep (>53°) Biancane slopes (Clarke and Rendell, 2006). Thus, we speculate that the mudflows explain how TTP is related to removal fragments in Badlands in Nanxiong Basin. Further research is now required to determine the precipitation threshold for mudflow initiation.

Gravity has been identified as the primary morphogenic process responsible for shaping slopes in badlands (Ciccacci et al., 2008). In Nanxiong, small slides of dry fragments under a small perturbation are another notable form of erosion. During the periods when no erosive rain events occurred, such small-scale slides resulted in the highest erosion rate of 195.64 Mg ha⁻¹ yr⁻¹, which was almost 65 times higher than the lowest recorded erosion rate of 2.92 Mg ha⁻¹ yr⁻¹. Parsons and Abrahams (1994) note that the maximum slope angles of badland surfaces are related to the angle of repose of dry weathered detritus. In the Nanxiong badlands, slope angle equals the angle of frictional resistance, thus promoting the transport of weathered fragments downslope. The initiation of small dry slides involves disturbing animals, such as birds and insects, and wind.

Badlands developed in sub-humid mountainous areas are subject to higher rates of denudation and more active dynamics compared to similar landscapes in arid or semiarid areas (Bull and Kirkby, 2001; Regüés and Gallart, 2004).

Previous studies demonstrated that wetting and drying cycles are more effective at causing the disintegration of clay-bearing rocks (Erguler and Shakoor, 2009; Yan, et al., 2019). However, in the field, precipitation amounts recorded between sampling periods did not significantly correlate with rock bulk density or surface mechanical resistance, indicating that regolith moisture does not depend solely on precipitation at this site but also related to temperature (Nadal-Romero et al., 2007). Thus, we speculate that the mean temperature contributes to increase the number of wetting and drying cycles. This might explain why, at our study site, mean temperature was found to contribute 14.6% of erosion rates, which was slightly higher than the contribution of total precipitation (12.6%).

It is apparent that mean temperature (including the mean lowest and highest temperatures) had a stronger relationship with erosion rates over monthly timescale than temperature difference. However, over a ten-day timescale, while the correlation was significant, the M-K tau coefficient was low. Remarkably, the mean temperature was found to have casual links with erosion rates rather than temperature differences. High temperatures were also found to be dominant in increasing erosion rates. Thus, it appears that the coefficient of the ten-day temperature difference and erosion rates resulted from a time-lag effect of temperature. Work is now needed using long-term datasets to quantify the nature of this time interval.

In this climate and lithology, mean temperature rather than temperature difference has more of an influence on the erosion rates of badland slopes. As for other dry badlands, weathering dynamics depend mainly on rainfall characteristics and water deficit (Sole´-Benet et al., 1997). For example, Gallart et al. (2002) reported that in the Vallcebre catchments of the Pyrenean ranges, the main controlling factor on badland development appears to be low temperatures rather than a lack of moisture. Indeed, at our subtropical study site, the temperature was found to have at least the same influence as precipitation on the erosion of badland slopes.

We applied the Granger causality method to examine the relationships between climate variables and erosion rates over short temporal scales. In this sense, the dataset we need for this test is not necessarily from two or more years, although we are aware that more extended datasets have preferable. Nevertheless, the impulse response has shown that the result of the impulse response is reliable. Thus we believe that studies with only one are fine.

Analysis based on M-K τ correlation tests suggests that temperature variables positively and significantly corelated with erosion rates in the Nanxiong badlands. Granger causality relationships were also derived for erosion rates and TMT and total TTP. The results of impulse response and variance decomposition analyses show that the relative contributions of TMT and TTP to erosion were 14.6 and 12.61% during the study period, respectively. This suggests that in this subtropical environment, high mean temperatures have at least the same influence on badland development as precipitation. This implies that mean temperature might be an important parameter for erosion forecasting.

During the year-long study, 22.5% of the total erosion amount occurred during periods when no erosive rain events occurred. During these periods, small slides of dry weathered fragments were important for the removal of material in this transport-limited humid badland landscape, while mudflow processes were important for transporting material during periods of continuous rain.