The empirical relationships derived in the literature linking rainfall event occurrence and land use to the surface runoff features are often too simplified. There is the need of more comprehensive tools able to account the inherent complex nature of erosion, which involves a series of processes (such as hydraulic erosion, sediment erosion, tunnelling, folding, pressure cracking, and mass movement processes, including slip) and their relations with the hillslope and soil dynamics. The integrated effects of these processes eventually lead to some degree of erosion and its nonlinear growth over the long term (Van Val Agham et al., 2005). The process-oriented approach has not been considered for all types of erosion. Accordingly, there is a pressing need to use physical models to deal with different erosional, hydraulic, and hydrological processes, such as tunnelling and landslides for a better control (Campo-Biscose et al., 2013; Tori and Poznan, 2014).
The present study allowed for the evaluation of gully erosion and expansion, a major erosion process, using physically related (i.e., erosion index) and conceptual (i.e., topographic threshold) approaches. In particular, the erosion index explicitly relates the erosion with runoff for a rainfall event using a distributed routing procedure and with other factors controlling the flow velocity (i.e., slope) and land use and soil physical conditions (i.e., derived from CN). In addition, also the gully head topographic threshold (Torri & Poesen, 2014; Torri et al., 2018; Rossi et al., 2015) concept holds a physical basis (Rossi et al., 2022). Thus, the selected model, even if simplified, allowed for a physical evaluation of the erosion occurrence, unlike other black box, empirical, conceptual, or regression models etc. This potentially enables the simulation of future events and their evaluation in terms of processes and physical principles.
Using LANDPLANER and exploiting the minimum, maximum and the reference curve number seasonal scenarios multiple simulations to estimate the erosion index and topographic threshold value in the studied watershed were run. As expected, the results (Fig. 7 to 10) shows that the gully occurrence is controlled by rainfall and land use and in turn the landscape shaping and development. Agricultural land was confirmed to be the most affected land use by soil erosion (Torri et al., 2018).
The simulated scenarios 1 to 3 used three daily rainfall intensities corresponding to 40, 60, and 80 mm/day. In the scenario 1, where the larger CN (a less permeable condition in dark red) are close to the morphological divide and the lower CN value (higher permeability) is close to the basin outlet (in green), the larger peak runoff values for both rainfall intensities are observed close to the basin divides. In contrast, in scenario 3, the results are the opposite. The different simulated rainfall intensities produce significantly different peak runoff values with their distribution in the basin strictly controlled by the land use/soil distribution. The hydrological connection inside the basin is possible only for higher rainfall intensities, but the land use/soil distribution firmly controls its geographical distribution. The larger the area, the less probable it is to reach the total connection of the basin. The more permeable conditions (lower CN in green) effectively reduce the amount of runoff reaching the basin outlet. The most critical condition for the gully formation is observed for scenario 3. All these results show that the gully triggering conditions, and potential gully formation locations, are strictly controlled by the combination of the land use/soil distribution and rainfall intensity.
Estimating gully occurrence in relation with topographical variables (e.g.., slope) unravels the portion of the study area more affected by gully erosion. The results obtained in this study are in line with previous research (Valentin et al., 2005; Lucà et al., 2011; Frankl et al., 2012; Bernatek-Jakiel et al., 2016). In comparison with the recent study carried out by Kariminejad et al., 2020, gully erosion density is nearly in the same slope aspect (south-east facing slopes) and the same elevation (around 200–300 m), but in gentler slope (within 15–30°), and for lower average accumulation value (< 100 m) (Fig. 6). They also explained that on average, the higher the CN, the higher the runoff, the higher the erosion index, and the larger should be the places predicted with gully erosion based on the topographic threshold approach. Here, the model simply predicts a larger (lower) gully occurrence depending on the higher (lower) CN values, possibly highlighting the evaluation of temporal gully occurrence variation. Hosseinalizadeh et al (2020) stated that gully erosion is common in different pedo-climatic regions of Iran with slope range of 2–27%. Higher values of topographic threshold and erosion index are also observed for the cold season in winter, where rainfall is highest and vegetation cover is lowest in the rangelands. This can also be extended to different types of erosion and even in determining the potential of the area for landslide (Hong and Adler, 2007; Liao et al., 2010; Liao et al., 2012).
Being the anthropic influence one of the main drivers of gully erosion occurrence, conventional agriculture should be replaced by conservation agriculture to reduce agricultural practices impact and machinery. In this case, soil organic matter and soil biodiversity should be increased. In modeling terms, this will correspond to a reduction of CN mainly due to the presence of permanent land cover but also to the induced soil properties change, with the primary effect of reducing runoff and hence surface erosion. This highlights also how gully erosion and improve soil health can be managed by applying a wise and shrewd land management. Besides, the possible increase of intense rainfall events due to climate changes, should potentially affect the erosion occurrence in the coming decades.
The LANDPLANER model, allowed a comprehensive comparison of the watershed response to environmental changes. This model has been successfully used elsewhere such as in Italy (Rossi et al., 2015; Agostini et al., 2022) to investigate erosion occurrence as a result of different landscape, climatic and vegetation changes. LANDPLANER can estimate the erosion process with some limitations in terms of model assumptions (i.e., using a rainfall event-based approach; using simplified erosion indices). Further work needs to be done to establish where and whether the model assumptions are appropriate and to verify the model applicability in different environmental conditions. Being open source, the software allows to modify or add aspects of the hillslope dynamics and integrate them in the modelling framework. Remarkably, the integration of the dynamic modification of the slope morphology and its effects on the possible related hillslope processes would enhance the model's potential in characterizing their dynamical mutual relations.
Gully detection and modelling are methodologically challenging (Grellier et al., 2012). Scientists need to cope with the problem exploiting adequate data and appropriate applicable methods (i.e., geophysical, geologic, pedological). for assessing hazards posed by gully erosion and for predicting their occurrence. This will be an essential knowledge both for land management and for the implementation of reliable early warning systems aimed at predicting erosion phenomena.
The results obtained in this study, suggest that specific conservation practices should be carried out in order to prevent the concentration of runoff from upstream areas leading to piping and gullying. Biological operations including minimum mechanical soil disturbance, permanent soil organic cover, and varied crop sequences (CN scenario) may be adopted to control gully occurrence or development.