Spatial point pattern analysis of piping erosion in loess-derived soils in Golestan Province, Iran

Point pattern analysis of collapsed pipes as a subsurface erosion in natural and manmade conditions plays an important role to understand their landforms/features, predisposing factors, and for prevention and forecasting purposes. Thus, the present study aimed to evaluate the spatial pattern and the associated land factors of piping erosion in loess-derived soils in a semiarid climate in Golestan Province, Iran by applying numerical summary statistics. To this end, a 105 ha area with homogeneous environmental conditions (gentle slope and loess-derived soil) was selected and the maps related to 101 collapsed pipes and their features were obtained by the aerial photos provided by an unmanned aerial vehicle (UAV) with a 50 cm resolution and field survey. The mean of spatial distances between pipe locations was 309 m, and the frequency of pipes per hectare was 0.97. Moreover, soil samples of the pipe locations were collected and physical-chemical soil properties were measured in laboratory. Approximately 95% simulation envelopes were selected using the 5th-lowest and 5th-highest values of 199 Monte Carlo simulations of the null model of homogeneous complete spatial randomness. Based on the results of univariate pair correlation function, the significant aggregation of 101 collapsed pipes was observed at a scale of 0–50 m in both rangeland and agricultural land use types. The bivariate pair correlation function, which is considered to be the most informative second-order summary characteristic, was used for analyzing the statistical correlations between collapsed pipes and linear phenomena including distance from drainage networks, ridges, and roads. Based on bivariate summary statistics, collapsed pipes had positively been affected by both the distribution of drainage networks and ridges. However, the negative statistical correlations occurred between pipes and roads at the scales of 1–50 m. Also, the correlations of soil characteristics (silt content, exchangeable sodium percentage (ESP), the weight of soil in a given volume (bulk density), soil electrical conductivity (EC), and organic matter (O.M.)) of neighboring collapsed pipes were evaluated by mark correlation function. Based on mark correlation function kmm(r), a significantly positive correlation was found between the pipes density and silt content, ESP, and bulk density, when they are more than overall average. In addition, less values of EC and O.M. were positively related to the aggregation of collapsed pipes. Similar to the results of summary statistics, the maps confirmed all statistical correlations. Consequently, the outcome of this study highlights the spatial pattern of collapsed pipes and their associations in the study area.