Evaluating and Modelling Soil Detachment Capacity in Rills After Treatment With Biochar From Solid Waste of Olive Oil Mills

Although being a viable substrate to reduce soil erosion, the effects of biochar from solid waste of olive oil mills (OMSW biochar) on soil detachment capacity in rills (D_c) has never been explored. Furthermore, no equations have been proposed to predict important parameters (soil erodibility factor, K_r, shear stress, τ, and its critical value, τ_c, for rills) of this hydrological process in treated soils. This study was conducted in agro-forest sites of Northern Iran to evaluate D_c and key properties of soils–organic carbon (OC), aggregate stability (MWD), bulk density (BD) and carbon exchange capacity (CEC)—treated with OMSW biochar in comparison to untreated sites through flume experiments. Moreover, regression models were developed to predict D_c, K_r and τ_c for both treated and untreated soils. Compared with the untreated soil, the application of this biochar noticeably increased OC (+85%), MWD (+51%) and CEC (+101%), and reduced BD (−11%) and D_c (−31%). The correlation analysis revealed significant but not high associations between physical properties on one the hand, and soil detachment capacity in rills, on the other hand. Overall, the soil treatment with OMSW biochar impacted agro-forest soils to a severe extent. Treated soils were discriminated from untreated sites into two distinct groups by the principal component analysis and agglomerative hierarchical cluster analysis. The linear equations interpolating D_c and τ estimated K_r, and τ_c with accuracy in treated and untreated soils (r² > 0.74, p < 0.05). The best prediction capacity of D_c was given by power equations applied to the stream power (r² > 0.78, p < 0.05). The multiple regression equation developed to estimate D_c from the water flow rate and soil slope was also very accurate (r² > 0.95 and NSE, coefficient of Nash and Sutcliffe, > 0.89). These results help land managers and hydrologists to control and predict rill detachment in long and steep hillslopes against the risk of soil erosion.