Comparative assessment of Ethiopian highland soil erodibility using nomograph along with soil color and simulated rainfall methods

Abstract

Soil erodibility (K) measures the inherent vulnerability of soils to detachment and transport by rainfall-runoff, and is a crucial input for empirical and process-based soil erosion models. Choosing appropriate methods to determine K are important to the reliable soil-loss prediction and for targeting conservation measures in erosion-prone landscapes. The aim of the study was evaluate the K-values of major soil types using nomograph, soil color, and simulated rainfall approaches. Runoff and sediment yield measurements were obtained using simulated-rainfall experiments; process-based K-values were determined from these results using established correlations between erodibility, runoff, and sediment yield. For empirical nomograph based K- values a total of 50 composite soil sample from the topsoil (0–30 cm) were collected in major soil types of the study watershed. Laboratory analyses followed standard procedures to determine particle-size distribution (sand, silt, and clay), soil organic carbon (SOC), and other variables required by the nomograph. Descriptive statistics and Pearson correlation were used in the data analysis to evaluate the connections between K and soil characteristics. The results showed that nomograph-derived K-values ranged from 0.030 to 0.053 t ha h MJ −1 mm −1 ha −1 .The soil color method produced higher K estimates, between 0.200 and 0.250 values. Process-based (simulated rainfall) K-values fell between these ranges, from 0.046 to 0.140 t ha h MJ −1 mm −1 ha −1 . Chromic Vertisols and Lithic Leptosols were classified as slightly erodible and showed comparable K-values when estimated by both nomograph and simulated-rainfall methods. Correlation analysis indicated that K was strongly negatively correlated with clay content (r = − 0.93), soil structural class (r = − 0.96), and permeability class (r = − 0.85), moderately negatively correlated with SOC (r = − 0.45), and positively correlated with silt content (r = 0.57). The soil color–based method consistently overestimated K relative to nomograph and simulated-rainfall estimates and is not recommended as a sole estimator for the studied soils. Determination of K from runoff plots or simulated-rainfall experiments, together with detailed soil characterization for nomograph application, yields more reliable and affordable K estimates. We recommend using nomograph-derived K values calibrated with targeted simulated-rainfall or plot-scale runoff measurements to improve erosion-model accuracy and to inform soil conservation planning in the upper Blue Nile Basin.