Improving the utility of erosion pins: absolute value of pin height change as an indicator of relative erosion

Erosion pins can be an inexpensive and intuitive method to estimate hillslope soil erosion and deposition. It is common practice to calculate annual erosion/deposition rates (also called ground advance/retreat or ground lowering) from pin measurements as the mean net change in pin height over a given area. However, many studies have found this net ‘real number’ change does not produce strong relationships with erosion rates estimated using other methods, or with variables expected to be highly correlated with erosion, calling into question the efficacy of this approach. Here we evaluate an alternative (or complementary) approach - using the absolute value of pin height change to capture the overall magnitude of soil movement as an indicator of erosion. We used measurements from erosion pins in experimental plots across different maize-bean production systems and forest-fallows in northern El Salvador to compare both the absolute and ‘real number’ change in erosion pin height against modeled erosion, related factors (e.g., slope and soil cover), and soil loss collected in erosion pits. We found that the absolute value of pin height change was strongly correlated (r = 0.67, p < 0.01) with erosion rates predicted from the Revised Universal Soil Loss Equations (RUSLE) and moderately correlated (r = 0.82, p < 0.10) with erosion measured in collection pits, while no relationships were found for the real number value. The absolute value was also strongly correlated with RUSLE factors related to slope and cover, while no correlations existed for the real number value. Statistically significant differences in RUSLE-predicted erosion were found between plots classified as having ‘high’, ‘medium’ and ‘low’ vegetative cover, and these differences were also detected using absolute value of pin height change. Conversely, such differences were not detected using the net real number value. We conclude that, when using erosion pins for comparative analysis between land management practices or monitoring changes in erosion over time, the absolute value of pin height change is likely a better indicator than net real number change. We encourage additional research using new and existing datasets to further evaluate the utility of absolute value of pin height change as an indicator of relative erosion.