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Isoerodent Mapping of the Conterminous United States
Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org
Citation: Soil Erosion Research Under a Changing Climate, January 8-13, 2023, Aguadilla, Puerto Rico, USA .(doi:10.13031/soil.23051)Authors: Ryan P McGehee, Dennis C Flanagan, Bernard A Engel, Puneet Srivastava, Chi-hua Huang, Mark A Nearing
Keywords: Erosion, Erosivity, Isoerodent, RUSLE, RUSLE2, USLE.
Abstract
Rainfall erosivity, or the capacity of rainfall to cause erosion, is a topic which has garnered substantial confusion over the years. This confusion stems from a number of discrepancies between isoerodent maps of the United States (and the world) and erosivity (or erosion index) benchmark values in the literature. Resolving these discrepancies is important since several of the world‘s soil erosion models, applications, and assessments are based on Universal Soil Loss Equation (USLE) technologies such as USLE, RUSLE, or RUSLE2. A brief history of these discrepancies is provided below.
McGregor et al. (1995) observed a 30% difference (greater) in erosivities obtained from 29 breakpoint gauges in the Goodwin Creek Watershed, Mississippi as compared to isoerodent maps for the same location as used in the RUSLE model. Similar findings had been reported prior to the development of RUSLE (McGregor et al., 1980) regarding erosion index (EI) values used in the USLE. There are a number of reasons for these differences. Climate change and climate variability may have contributed, but there is also the potential impact of differing data types, which is discussed more below.
An updated isoerodent map was prepared for the arrival of RUSLE2, which used more recent climate data (1960-1999). However, this update switched from using breakpoint precipitation data to a less precise and more ‘lossy‘ fixed-interval precipitation data for erosivity calculations (McGehee et al., 2021). This more commonly available data type does not preserve precipitation characteristics as well as the breakpoint data type, and thus, introduces bias/error in erosivity calculations when compared to the approach of Wischmeier & Smith (1958) and Wischmeier (1959), who discovered the original relationship of erosivity to soil loss. The RUSLE2 erosivity map values were greater than those from RUSLE and USLE by about 10%, but these values still fall short of benchmark erosivity values from McGregor et al. (1995) and Flanagan et al. (2020) by 19% and 32%, respectively. It is unclear how much of these differences are a result of the differing time periods, measurement technologies, or erosivity calculation, gap-filling, and interpolation methods.
McGehee et al. (2021) demonstrated the impact of different data types (e.g., breakpoint vs. fixed-intervals for common gauge precisions) on erosivity calculations, and McGehee et al. (2022) updated the national isoerodent map of the United States. Erosivity estimates based on fixed-interval precipitation data continued to fall below breakpoint-derived benchmarks, but these differences were substantially less than existing erosivity maps. These results point to potential insufficiencies in currently recommended intensity dampening correction and storm omission practices when using fixed-interval precipitation data to estimate erosivity values and map them over large areas.
Work aimed at resolving these issues is ongoing. This presentation will provide a brief history of erosivity estimation and mapping in the United States, the most recent progress towards a more reliable dynamic national erosivity map for the United States and the world, and a glimpse of the future of these efforts.
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