R, you correct? The Curious Case of Arnoldus (1977). Response to “Comment on ‘Towards improved USLE-based soil erosion modelling in India: A review of prevalent pitfalls and implementation of exemplar methods’ by Majhi et al. (2021), Earth-Science Reviews 221, 103,786” by Chen and Bezak (2022)
Section snippets
Form of the R factor estimating equation of Arnoldus (1977)
Included in the Annex IV of the FAO Soils Bulletin 34 entitled “Assessing Soil Degradation”, an R factor equation devised by Arnoldus (1977) reads:where, P is the annual precipitation and Pi is the monthly precipitation (N.B. the sum of squared monthly to annual precipitation ratios is termed the “Modified Fournier Index” (MFI)).
Chen and Bezak (2022) contend that the above equation actually takes the form –and that since the
Conundrum on its unit (conversion)
Arnoldus (1977) only stated that the equation in question estimated the R factor in metric units, which was left undesignated. Renard and Freimund (1994) surmised that its unit is t-m cm ha−1 h−1 yr−1, as it was the metric unit mentioned in the Agricultural Handbook No. 537 (Wischmeier and Smith, 1978). To convert from the imperial to metric units, Arnoldus (1977) used a factor of 1.735, while Arnoldus (1980) rounded it to 1.74. Chen and Bezak (2022) are of the opinion that Arnoldus (1977) got
Its regional specificity and applicability
Both Arnoldus (1977) and Arnoldus (1980) explicitly stated that the creation of rainfall erosivity estimators using the MFI necessitates delineation of climatically homogeneous regions. Naturally, this implies that various climatic regions will have different regression equations of various levels of correlation.
Arnoldus (1977) formulated the equation in question using data from 14 stations of West Africa and applied it to estimate the rainfall erosivity for 112 locations in Morocco, subsequent
Conclusion
The regional specificity of R factor equations that link rainfall erosivity to precipitation amounts or MFI dictate that the usage of such methods in geographic regions other than where they were developed is fundamentally inappropriate and fraught with errors and discrepancies. This is simply because two locations may receive different amounts of rainfall annually, yet the rains may be similarly erosive at both locations, thereby implying that the regression equations will necessarily differ
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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