Temporal and elevation trends in rainfall erosivity on a 149 km2 watershed in a semi-arid region of the American Southwest

Authors: Nearing, Mark; Unkrich, Carl; Goodrich, David - Dave; Nichols, Mary; Keefer, Timothy

Submitted to: International Soil and Water Conservation Conference
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/29/2015
Publication Date: 8/1/2015
Citation: Nearing, M.A., Unkrich, C.L., Goodrich, D.C., Nichols, M.H., Keefer, T.O. 2015. Temporal and elevation trends in rainfall erosivity on a 149 km2 watershed in a semi-arid region of the American Southwest. International Soil and Water Conservation Conference. 3:77-85. https://doi.org/10.1016/j.iswcr.2015.06.008.
DOI: https://doi.org/10.1016/j.iswcr.2015.06.008

Interpretive Summary: Rainfall erosivity is the term used to describe the power or capability for rain storms to cause soil erosion. Scientific studies have shown that the amount of soil erosion caused by a storm is a function of the total energy of a storm and its intensity, given conditions on the ground surface, such as soil type, topography, and vegetation, being equal. The energy of a storm is related to the size of raindrops, how fast they fall, and how long the storm lasts. The intensity is defined and calculated by the maximum amount of rain that falls during any 30 minute period of the storm. With climate change there is an expectation that rainfall erosivity is changing. Generally across the world scientists have observed an increase in the number of extreme rainfall events, which means we are seeing more storms with more energy and probably also higher intensity for a longer period of time during the event. In this study we used data from our ARS watershed in Tombstone Arizona to look at the rainfall erosivity and how it may have changed over the past 50 years. We can do this at our ARS watershed, called Walnut Gulch, because at this site we have a very long record of highly detailed rainfall information from many raingages. This gives us the long record needed for finding temporal trends in the data, as well as the very high resolution of data needed to accurately calculate erosivity. An added bonus, and very important for the study from a statistical standpoint, is that we have there more than 80 raingages spread across the watershed. It is a highly replicated set of data. The results of the study did not show any temporal trends in the rainfall erosivity. For this particular location the rainfall patterns do not seem to have significantly changed, at least in a way that has caused a significant change in rainfall erosivity. We did find a spatial trend in the data, showing that higher elevations tended to slightly increasing erosivity. This was due to something called the orographic effect, which basically results in the fact that more rainfall comes in higher elevations. This was true even though the maximum elevation difference in the Walnut Gulch watershed was only of the order 1300 feet. The study provides a basis and methodology that can be used at other ARS watersheds to determine if trends exist in other parts of the country.

Technical Abstract: Temporal changes in rainfall erosivity can be expected to occur with changing climate; and because rainfall amounts are known to be in part a function of elevation, erosivity can be expected to be influenced by elevation as well. This is particularly true in mountainous regions such as are found over much of the western United States. The objective of this study was to identify temporal and elevation trends in rainfall erosivity on a 149 km2 (58 mi2) watershed in a semi-arid region of southeastern Arizona. Data from 84 raingages for the years 1960 through 2012 at elevations ranging from 1231 to 1644 m (4038 to 5394 ft) were used in the analyses. The average annual erosivity averaged over the watershed as a whole was 1104 MJ mm ha-1 h-1 yr-1 (65 hundreds of foot ton inch acre-1 h-1 y-1), and ranged from approximately 950 to 1225 MJ mm ha-1 h-1 yr-1 (56 to 72 hundreds of foot ton inch acre-1 h-1 y-1), with a statistical trend showing greater erosivity at the higher elevations. No statistically significant temporal changes in annual or summer erosivities were found. This result stands in contrast to recent modeling studies of runoff and erosion in the area based on downscaled GCM information that project significant levels of erosivity changes over coming decades. These results are consistent with known orographic rainfall effects, but contrast with recent studies that presented projections of significant trends of increasing erosivity in the future based on downscaled GCM outputs for the area. The results illustrate the need for testing and developing improved techniques to evaluate future erosion scenarios for purposes of making targeted soil conservation decisions.