Effects of the largest daily events on total soil erosion by rainwater. An analysis of the USLE database

Abstract

Soil erosion by water (rain and wash) is a discrete process, and there are several questions relating to this that have yet to be answered. For instance, for how long should measurements be taken in order to obtain realistic mean erosion rates, and what is the effect of the largest daily events on soil erosion, are questions still not fully answered. To make progress on these issues, detailed information is needed, but usually not available. This work has analysed the USLE database compiled by the United States Department of Agriculture. A total of 27 857 daily erosion events were examined, monitored over 310 erosion plots representing 3195 plot-years of soil erosion measurements. Periods of measurement varied between plots, ranging from 2 years to 32 years. Data have been analysed by calculating the percentage of soil eroded by the largest to the tenth largest daily events over the entire length of the record. The percentage of soil eroded during the n-largest events follows a power function y = x −b , where y is the percentage of soil eroded in the selected n-largest event, and x the total of measured daily events. Results showed that the top 10% of total daily erosive events produce a mean of 50% of eroded soil. Soil erosion measured over short periods, e.g. typically less than 5 to 10 years, is compressed into a few daily events, whatever their magnitude; therefore, mean erosion rates estimated under such small time frames cannot be taken as a good descriptor of the real processes. Analysis suggests that to weight the dependence of soil erosion on the largest daily values, a minimum number of events should be considered. In general, if it is wished to reduce the effect of the largest events to less than 50% of total soil erosion, the mean number of recorded daily events should be between 75 and 100. They represent, on average, 10 years of measurements for the database used here.

Contribution of the largest events to suspended sediment transport across the USA

“…The ratio indicates that the suspended sediment produced by the n-largest daily events, representing 1 per cent of the total daily events recorded, accounts for a mean of 30 per cent of the total sediment load (Table II). The ratio follows a similar pattern to those described by Gonzalez Hidalgo et al (2007b) derived from USLE erosion plots.…”

“…These seem to be the main results stated by the empirical function reported in this paper, in which the percentage of suspended sediment load produced by n-largest daily events is related to the total number of daily events recorded. The power-law function is very similar to those proposed by Gonzalez-Hidalgo et al (2007b) on soil erosion after an analysis of the USLE database, and they suggest a minimum record time to avoid bias due to short-term series. In this case, i.e.…”

“…suspended sediment load in rivers all over the USA, a minimum number of 2500 events should be analysed to avoid the 25-largest daily events exceeding 30 per cent of the total load. This period is more or less equivalent to 10 years, and coincides with the minimum period estimated from the USLE database (Gonzalez Hidalgo et al, 2007b). Marginal means (cofactors catchment area and daily events).…”

The effect of intense rainstorm events on the suspended sediment response under various land uses : the Aísa Valley experimental station

“…A similar result was found by González-Hidalgo et al (2007) in western Mediterranean areas, where three daily erosive events per annum represented more than 50% of soil erosion, regardless of the total amount of soil eroded. Thus, González-Hidalgo et al (2009) reported that a relatively long period of collection of sedimentary records is necessary to provide realistic estimates of average erosion rates. Such events can exhaust sediment at the basin scale, reducing sediment supply for many years (Alvera and García-Ruiz, 2000) and can result in marked changes to hillslope and channel dynamics, producing deep-seated and shallow landslides, gullies, channel erosion, braiding-dominated channels, and large volumes of suspended sediment load and bedload .…”

“…Many studies of soil erosion have highlighted that a small number of short-term erosive events are responsible for a large proportion of the geomorphological effects and the sediment load (see González-Hidalgo et al, 2009, 2010). This phenomenon is related to "time compression", which is defined as the shortest temporal interval producing most of the soil erosion or sediment load in a catchment during a defined period (one year).…”