CALCULATION OF SHORT-TERM EROSION RATES 967 Copyright © 2003 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 28, 967–977 (2003) Earth Surface Processes and Landforms Earth Surf. Process. Landforms 28, 967–977 (2003) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/esp.509 SHORT-TERM EROSION RATES FROM A 7 BE INVENTORY BALANCE CHRISTOPHER G. WILSON,* GERALD MATISOFF AND PETER J. WHITING Department of Geological Sciences, Case Western Reserve University, Cleveland, OH, USA Received 5 February 2002; Revised 19 December 2002; Accepted 7 February 2003 ABSTRACT Detailed soil erosion studies benefit from the ability to quantify the magnitude of erosion over time scales appropriate to the process. An inventory balance for 7 Be was used to calculate sediment erosion in a 30·73 m 2 plot during a series of runoff- producing thunderstorms occurring over three days at the Deep Loess Research Station in Treynor, Iowa, USA. The inventory balance included determination of the pre- and post-storm 7 Be inventories in the soil, the atmospheric influx of 7 Be during the event, and profiles of the 7 Be activity in the soil following the atmospheric deposition. The erosion calculated in the plot using the 7 Be inventory balance was 0·058 g cm -2 , which is 23 per cent of the annual average erosion determined using 137 Cs inventories. The calculated erosion from the mass balance is similar to the 0·059 g cm -2 of erosion estimated from the amount of sediment collected at the outlet of the 6 ha field during the study period and the delivery ratio (0·64). The inventory balance of 7 Be provides a new means for evaluating soil erosion over the time period most relevant to quantifying the prediction of erosion from runoff. Copyright © 2003 John Wiley & Sons, Ltd. KEY WORDS: erosion rates; runoff; 7 Be; Iowa INTRODUCTION The use of radionuclides, such as 137 Cs (t 1/2 = 30·17 years), to investigate erosion is well established (Ritchie and McHenry, 1990). 137 Cs is an excellent tracer for studying sediment movement due to its 30-year half-life, known fallout history, and strong affinity to soil particles (Ritchie and McHenry, 1990; Walling and Quine, 1990; Higgitt, 1995). The presence of 137 Cs in North America is a result of atmospheric nuclear weapons testing during the 1950s and 1960s. After its release, 137 Cs was circulated globally in the upper atmosphere and was deposited as wet and dry fallout in North America between 1952 and 1984; deposition reached a maximum in 1964 (Ritchie and McHenry, 1990). Once deposited, 137 Cs was strongly adsorbed to soil particles (Schulz et al., 1960; Rogowski and Tamura, 1970; Owens et al., 1996). The current redistribution of 137 Cs within soils results primarily from movement of the soil particles to which the 137 Cs is attached (Higgitt, 1995; Owens et al., 1996; He and Walling, 1997). Studies of soil erosion and deposition using 137 Cs have been conducted in a wide range of terrestrial settings because of certain sampling advantages (Ritchie and McHenry, 1990). Erosion rates can be determined from samples collected during only one site visit (Walling et al., 1999). In addition, infield sampling provides local estimates of soil erosion rates, in contrast to other methods that provide spatially averaged values. 137 Cs can be measured non-destructively using modern HPGe gamma detectors; hence, the samples are available for other analyses. Finally, sample preparation is minimal for modern detectors (Ritchie and McHenry, 1990). One limitation of using 137 Cs to evaluate sediment redistribution is the inability to quantify soil movement over the short term, especially over single-runoff events. Surface soils have low 137 Cs activities because there is no longer atmospheric replenishment of 137 Cs and because 137 Cs has migrated down in the soil column. In agricultural fields, tillage will have homogenized the 137 Cs profile throughout the plough layer. Since individual runoff events typically remove only a small fraction of the soil column, especially when sheet erosion is dominant over rilling, a change in the total 137 Cs inventory is undetectable over short time scales. Longer time * Correspondence to: C. G. Wilson, Department of Geological Sciences, Case Western Reserve University, 10900 Euclid Ave., Cleveland, OH 44106, USA. E-mail: cgw3@pop.cwru.edu