The effect of El Niño Southern Oscillation cycles on the decadal scale suspended sediment behavior of a coastal dry‐summer subtropical catchment

Abstract

Rivers display temporal dependence in suspended sediment–water discharge relationships. Although most work has focused on multi‐decadal trends, river sediment behavior often displays sub‐decadal scale fluctuations that have received little attention. The objectives of this study were to identify inter‐annual to decadal scale fluctuations in the suspended sediment–discharge relationship of a dry‐summer subtropical river, infer the mechanisms behind these fluctuations, and examine the role of El Niño Southern Oscillation climate cycles. The Salinas River (California) is a moderate sized (11 000 km2), coastal dry‐summer subtropical catchment with a mean discharge (Qmean) of 11.6 m3 s−1. This watershed is located at the northern most extent of the Pacific coastal North America region that experiences increased storm frequency during El Niño years. Event to inter‐annual scale suspended sediment behavior in this system was known to be influenced by antecedent hydrologic conditions, whereby previous hydrologic activity regulates the suspended sediment concentration–water discharge relationship. Fine and sand suspended sediment in the lower Salinas River exhibited persistent, decadal scale periods of positive and negative discharge corrected concentrations. The decadal scale variability in suspended sediment behavior was influenced by inter‐annual to decadal scale fluctuations in hydrologic characteristics, including: elapsed time since small (~0.1 × Qmean), and moderate (~10 × Qmean) threshold discharge values, the number of preceding days that low/no flow occurred, and annual water yield. El Niño climatic activity was found to have little effect on decadal‐scale fluctuations in the fine suspended sediment–discharge relationship due to low or no effect on the frequency of moderate to low discharge magnitudes, annual precipitation, and water yield. However, sand concentrations generally increased in El Niño years due to the increased frequency of moderate to high magnitude discharge events, which generally increase sand supply. Copyright © 2014 John Wiley & Sons, Ltd.

Conversion to drip irrigated agriculture may offset historic anthropogenic and wildfire contributions to sediment production

“…Furthermore, factors affecting watershed-scale sediment production operate over a wide range of time scales, with even seemingly discrete events generating legacy effects that may last for years or decades (Warrick et al, 2012;Warrick et al, 2013;Gray et al, 2014). Semi-arid basins in particular have been found to display persistent dependence on climatically driven antecedent basin conditions, such as storm/flood and wildfire histories (Abraham, 1969;Tanji et al, 1980;Lenzi and Marchi, 2000;Lana-Renault et al, 2007;Warrick and Rubin, 2007;López-Tarazón et al, 2011;Gray et al, 2015a). The addition of human influences further complicates sediment flux controls in the highly developed portions of the world that are the most intensively studied (Walling, 2006;Syvitski and Milliman, 2007).…”

“…Niño Southern Oscillation (ENSO) cycles (Inman and Jenkins, 1999;Farnsworth and Milliman, 2003;Gray et al, 2015a). Integrated expressions of external and internal factors in the form of antecedent hydrologic conditions (i.e.…”

“…Integrated expressions of external and internal factors in the form of antecedent hydrologic conditions (i.e. discharge and drought history) that are affected by ENSO have also been examined as controls on suspended sediment discharge regimes (Gray et al, 2014;2015a;2015b). One previous study has also addressed the importance of wildfire in controlling the sediment export from a major Salinas tributary (Warrick et al, 2012).…”

“…Gray et al (2014;2015a;2015b) thoroughly investigated suspended sediment dynamics in the Salinas River in relation to climatic and hydrological drivers. It was found that C SS -Q behavior was influenced by antecedent hydrologic conditions at event, annual, interannual and 13 decadal time scales (Gray et al, 2014;2015a), and that the temporal trend in discharge-corrected suspended sediment concentrations was negative over the 1967-2011 period of record (Gray et al, 2015a). Notably, no change in the relationship between precipitation and discharge was found over this time period (Gray et al, 2015a).…”

“…It was found that C SS -Q behavior was influenced by antecedent hydrologic conditions at event, annual, interannual and 13 decadal time scales (Gray et al, 2014;2015a), and that the temporal trend in discharge-corrected suspended sediment concentrations was negative over the 1967-2011 period of record (Gray et al, 2015a). Notably, no change in the relationship between precipitation and discharge was found over this time period (Gray et al, 2015a). By taking these factors and the temporal dependence of sediment behavior into consideration, the average annual suspended sediment flux was estimated as 2.1 to 2.4 Mt (Gray et al, 2015b).…”