Book 2.indb - US Climate Change Science Program

Abrupt Climate Changefunctions; Kochel and Baker, 1982; Baker etal., 1988), paleoflood hydrology also providesinformation on the response of watersheds tolong-term climatic variability or change (Ely,1997; Ely et al., 1993; Knox, 2000), or to jointhydrological-climatological constraints onflood magnitude (Enzel et al., 1993).Knox (2000, see also Knox, 1985, 1993) reconstructedthe relative (to present) magnitude ofsmall floods (i.e., those with frequent returnintervals) in southwestern Wisconsin during theHolocene using radiocarbon-dated evidence ofthe size of former channels in the floodplains ofsmall watersheds, and the magnitude (depth) oflarger overbank floods using sedimentologicalproperties of flood deposits. The variationsin flood magnitude can be related to the jointeffects of runoff (from precipitation and snowmelt)and vegetation cover (Fig. 3.13). The largestmagnitudes of both sizes of floods occurredduring the mid-Holocene drought interval,when tree cover was low, permitting morerapid runoff of flood-generating snowmelt andprecipitation (see Knox, 1972). As tree coverincreased with increasing moisture during theinterval from 6 ka to 4 ka, flood magnitudesdecreased, then increased again after 3.5 ka aseffective moisture increased further in the lateHolocene.The paleoflood record in general suggests aclose relationship between climatic variationsand the flood response. This relationship maybe quite complex, however, inasmuch as thehydrologic response to climate changes ismediated by vegetation cover, which itselfis dependent on climate. In general, runofffrom forested hillslopes is lower for the sameinput of snowmelt or precipitation than fromless well-vegetated hillslopes (Pilgrim andCordery, 1993). Consequently, a shift from dryto wet conditions in a grassland may see a largeresponse (i.e., an increase) in flood magnitudeat first (until the vegetation cover increases),while a shift from wet to dry conditions maysee an initial decrease in flood magnitude,followed by an increase as vegetation coveris reduced (Knox, 1972, 1993). This kind ofrelationship makes it difficult to determine thespecific link between climate variations andpotentially abrupt responses in flood regimewithout the development of appropriate processmodels. Such models will require testing underconditions different from the present, as is thecase for models of other environmental systems.Paleoflood data are relatively limited relative toother paleoenvironmental indicators, but workis underway to assemble a working database(Hirschboeck, 2003).6.3 Floods and Global Climate ChangeOne of the main features of climate variationsin recent decades is the emergence of a packageof changes in meteorological and hydrologicalvariables that are consistent with global warmingand its impact on the hydrological cycle andthe frequency of extreme events (Trenberthet al., 2007, IPCC AR4, WG4, Ch. 3). Themechanisms underlying these changes includethe increase in atmospheric moisture, the intensityof the hydrologic cycle, and the changesin atmospheric circulation as the atmospherewarms (Knight et al., 2008). As described inone of the key findings of Gutowski et al. (2008;CCSP SAP 3.3, Ch. 3), “Heavy precipitationevents averaged over North America haveincreased over the past 50 years, consistentwith the increased water holding capacity of theatmosphere in a warmer climate and observedincreases in water vapor over the ocean.” (Seealso Easterling et al., 2000, Kunkel, 2003;Kunkel et al., 2003.) In addition, the frequencyof season-long episodes of greater-than-averageprecipitation is increasing (Kunkel et al., 2008;CCSP SAP 3.3, Sec. 2.2.2.3), and the timing ofsnowmelt is changing in many parts of the country(see Sec. 7). All of the meterological controlsof flooding (short- and long-duration heavyprecipitation, snowmelt) are thus undergoing111