2000 years of Drought Variability in the Central United States

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Z 0 o= a Central Kansas sx r y FIG. 6. Palmer Drought Severity Index records (1700 -1979) for eastern Mon- tana (grid point 7 in Fig. 4a) (top), central Kansas (grid point 27) (middle), and north - central Texas (grid point 42) (bottom), as reconstructed using tree -ring data (Cook et al. 1996; see also the NOAA/NESDIS Web site (see text)]. Also shown are the observed PDSI values for grid points (light gray lines). The colored verti- cal bars indicate years of drought from historical accounts. Graumlich 1996). A flow reconstruction for the Truckee River in eastern California reflects this sixteenth - century drought (Hardman and Reil 1936), as do reconstructions of precipitation for northern and southern regions of California west of the Sierra Ne- vada (Haston and Michaelsen 1997). Additionally, reconstructions of western U.S. regional precipitation indicate a drought beginning in the southwest around 1565 and spreading to the entire western United States by 1585 (Fritts 1965), corresponding to drought evi- dence in both lake sediment data from western Min- nesota (Dean et al. 1994) and scarcity of old, living conifers established before about 1600 in the south- west (Swetnam and Betancourt 1998). Recent analy- sis of eolian sedimentation dates in the Wray dune field of eastern Colorado by Muhs et al. (1997) esti- mate the most recent period of eolian activity to have occurred in the past —400 years (14C yr before present), while Stokes and Swinehart (1997) docu- ment an optically dated period of eolian activity in the Nebraska Sand Hills that also coincides with the late sixteenth - century drought. Eolian activity is prima- rily due to drought severe enough to remove vegetation (Muhs and Holliday 1995), and the late 1500s drought was likely severe and long enough to have cleared sand deposits of live vegetation. The other megadrought of the thir- teenth to sixteenth centuries occurred in the last quarter of the thirteenth century. The time line in Fig. 8 shows the tree - ring records that reflect this drought, while Fig. 10 shows the proxy records that reflect this drought in a coarser tem- poral context (locations of the proxy records are shown in Fig. 9b, key in Table 2). Most of the proxy records mentioned for the sixteenth - century drought that extend back to the thirteenth century also record this severe multi - decadal drought, including tree -ring chronologies and /or reconstructions for southwestern Nebraska (Weakly 1965), northern New Mexico (Grissino- Mayer 1996), the Four Corners area (Rose et al. 1982), and the White Moun- tains (Hughes and Graumlich 1996). Weakly (1965) reported a 38 -yr drought from 1276 to 1313 in his southwestern Nebraska tree -ring chronology, the longest drought in the past 750 years. Other less finely resolved proxy data also testify to the occurrence of this drought, which in some areas appears to have ri- valed or exceeded even the sixteenth - century drought and was almost certainly of much greater intensity and duration than any drought of the twentieth century. Recent analysis of eolian sediments in the Nebraska Sand Hills suggests an onset of eolian activity begin- ning within the past 80014C years (Muhs et al. 1997). A period of drought at this time is documented in the varve record of western Minnesota (Dean et al. 1994). Archaeological data from the Great Plains and Four Corners areas also provide documentation of this drought (Bryson et al. 1970; Lehmer 1970; Wendland 1978; Euler et al. 1979; Dean et al. 1985; Dean 1994; Peterson 1994). In the Southwest this drought, some- times referred to as the "Great Drought," coincided with the abandonment of Anasazi settlements, redis- 2702 Vol. 79, No. 12, December 1998