ABSTRACT. The availability of 1961-1990 precipitation data and the average increase in precipitation from the 1925-1960 data stimulated preparation of a new isohyetal map for Tennessee and adjacent area. Most of West and Middle Tennessee and the Sequatchie Valley receive 130-150 cm; the central and southern Cumberland Plateau receives over 150 cm annually. The Ridge and Valley receives 110-140 cm. Mountainous regions of East Tennessee and western North Carolina receive 110-over 200 cm annually but the paucity of measurement stations from these areas has abrogated accurate mapping of isohyates there.

No external factors influence the human condition more than those of weather and climate, and those factors related to them. The world’s human populations are clustered in relation to the equitibility of climate and its food and shelter provision. Similarly the terrestrial biotic elements, flora and fauna are arranged in a general pattern responsive to the types of climate (Walter 1984, Trewartha 1961). Biotic units, as biomes, are arrayed around areas of unique climate (Vankat 1979, Barbour and Billings 1988). The occurrences of drought and flood focus attention on the water regime as a climatic element. Precipitation lows producing drought may change human history (Bryson and Murray 1977, Rosenberg 1978). The grasslands in particular have been examined for effects of drought stress (Weaver 1954). Floods, normal in river valley topography, may cause severe devastation to societal infrastructure (Tennessee Valley Authority 1949), and damage vegetation normally upslope from the flood zone (Lindsay et al. 1961).

Average precipitation, as shown on isohyetal maps, is the baseline for understanding drastic variation resulting in droughts and floods. Past isohyetal maps for Tennessee have been based on 1925-1955 data (Dickson 1960) and 1931-1960 data (United States Department of Commerce 1968, Baldwin 1973). The new map included here chiefly uses data from 1961-1990. The need for this is occasioned by higher precipitation averages in recent decades. Thirty-four Tennessee stations with 1925-1955 data (Dickson 1960) show an average increase of 5.5 cm in the more recent data; 16 western North Carolina stations show an average 3.6 cm increase when comparing 1925-1955 (Carney 1960) and modern data. The 1925-1955 data includes the droughts of the 1930s and 1950s (Vaiksnoras and Palmer 1973.)

     Average total annual precipitation, for 1961-1990, for Tennessee and an adjacent area for 165 stations was obtained on the Internet by Schmidt from, i.e., Owenby and Ezell (1992) and United States Department of Commerce (1992). Station data were placed on a map display of stations and isohyetal lines were drawn freehand (Figure 1). Lines ("contours") are with variable rather than constant intervals of centimeters. These lines were then compared to those adjacent states in Climatography of the United States (United States Department of Commerce 1960). Agreement was fairly good except for the mountainous eastern edge of Tennessee and western North Carolina and northwestern South Carolina. In the new data set there are only 14 stations with precipitation over 158 cm. In this mountainous area, the map was compared with that by the Tennessee Valley Authority (1959), using data from the years 1935-1955. For these data, the precipitation gauge network was about one per 199 square kilometers versus one per more than twice that many square kilometers in Middle Tennessee. This network includes a few stations on mountains of high precipitation (there are 44 peaks over 1830 meters in Tennessee and North Carolina). Precipitation data from a few peaks have been added to Figure 1 using Tennessee Valley Authority (1959) data:

Graham County, NC (Hawk Knob, Haywood Gap)
Sevier County, TN (Clingman’s Dome, Great Smoky Mountains National Park)
Carter County, TN and Madison County, NC (McKinley Gap, Roan Mountain area)
Yancey County, NC (Mt. Mitchell)
Buncombe County, NC (Mt. Pisgah)

     An irregular precipitation trend between southeastern Missouri and eastern Middle Tennessee (120-150 cm) reflects part of the mid-continental trend of increasing precipitation from the eastern foot of the Rocky Mountains eastward (United States Department of Commerce 1968, 1992). A less well-defined trend from southern to northern Middle Tennessee reflects part of the Gulf of Mexico to central southeastern decreasing trend (United States Department of Commerce 1968, 1992). The westernmost 130 cm line marks the loess bluffs area of West Tennessee with over 31-meter bluffs over the Mississippi River alluvial plain (Springer and Elder 1980). The elevated central and southern Cumberland Plateau exhibit precipitation of 150 cm and above. However, the Sequatchie Valley (Pikeville) and that valley in northeastern Alabama, lower topographically, exhibit lower precipitation than the Plateau surface (also see Long 1959).

     The Ridge and Valley, lying in the rain shadow of the Appalachian (Cumberland) Plateaus to the north and west, and also being sheltered by the Blue Ridge Mountains from the moist winds blowing from the south, include areas of lower precipitation (110-140 cm) and the isohyates extend into North Carolina in the French Broad and Pigeon River valleys (also seen on the map by Carney 1960).

     Since few precipitation measurements are made on the little-accessible mountain peaks and ridges, mountainous areas do not show precipitation corresponding to the intricacies of the topography. In Figure 1, no precipitation increase is shown for the Cumberland Mountains, with ridges over 1068 meters in Tennessee; nor is one shown for the Cumberland Mountains of Kentucky with peaks over 300 meters higher (Anderson 1959). Blue Ridge precipitation varies from <110 to >200 cm.

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     The addition of Tennessee Valley Authority data from five isolated areas of high precipitation of earlier decades only slightly improves the map quality. These data do not suffice for the generally inadequate high elevation data. Expected annual precipitation can be predicted from precipitation/ elevation relationships (Dickson 1959), but great seasonal annual and topographic variability is to be expected (Smallshaw 1953). Perhaps in the future, a new, cheap instrumental unit will become available to place in usually inaccessible areas to provide routine precipitation data.

     A map of annual precipitation levels is one useful descriptor of a geographic area as are those of other climatic descriptors (as temperature). This hybrid map uses precipitation chiefly from the 1961-90 period but five stations are included with data from earlier decades. However, no averages predict annual conditions, variability is great and is to be expected (see data in Martin 1930). The paucity of measurements on uplands in mountainous topography make assessment of the true pattern of mountain precipitation nearly impossible. The probability that our climate is undergoing at least partly man-induced changes means that known averages (and extremes) of precipitation measures may be expected to continue to change (Budyo and Izrael 1991, Gates 1993, Lashot and Tirpak 1990, Schneider and Temkin 1977).

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