Chapter Outline
Introduction Climate, topography and biotic factors, operating on rock and other parent materials over time, determine rates of soil formation and the resulting profile morphology (Jenny, 1941). Weathering converts minerals in parent material to soil minerals with temperature and precipitation being major determinants of the rate of these chemical and physical transformations (Paton, 1978; Paton et al., 1995). The climate of the southern United States has several contrasting zones, including continental, marine, arid and mountain. Portions of the states that border the Gulf of Mexico or the Atlantic Ocean are moist, having high rainfall and high humidity during summers. Areas in far-western Texas are arid while the plains of western Oklahoma and Texas are classified as semiarid.
There are several Internet sites that provide specific climatic and weather data for the southern region. In particular, the National Oceanic and Atmospheric Administration (NOAA) web site (http://www.cdc.noaa.gov/ClimateInfo) is an excellent source of climatic data. At this web site, data including heaviest rainfall, maximum temperatures, minimum temperatures, and maximum wind speed are available. A useful web site for land surface maps with links to soil information is provided by the United States Geological Survey (USGS) at "http://edcwww.cr.usgs.gov/webglis". Shaded relief maps for any state showing terrain features may be accessed on the Internet at "http://fermi.jhuapl.edu/states/states.html." These relief maps indicate where orographic lift may enhance precipitation or where terrain may induce a rain shadow.
Precipitation Precipitation varies greatly across the Southern Region from about 200 mm/year in the semi-arid zone of western Texas to over 1900 mm/year in the Southern Appalachian Mountains of South Carolina, North Carolina, and Tennessee (Table 1). A large proportion of the southern region has annual rainfall in excess of 1000 mm. Gradients in rainfall occur in several states in association with changes in topography and proximity to the coast. Mean monthly and annual rainfall distribution maps for the Southern Region can be accessed at http://www.ocs.orst.edu/prism/prism_new.html. These maps were generated from weather records from national and some state monitoring stations for the 1961-1990 period. An interpolation technique, accounting for orographic effects on precipitation (Daly et al., 1994), provides precipitation estimates for locations without monitoring stations. The spatial distribution of mean annual rainfall for the southeast is provided in Fig. 1.
Fig. 1. Composite annual mean precipitation from 1950 to 1995. (Source: NOAA-CIRES/Climate Diagnostics Center.)
Air Temperature The range of minimum January and maximum July temperatures (Table 1) shows that parts of all but two states (Florida, Louisiana) have average January minimum temperature at or below freezing, whereas all states have areas above 30°C as a maximum average temperature in July. Most of the southern regional soils are thermic (15.0 to 22.2°C) (Daniels et al., 1973). Southern Florida and southern Texas have extensive hyperthermic (greater than 22.2°C) soils. Louisiana has a relatively small area with a hyperthermic regime. Soils in the mesic temperature class (8.3 to 15.0°C) occur in the northern portions of the region. The spatial distribution of mean annual temperatures for the southeast is provided in Fig. 2.
A 25-year mean annual air temperature map may be accessed at http://www.cs.utk.edu/~imp/maps/maps.html. This map was generated by interpolation of weather station data obtained from the NOAA web site. In this process temperature data from meteorological stations were adjusted to equivalent values at sea level with the use of a lapse rate for the change in temperature with elevation. Spline interpolation was applied to estimate the sea-level-equivalent temperatures for locations without meteorological stations. Finally, the lapse rate was applied to the sea-level-equivalent temperatures to adjust for land elevation as given by the USGS digital elevation model for the 13-state region. At the same Internet site, monthly mean air temperature maps for the continental United States have been developed from data for the 1961-1990 period. These maps are displayed as a movie to show the seasonal progression in temperature using a color scale to represent degrees Fahrenheit x 10.
Other Climatic Factors Relative humidity, solar radiation and wind speed are other climate variables, in addition to temperature and precipitation, that determine the water budget of an area through effects on evapotranspiration. Relative humidity is highest in the coastal regions near the Gulf of Mexico and the Atlantic Ocean. Sunshine is abundant in the western part of the region and decreases eastward. The percentage of sunshine received depends on cloudiness. Records of cloud cover may be obtained on the Internet at http://cdiac.esd.ornl.gov/ndps/ndp026a.html from the Carbon Dioxide Information Analysis Center. Along with the western portions of the region, Florida receives sun about two-thirds of the daylight hours during the year. The sun also shines longer in Florida in the winter than in the northern parts of the country reinforcing the sunshine state identity.
Two climatological factors that influences crop growth, but not soils directly, are frost-free days and growing degree days. Growing degree days is the summation of mean daily air temperature above a threshold of 5.55°C. Annual sums range from a low of about 3000 in Virginia to a high of over 6000 in southern Florida and southern Texas. Estimated monthly actual and potential evapotranspiration for the continental United States have been determined and are available on the Internet at http://www.cs.utk.edu/~imp/maps/maps.html. These maps are displayed as a movie. Similarly a water budget for the continental United States is developed as a movie in monthly time steps. The water budget shows the difference between precipitation and estimated actual evapotranspiration with surpluses shown in shades of green and deficits shown in shades of red.
Extreme Events Intense rainfall causes runoff and water erosion as well as damaging floods. Torrential rains accompanying hurricanes or tropical storms cause severe erosion as they cross coastal beaches or experience the orographic lift of the Appalachian or Rocky Mountains. Slopes with little vegetation cover such as tilled fields, clearcut forest areas and surface mining sites are particularly susceptible to erosion from intense rainfall. Some of the more severe thunderstorms may be accompanied by tornadoes and damaging hail. While tornadoes typically occur in the spring, they can also occur as a winter phenomenon. Some are also spawned by hurricanes during the late summer. Hurricanes and tropical depressions typically occur in late summer and fall. These may cause severe crop loss and forest damage from high wind speeds. Snowfall is usually of minor importance in the Southeast except in the Southern Appalachian Mountains where significant snowfalls regularly occur through the winter period. Occasional snow storms blanket the southern region, except for the southern portions of Florida and Texas. Blizzards, characterized by subfreezing temperatures, very strong winds and drifting snow, sometimes occur in the western plains. Localized episodes of freezing rain and ice storms occur periodically causing extensive damage in some cases.
Flooding Damaging floods have affected much of the South particularly prior to dam construction and flow management of major river systems. The risk assessments associated with flooding, hurricanes, and wind are presented in Table 2. Harmful floods are rare, however, areas within most states are subject to local heavy rains. In Florida, the impacts from heavy rains are sometimes just as severe as droughts due to limited surface runoff gradient. Flash floods, a frequent result of high intensity thunderstorms, may occur in several southern states. In Louisiana, flooding rains can occur during any month of the year, but are most frequent in late winter and early spring. The flood season in Mississippi is usually during the first six months of the year. Due to persistent thunderstorms or heavy rains that originate in the Gulf, flooding can also occur during late summer and early fall. Flooding in Oklahoma occurs in the late spring and early summer. Minor flooding occurs in South Carolina every year; major floods occur about once every seven or eight years. The winter and early spring are Tennessee’s high flood season due to the frequent storms that bring high intensity rains. Widespread flooding and local flash floods can occur. In Texas, flood stage is reached on some streams almost every year. Floods can occur in all months in Virginia, mostly in late winter and early spring with snowmelt being a factor.
Drought Drought is a recurring phenomenon in the southern states. Severe local droughts occur almost every year, but severe statewide droughts are relatively rare except in Texas and Oklahoma. Persisting conditions of strong and hot winds added to the daytime heat produce high evaporation that can lead to severe droughts in these two states. Even though Florida and Louisiana receive relatively high rainfall, droughts are not uncommon. Though statewide droughts during the summer are rare, portions of a state are often affected. If rainfall does not occur during the summer months, a drought may develop from soil water depletion by evaporation and transpiration.
Climate Change Global warming, whether anthropogenic or natural, will impact physical, chemical and biological processes in soil. Use of soils in the southern region may be impacted by warming. Global warming could increase desertification in MLRAs 77, 78, 81, and 83. Schlesinger et al. (1990) suggest global warming favors shrub invasion in semi arid climates leading to increased wind erosion, water erosion, and nutrient loss. Global warming may specifically influence MLRAs along the Atlantic or Gulf of Mexico coasts by increasing evaporative demand and possibly lessening precipitation.
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