Frequency of Moisture Deficits and Surpluses in the Humid Subtropical Climatic Region of the United States

FREQUENCY OF MOISTURE DEFICITS AND SURPLUSES IN THE HUMID SUBTROPICAL CLIMATIC REGION OF THE UNITED STATES Robert A. Müller" The professional literatures indicate continued interest of geographers and environmental scientists in the Thornthwaite potential évapotranspiration and water balance models. For example, Malmström recently published a much simplified version of a Thornthwaite climatic classification. (J ) Nevertheless, the Thornthwaite climatic classifications have not replaced the Koppen classification or its many modifications in the geographical literature. One apparent reason for this is that for teaching regional or global climates, geographers prefer to relate the Koppen climatic types to simple dynamic atmospheric controls, as, for example, in the recent physical geography texts by Strahler. (2) The potential évapotranspiration concept and derived water balance components have particular applications for environmental scientists and resource managers not found in the Koppen type classifications. This is because the water balance methodology indexes and evaluates the interactions between two of the most active factors of the local climate—energy (potential évapotranspiration ) and moisture (precipitation ). In recent years water balance methodology has been applied to, for example, drought by Palmer, (3) crop yields by Mather, (4) and increased runoffdue to urbanization by Müller. (5) These specialized applications of water balance components require use of a "continuous" water balance methodology, but it is the "average" water balance which appears most frequently in the geographic literature. The continuous balance represents a series of balances calculated in sequence through time for particular intervals such as a month, week, or day. By means oftables, (6) calculations of continuous balances are relatively simple, but, nevertheless, tedious. Computer programs are available for monthly or daily balances. In addition, each Weather Bureau State Climatologist receives a computer print-out of monthly water balance components by climatic divisions within his state as part of the calculations of the Palmer Drought Index. This paper focuses on the continuous water balance and how particular components differ from the same components calculated from the average water balance. The analysis is structured by a comparison of water balance data for two cities near the northern and southern margins of the humid subtropical climate (Cfa ) in the United States—New Brunswick, New Jersey, *Dr. Muller is associate professor of geography at Louisiana State University, Baton Rouge. The paper was accepted for publication in December 1969. Vol. X, No. 1 31 and Baton Rouge, Louisiana. This exploratory study is intended to encourage further analysis, not to be a finished description of environmental factors. Average water balances for New Brunswick and Baton Rouge are shown in Table 1. The balances are based on average monthly temperatures and precipitation for the 24 years between 1945 and 1968. The four-inch soil moisture storage capacity at New Brunswick and the six-inch capacity at Baton Rouge are assumed to approximate storage capacity within the rooting zones of agricultural crops in the clay soils in the vicinity of the two cities. In the table (PE ) represents potential évapotranspiration, an index ofclimatic energy availability, (P ) represents precipitation, (ST ) represents soil moisture storage within the rooting zone. (AE ) represents actual évapotranspiration, (D ) represents the moisture deficit, or the difference between (PE ) and (AE), and finally, (S) represents the moisture surplus, or the precipitation which is calculated to be available for groundwater recharge or surface runoff into stream channels. Table 1 shows that much more precipitation is needed at Baton Rouge than at New Brunswick to meet the "demand" of PE for moisture. As a consequence , the surplus (S ) is approximately the same at both places. Despite the relatively high rainfall at both places, the average balance for both shows a moisture deficit. For these years of record, the average water balance components agree well with annual average maps for the eastern United States, (7) and with tables of long-term average water balances for the United States. (8) Using the Thornthwaite climatic classification categories as modified in 1955, (9) New Brunswick and Baton Rouge work out to be respectively B2B'2 and BiB'4, both humid categories. Finally, the average balances show that the deficits and surpluses are largely seasonal: deficits occurring during summer and fall months and surpluses being generated during winter and spring. Resource managers...