THE ASSESSMENT OF DROUGHT INTENSITY BY A NEW INDEX James A. Shear and Donald Steila* Drought is a phenomenon of widespread significance. The use of the term in both professional and non-professional literature, however, is extremely varied. To the forester, drought is thought of primarily in terms of the susceptibility of surface conditions to the threat of fire. To the agriculturist, the concern of drought centers about his crop’s water-needs and soil moisture conditions. To the hydrologist, drought is indicated by lowered water levels in wells, streams, and reservoirs. Departures of precipitation during any period of time (for a given area) from the normal amount for that period are expected occurrences. Rarely do average climatic data express persistent conditions over extensive time periods. If a particular moisture departure does not significantly deviate from mean moisture conditions, the environment may well accommodate that departure without deleterious effects (1), but devia tions that reach the stress limits of the physical environment are of major consequence to man. Basic to most physical definitions of drought is a negative departure from the expected moisture availability of an area. Several statistical techniques have been employed in an effort to derive a quantitative measure of drought intensity. The most widely applied and compre hensive of these, to date, is Palmer’s Index. (2) Palmer attempted to divorce himself from the subjectivity and narrowness of scope associated with previous drought measure (e.g., agricultural and hydrological) and characterized drought solely in terms of meteorological phenomena. He defined a drought period as: An interval of time generally of the order of months or years in duration, during which the actual moisture supply at a given place rather consistently falls short of the climatically expected or climatically appropriate moisture supply. Further, the severity of drought may be considered as being a function of both the duration and magnitude of the moisture defi ciency. (3) Palmer’s methodology, very briefly summarized, consists of the follow ing: 1. Utilizing the Thomthwaite model for computing potential évapo transpiration, a hydrologie accounting by months is accomplished Dr. Shear is professor and chairman of the Department of Geography at the Uni versity of Georgia. Dr. Steila is assistant professor of geography at East Carolina University. The paper was accepted for publcation in Ju ly 1972. V ol. XIII, No. 1 13 for a long series of years and includes measures of: (1) potential soil recharge, (2) potential runoff, and (3) potential loss, i.e., the amount of moisture that could be lost from the soil provided the precipitation during the period was zero. 2. The data obtained in Step 1 are utilized to obtain certain coeffi cients which are dependent upon the climate of the area being analyzed. They are: the coefficient of évapotranspiration (d) d = E T /PE the coefficient of recharge (B) B = ___R /PR the coefficient of runoff (y ) Y = RO/PRO the coefficient of loss (§) & = L/PL where the variables utilized are averages for évapotranspiration (ET), potential évapotranspiration (PE), recharge (R), potential recharge (PR), runoff (RO), potential runoff (PRO), loss (L), and potential loss (PL). 3. The series of basic data are then reanalyzed using the derived coefficients to determine “CAFEC” (Climatically Appropriate For Existing Conditions) quantities—denoted by a circumflex— for each of the major components of the water budget: ET = PE, 1 =PR’ RO = PRO, and ' t = PL. The foregoing values are then utilized to determine CAFEC precipitation: ^ = ET + 6 o — L, Calculation of CAFEC precipitation, according to Palmer, provides the “amount of pre cipitation that would have maintained the water resources of the area at a level appropriate for the established economic activity of the area.” (4) 4. CAFEC precipitation is compared with actual precipitation to provide a measure of the departure cjf the moisture aspect of the weather from normal, i.e., d = P — P. 5. A climatic characteristic (k) is calculated for use as a weighting factor: k = (PE + R) (P + L). 6. A moisture anomaly index ( z ) is calculated and provides a meas ure of moisture departure: z = dk. These values cannot be re garded as inches of moisture departure, but only as index num bers. (5) Palmer’s methodology has gained...