Abstract
Because of its ease of calculation, the de Martonne aridity index (IdM) is used frequently by investigators for identifying susceptibility to drought conditions. However, application of the classical IdM formula in some geographical areas does not lead to a correct or complete configuration of drought occurrence. This paper proposes a modified index (IMod) with consideration of a general water-budget equation, instead of precipitation alone as in the IdM. This equation may contain different components in accordance with the season to which it refers; a pattern of three seasons is considered for the Napa Basin area. Equation inputs are averaged monthly values. Assessment is affected by a long series of factors, such as meteorological conditions, evapotranspiration, land morphology and runoff, subsurface influx and drainage capacity, soil cover/use, and irrigation. Since geographical conditions dramatically change with the seasons, the IMod variation is large. In comparison with the Palmer drought severity index, the main advantage of the newly derived formula is its ease of use under such seasonally changing conditions. It is concluded that use of the IMod allows a more detailed description of drought character, enlarging the range considered for the classical IdM categories. The main objective of this approach is to provide a basis for a more comprehensive consideration of diversification in water- and land-resource planning. Although applied to the Napa Basin, California, the general water-budget equation included in the IMod could be adapted successfully to different landscapes. [Key words: de Martonne aridity index, Palmer drought severity index, drought, Napa Basin, California.]