Modified de Martonne aridity index: Application to the Napa Basin, California

Because of its ease of calculation, the de Martonne aridity index (I-dM) is used frequently by investigators for identifying susceptibility to drought conditions. However, application of the classical I-dM formula in some geographical areas does not lead to a correct or complete configuration of drought occurrence. This paper proposes a modified index (I-Mod) with consideration of a general water-budget equation, instead of precipitation alone as in the I-dM. 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 I-Mod 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 I-Mod allows a more detailed description of drought character, enlarging the range considered for the classical I-dM 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 I-Mod could be adapted successfully to different landscapes.