Monitoring drought severity in agriculture through a synthetic index based on dry periods: a case study in the Mediterranean basin

Drought is usually defined as a significant, temporary reduction in water availability below the expected amount for a specified period and for a defined climatic zone. Drought episodes may be described by means of different characteristics, namely drought duration and intensity, considered as the two dimensions of this agro-climatic phenomenon. Among the several methods proposed for describing drought, the run method appears as suitable to provide an objective characterisation of drought events. A simple drought index, obtained from only two input variables (rainfall and reference evapotranspiration), able to describe different dimensions of drought phenomena (duration, intensity and severity), and to recognise "normal" conditions, both in statistical and ecological terms, is meaningful. Our interest is therefore to develop such a drought index, able to produce information on a detailed geographic and time scale.The procedure we have built up consists in calculating for each day, considered as dry or wet, the length of the present dry spell (LH) and water deficit (DF). The same variables were computed over a six-month period preceding the evaluation day (LH₆ and DF₆). For each day to be evaluated we associated each agro-climatological variable with the corresponding percentile value obtained from the climatology data set. Two partial indicators depicting drought duration (LHS) and intensity (DFS) were introduced. LHS and DFS were computed respectively as the maximum daily value between LH and LH₆, and between DF and DF₆. A synthetic index of drought severity (DSI) was finally obtained as the geometric mean of LHS and DHS indicators.Using dry spell climatology we estimated duration, intensity and severity of drought on a daily basis over the test period 1988-2003. The most arid period in Italy was recorded during spring-summer 2003, when especially dry conditions occurred in northern and central Italy. DSI correctly indicates a significant increase of drought from March to July 2003 throughout Italy: drought severity reaches the 90 percentile in all the stations from Po plain, Apennines, and coastal areas of central Italy.In conclusion, DSI may be used to assess the agro-climatic conditions occurring in a certain location through only two input variables. It allows a synthetic description of drought episodes in terms of both dry spell length and water balance.