Characterization of agricultural drought propagation on the Iberian Peninsula through non-parametric indices

This study presents a multi-temporal analysis of the propagation mechanism from meteorological to agricultural drought events over the Iberian Peninsula. The analysis was carried out using a multi-scale reanalysis dataset covering the period from 1950 to 2021, which contained multiple drought events. To identify meteorological drought episodes, the Standardized Precipitation-Evapotranspiration Index (SPEI) was used, while three different non-parametric agricultural drought indices were adopted for the detection of agricultural droughts. These indices range from the univariate Standardized Soil Moisture index (SSI) to multi-variate indices such as the Multivariate Standardized Drought Index (MSDI) and the Standard Precipitation Evapotranspiration and Soil Moisture index (SPESMI) in order to separately consider the physical quantities involved in the process. Additionally, a new Combined agricultural drought index (COMB) was proposed. The statistical approach based on run theory was employed, and several characteristics of the drought propagation were analyzed, including the response time scale, propagation rate, propagation probability, and lag time, both over the entire dataset period and specifically over the representative 2005 drought episode. The results showed a fast response time scale of about 1 or 2 months for agricultural drought events, in agreement with other studies based on in-situ measurements. There was a high probability of occurrence when considering the transition from seasonal or monthly meteorological to monthly agricultural drought events. The duration of agricultural drought was found to be shorter than that of meteorological drought, with a delayed onset but the same term. The propagation probability was found to increase according to the severity of the originating meteorological drought. The results obtained by multi-variate indices showed a more rapid propagation process and a tendency to identify more severe events with respect to the univariate, implying that the contribution of other variables accelerated the response compared to the soil moisture alone. The newly developed combined agricultural drought index was found to be a useful tool for balancing the characteristics of other adopted indices. These findings could serve as a cue for future studies involving ensembles of indices to overcome the issue related to the specificity of single drought indices. Additionally, the adopted methodology can be useful for carrying out future investigations dedicated to a global warming environment, assuming non-stationary conditions.