Hydrological drought monitoring in the Ebro basin: Standardized Soil Moisture Index
Authors: Sánchez Alcalde Guillem , Escorihuela Maria José , Paolini Giovanni 
Published: February 2023

Recent studies manifest that the frequency and severity of droughts are increasing due to climate change. Drought stands as a major climate risk; thus, its understanding and study are of utter importance. Such phenomenon results from complex interactions between the atmosphere, the continental surface and water resources management, and it can lead to large socioeconomic impacts.Following the work of Wilhite and Glantz, droughts can be categorized based on their severity as: meteorological, agricultural, hydrological, and socioeconomic (Wilhite, D.A.; and M.H. Glantz, 1985). The first three approaches are described by the physical impact of the drought, while the latter deals with drought in terms of supply and demand (e.g., the lack of energy, food or drinking water).Meteorological drought is associated with a precipitation deficiency period, which can also be accompanied by high temperatures or low relative humidity. If such a period persisted in time, we would start observing a deficiency in soil moisture, and a reduction in crop population and yield. Such circumstances would indicate that we are under the influence of an agricultural drought, with the potential to evolve into a hydrological drought with time. The frequency and severity of hydrological drought are defined typically on a river basin scale, with an impact on the surface and subsurface water supply (i.e., reduced streamflow or inflow to reservoirs, lakes and ponds).Due to the effects and frequency of droughts, monitoring them is of sheer importance. Different indices have been developed for the study of droughts, based on variables such as precipitation or vegetation status. One of the most used indices is the standardized precipitation index (SPI), which shows the deviation from average precipitation. Hence, it is related to drought hazards. Each index provides different information about the drought; therefore, a combination of indices is required to identify and assess them.Drought indices can also be obtained from L-band (21 cm, 1.4 GHz) radiometers, which provide soil moisture data, among other variables. Soil moisture plays a key role in agricultural monitoring and drought forecasting. While vegetation-based drought indices can only be applied once the drought is already causing vegetation damage, soil moisture observations can forewarn of impending drought conditions.The main drawback of precipitation-based drought indices is that they require in-situ data, providing a discrete image of the drought. Despite precipitation indices based on theoretical models providing a continuum picture of the drought, their performance and reliability should be taken with a grain of salt. On the other side, soil moisture data not only does not depend on any model but also displays a continuum image of the drought.In this presentation, we will study the performance of a variety of drought indicators based on precipitation and soil moisture data in the Ebro basin region and show how they manifest hydrological drought. Namely, we have developed the standardized soil moisture index (SSI). The SSI is based on the SPI method, and we have tested this index for different integration times.