Drought is one of the most complex climate-related phenomena and is expected to progressively affect our lives by causing very serious environmental and socioeconomic damage by the end of the 21st century. In this study, we have extracted a dataset of exceptional meteorological drought events between 1975 and 2019 at the country and subregional scales. Each drought event was described by its start and end date, intensity, severity, duration, areal extent, peak month and peak area. To define such drought events and their characteristics, separate analyses based on three drought indices were performed at 12-month timescale: the Standardized Precipitation Index (SPI), the Standardized Precipitation Evapotranspiration Index (SPEI), and the Reconnaissance Drought Index (RDI). A multivariate combined drought index (DXI) was developed by merging the previous three indices for more understanding of droughts’ features at the country and subregional levels. Principal component analysis (PCA) was used to identify five different drought subregions based on DXI-12 values for 312 Mediterranean stations and a new special score was defined to classify the multi-subregional exceptional drought events across the Mediterranean Basin (MED). The results indicated that extensive drought events occurred more frequently since the late 1990s, showing several drought hotspots in the last decades in the southeastern Mediterranean and northwest Africa. In addition, the results showed that the most severe events were more detected when more than single drought index was used. The highest percentage area under drought was also observed through combining the variations of three drought indices. Furthermore, the drought area in both dry and humid areas in the MED has also experienced a remarkable increase since the late 1990s. Based on a comparison of the drought events during the two periods—1975–1996 and 1997–2019—we find that the current dry conditions in the MED are more severe, intense, and frequent than the earlier period; moreover, the strongest dry conditions occurred in last two decades. The SPEI-12 and RDI-12 have a higher capacity in providing a more comprehensive description of the dry conditions because of the inclusion of temperature or atmospheric evaporative demand in their scheme. A complex range of atmospheric circulation patterns, particularly the Western Mediterranean Oscillation (WeMO) and East Atlantic/West Russia (EATL/WRUS), appear to play an important role in severe, intense and region-wide droughts, including the two most severe droughts, 1999–2001 and 2007–2012, with lesser influence of the NAO, ULMO and SCAND.

/pdf/mediterranean-scale-drought-regional-datasets-for-168vk561d2.pdf

Mediterranean-Scale Drought: Regional Datasets for Exceptional Meteorological Drought Events during 1975-2019

Soil moisture drought, as one of the most important drought categories, is determined by both water supply (e.g., precipitation) and demand (e.g., potential evapotranspiration). To shed light on the underlying mechanisms driving soil moisture drought, the statistical multiple linear regression, machine learning, and modeling experiments methods have been pervasively used in early studies. However, these methods neglect the collinearity and interactions of climate variables, and thus cannot reflect the direct and indirect interaction of factors leading to soil moisture drought. To reveal the synergistic effects of water supply and demand on soil moisture drought, this study quantified the contributions of key drivers to the change of soil moisture drought by a path analysis method to exhibit the relationships between atmospheric movement state and soil moisture drought. Prior to applying the systematic path analysis model, we identified the spatial patterns of soil moisture droughts at different depths by using a state-of-art three-dimensional drought recognition method in China. Our results showed that precipitation deficits dominated the interannual variation of soil moisture drought while increasing potential evapotranspiration only had marginal intensification in drought. The response of soil moisture drought to potential evapotranspiration was magnified by drought deterioration, especially in basically severe drought conditions. The total column water vapor and the horizontal divergence of the vapor flux, as well as temperature, directly affected precipitation and potential evapotranspiration and led to soil moisture drought through various direct and indirect processes. This study highlighted that the interactions among precipitation, potential evapotranspiration, and atmospheric vapor movement state in space and time were important for understanding the drought development mechanisms. 

A pathway analysis method for quantifying the contributions of precipitation and potential evapotranspiration anomalies to soil moisture drought

The quantification of subsurface discharges may significantly be affected by multiple sources of uncertainty, especially in highly heterogeneous aquifers. In this work, subsurface discharges within alluvial aquifers located in the province of Lecco (Lombardy, Northern Italy) are quantified relying upon a Monte Carlo (MC) framework under geological and conditioning data uncertainty sources. These are respectively employed through different geological conceptual models, whose facies spatial distributions are simulated through Sequential Indicator Simulations methodology (SISIM), and by conditioning stochastic facies simulations with different data constraints. Stochastic porosity fields are then achieved upon facies simulated fields by associating average porosity values to each local simulated facies. Then, stochastic subsurface discharge fluxes are quantified under steady hydraulic conditions upon saturated hydraulic fields, inferred from porosity ones. Then, the study of subsurface discharges is pursued via the Generalized Extreme Values (GEV) approach. This method enables both to interpret peak-over-thresholds (POTs) discharges spatial distribution, and to study the role geological and data uncertainty sources on their quantification, for example studying discharges conveyed towards a lake through the surrounding aquifer. These uncertainty-based analyses show that subsurface discharges clearly convey in correspondence of main alluvial aquifers, as well as the tendency to be collected along preferential pathways. Furthermore, the inspection of POTs empirical probability spatial distributions highlights that the geological uncertainty plays as a primary uncertainty source rather than the conditioning data. The latter plays a minor role, e.g. between different employed models, only in presence of a relevant geological difference between analyzed ones. These results are consistent with other kinds of information, such as hydrogeological sections and available piezometric maps, providing novel contributions to understanding how to quantify subsurface discharges in poorly monitored aquifers or how to treat various sources of uncertainty. 

The role of different sources of uncertainty on the stochastic quantification of subsurface discharges in heterogeneous aquifers

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2020WR029079

Effects of Heterogeneous Stream-Groundwater Exchange on the Source Composition of Stream Discharge and Solute Load

A New Approach to Three-Dimensional Flow in a Pumped Confined Aquifer Connected to a Shallow Stream: Near-Stream and Far-From-Stream Groundwater Extractions

Evaluating Baseflow Simulation in the National Water Model: A Case Study in the Northern High Plains Region, USA

The aim of this study was to develop a soil moisture simulation model equipped with meteorological data enhanced by WRF (Weather Research and Forecast) model, and this soil moisture model was applied for quantifying soil moisture content and irrigation requirement. The WRF model can provide grid based meteorological data at various resolutions. For applicability assessment, comparative analyses were conducted using WRF data and weather data obtained from weather station located close to test bed. Water balance of each upland grid was assessed for soils represented with four layers. The soil moisture contents simulated using the soil moisture model were compared with observed data to evaluate the capacity of the model qualitatively and quantitatively with performance statistics such as correlation coefficient (R), coefficient of determination (R2) and root mean squared error (RMSE). As a result, R is 0.76, is 0.58 and RMSE 5.45 mm in soil layer 1 and R 0.61, 0.37 and RMSE 6.73 mm in soil layer 2 and R 0.52, 0.27 and RMSE 8.64 mm in soil layer 3 and R 0.68, 0.45 and RMSE 5.29 mm in soil layer 4. The estimated soil moisture contents and irrigation requirements of each soil layer showed spatiotemporally varied distributions depending on weather and soil texture data incorporated. The estimated soil moisture contents using weather station data showed uniform distribution about all grids. However the estimated soil moisture contents from WRF data showed spatially varied distribution. Also, the estimated irrigation requirements applied WRF data showed spatial variabilities reflecting regional differences of weather conditions.

Estimation of Soil Moisture and Irrigation Requirement of Upland using Soil Moisture Model applied WRF Meteorological Data

Application of high-resolution meteorological data from NCAM-WRF to characterize agricultural drought in small-scale farmlands based on soil moisture deficit

Understanding groundwater storage variations as a result of effects from both the vadose zone and the river is of critical importance in the hydraulically connected surface‐subsurface system. In this study, we present a novel Bidirectional Exchange Scheme in Surface and Subsurface (BE3S) that represents bidirectional exchanges between the vadose zone, phreatic aquifer, and river. The approach enables explicit representations of each flow regime while conserving mass, and successfully yields solutions of the coupled system for multiple temporal resolutions. We test the scheme by comparing the BE3S‐derived outputs against other models and corresponding observational data. We apply the scheme to simulate hydrologic states in a reach of the Brazos River in Southeast Texas, such as soil moisture content, groundwater level, and river stage as well as net subsurface discharge fluxes. Good agreements between the simulated and observed data for all the components show the suitability of the proposed scheme in modeling the bidirectional flows and exchanges. We also assess how the bidirectional hydrologic exchanges are affected by adjacent flow domains. We find that vertical percolation is significantly affected by unsaturated soil thickness, resulting in the spatial variability of vertical percolation across sloping topography. Hilltop‐to‐valley convergent groundwater flow is also found to impede vertical percolation in river valleys due to shallow water table while facilitating percolation in the hilltops. The capability of the presented scheme that accounts for the topographically driven lateral groundwater flow and drainage dynamics provides the potential to enhance the representation of the surface‐subsurface system in Earth System Models (ESMs).

An Explicit Scheme to Represent the Bidirectional Hydrologic Exchanges Between the Vadose Zone, Phreatic Aquifer, and River

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Evaluating Baseflow Simulation in the National Water Model: A Case Study in the Northern High Plains Region, USA

Estimation of Soil Moisture and Irrigation Requirement of Upland using Soil Moisture Model applied WRF Meteorological Data

Application of high-resolution meteorological data from NCAM-WRF to characterize agricultural drought in small-scale farmlands based on soil moisture deficit

An Explicit Scheme to Represent the Bidirectional Hydrologic Exchanges Between the Vadose Zone, Phreatic Aquifer, and River