Afghanistan is suffering from periodic events of drought, which has exacerbated in recent years due to extreme climate events in the region. Having an arid to semi-arid climate, the country faces significant challenges of water resources management, especially for irrigation as reliance on agriculture is cumbersome. This study is undertaken to characterize historical meteorological drought in Afghanistan to provide an insight on where and when meteorological drought events happened in different River Basins (RBs). The study mainly employs the gamma-Standardized Precipitation Index (gamma-SPI) to analyze historical meteorological droughts across Afghanistan from 1979 to 2019. Monthly precipitation data is obtained from the Ministry of Energy and Water (MEW) of Afghanistan, which is a combination of observed data from ground stations and gap-filled data by the MEW for the study period. Gridded gamma-SPI values are interpolated and mapped to visualize patterns of spatial drought across the entire country. The results indicate that countrywide extreme drought events occurred in 1999, 2000, 2001, 2010, 2016, 2017, and 2019, particularly affecting southern, western, and southwestern regions. Decreasing rainfall occurred in all five RBs, with the most considerable decline observed in the 1999–2008 period. The study reveals the increasing frequency and severity of meteorological droughts in Afghanistan. It also emphasizes on the vulnerability of agriculture and water sectors due to the drought events. The findings of the study suggest the need for better drought monitoring, preparedness, awareness, and adaptation of strategies to ensure water security and agricultural sustainability in the face of climate change.

Full waveform inversion (FWI) updates the subsurface model from an initial model by comparing observed and synthetic seismograms. Due to high nonlinearity, FWI is easy to be trapped into local minima. Extended domain FWI, including wavefield reconstruction inversion (WRI) and extended source waveform inversion (ESI) are attractive options to mitigate this issue. This paper makes an in-depth analysis for FWI in the extended domain, identifying key challenges and searching for potential remedies towards practical applications. WRI and ESI are formulated within the same mathematical framework using Lagrangianbased adjoint-state method with a special focus on time-domain formulation using extended sources, while putting connections between classical FWI, WRI and ESI: both WRI and ESI can be viewed as weighted versions of classic FWI. Due to symmetric positive definite Hessian, the conjugate gradient is explored to efficiently solve the normal equation in a matrix free manner, while both time and frequency domain wave equation solvers are feasible. This study finds that the most significant challenge comes from the huge storage demand to store time-domain wavefields through iterations. To resolve this challenge, two possible workaround strategies can be considered, i.e., by extracting sparse frequencial wavefields or by considering time-domain data instead of wavefields for reducing such challenge. We suggest that these options should be explored more intensively for tractable workflows.

Abstract A key element of successful nuclear monitoring involves the assessment of any newly acquired data in the light of the vast amount of data previously recorded from historical nuclear explosions. To support this effort, the “Waveforms From Nuclear Explosions (WFNE)” repository was developed and is maintained by Leidos under sponsorship of the Defense Threat Reduction Agency (DTRA) to provide convenient access to such historical data. It is built as a trusted data set, starting from the previous data repository “Nuclear Explosion Database (NEDB)” that was accessed in the past by numerous users in the US and international nuclear explosion monitoring community. WFNE includes detailed information (origin, bulletin, other geophysical data) on all the known 2157 atmospheric, underground, and underwater nuclear explosions detonated in the world between 1945 and 2017. It includes over 82,000 waveforms associated to 802 of the nuclear explosions ranging from digitized analog recordings for the oldest explosions to recent International Monitoring System (IMS) data, in CSS and SAC formats, and their station/instrument information, as collected from many sources. The web-based access and the presentation were updated and modernized and rendered ready for active user access. Users can search, visualize, and download data of interest for their own research. Data continues to be collected from newly identified sources. Recent efforts to rescue pre-digital seismic data via scanning and digitization provide interesting information to be added to WFNE, after completeness and quality checks. WFNE is open for the research community’s access to source parameter data and associated waveforms from worldwide nuclear explosions.