Publications

This study compares the accuracy and error characteristics of surface soil moisture (SSM) estimates obtained from various satellite and model-based data products over vegetated and irrigated regions. The study employed triple collocation analysis (TCA) and conventional error metrics to evaluate the accuracy of six different products: Advanced Scatterometer (ASCAT), Soil Moisture and Ocean Salinity (SMOS), Advanced Microwave Scanning Radiometer 2 (AMSR2), Soil Moisture Active Passive (SMAP), European Centre for Medium-Range Weather Forecasts Reanalysis 5 (ERA5), and Global Land Data Assimilation System (GLDAS). The results show that satellite-based SSM estimates from ASCAT, SMAP, and SMOS had fewer errors than ERA5 and GLDAS SSM products over vegetated areas, and over irrigated areas, ASCAT, SMOS, and SMAP outperformed other SSM products. The study also found that the limitations in satellite and model-based SSM data can be overcome by the synergistic use of satellite and model-based SSM products. The study suggests that the probability of obtaining SSM with a stronger signal than noise can be close to 100% when four satellite and model data sets are used selectively.

In recent years Vietnam has experienced historical drought events possibly affected by climate change, but the analysis is challenging due to lack of necessary observations for monitoring drought conditions. The goal of this study is to analyze the characteristics of droughts over a 30-year period, using three spatial-resolution MERRA-2 datasets in Vietnam. The Standardized Precipitation Evapotranspiration Index (SPEI) was used as an index for drought based on precipitation and temperature. We also estimated the impacts of drought on agriculture using annual land cover datasets.

This study compares the accuracy of six types of portable electromagnetic (EM) sensors for measuring soil water content (SWC). The study found that all SWC probes met the target accuracy after onsite correction with an RMSD of less than 0.025 m3 m−3. The study also observed that SWC data obtained from similar electrode lengths and from different manufacturers showed similar distributions over time with the same mean. Furthermore, combining SWC data from two different types of sensors using the maximize R method increased the accuracy of the results. The study found that the Pearson's correlation coefficient (R value) and RMSD values improved when datasets from two different types of sensors were combined, with an average R value improvement from .930 to .945, and the RMSD decreasing from 0.036 to 0.018 m3 m−3. These findings suggest that using multiple manufacturers’ EM-based SWC probes with site-specific correction can improve the accuracy of SWC measurements.

This study investigates the impact of air-borne dust on the Middle East and North Africa region, and the effects of dust emissions originating from the Tigris-Euphrates basin on the air quality of the entire Arabian Peninsula. The study uses a newly developed high-resolution (~500 m) source function in Weather Research and Forecasting model coupled with chemistry (WRF-Chem) to simulate dust emission and evaluate results against observations. The study finds that the use of the new source function provides reasonable estimates of dust optical depth and concentrations, and that the atmospheric dust originating from the Tigris-Euphrates basin exceeds the particulate matter 10 air quality standards in several downwind cities. The study suggests that coordinated management of the Tigris-Euphrates basin is necessary to maintain good air quality across the Arabian Peninsula and highlights potential environmental implications related to mobilization of depleted uranium deposited in Kuwait and Southern Iraq during the Gulf War (1991).

This study proposes a method to detect global irrigated areas by combining satellite and reanalysis datasets. The proposed method assumes that irrigation is an unmodeled land surface process, while satellite observations can effectively detect irrigation signals in near real-time. The study uses three irrigation-dependent variables, soil moisture (SM), land surface temperature (LST), and surface albedo (AL), to derive the spatial extents of irrigation by calculating the difference between the remotely sensed and reanalysis datasets. The proposed irrigation map is compared to commonly used global irrigation maps, and the study finds that combining the individual detection maps shows reasonable agreement with the reference irrigated maps, overlapping with approximately 70% of the irrigated areas. The study suggests that the proposed method, alone or in combination with existing irrigation maps, can benefit studies regarding water and energy balance closure in near-real time for large-scale land surface models, minimizing uncertainties in model parameterization. Overall, the study highlights the importance of understanding the reliable extent and distribution of global irrigated areas, given the significant role of irrigation in meeting the world's food demand and modifying water and energy cycles.

This study explores the amount of precipitation stored in the topsoil layer (0-10 cm) across different vegetation and aridity indices on a global scale. The study uses data from four satellites and two reanalysis data sets to investigate spatial trends of stored precipitation. The study finds that drier and less vegetated soil retains more precipitation in the top layer of the soil, while wet and forested areas have a lower retention rate due to large runoff fluxes and plants intercepting water. Specifically, the topsoil retains 37% ± 11% of precipitated water three days after a rainfall event where the aridity index was greater than 5, while wet and forested areas retain 21% ± 2%. The study also conducts a sensitivity analysis of different sampling frequency values using modeled data sets to calculate the stored precipitation fraction metric. Overall, the study highlights the importance of understanding the spatial trends of stored precipitation in the topsoil layer for better land-atmosphere interactions.