Publications

Check our peer-reviewed journal papers and conference papers.

Investigating the vulnerability and resilience of different land cover types to flash drought: A case study in the Mississippi River Basin

Journal of Environmental Management

July 1, 2025

Flash droughts are fast-developing droughts that can seriously affect both nature and human activities. This study examines how different types of land, such as forests and farmland, respond to flash droughts in the Mississippi River Basin (MRB) from 2000 to 2022. Using a drought index (SAPEI) to detect drought events and plant growth data (GPP) to measure recovery time, researchers identified 315 flash droughts and analyzed the 10 most severe cases. Recovery times varied widely, from 8 to 120 days, with the longest delays occurring in extreme drought years like 2006, 2012, and 2022. Forested areas bounced back quickly, while farmland, especially rain-fed crops, took the longest to recover, showing their high vulnerability to sudden moisture loss. The Upper MRB, with drier conditions and heavy agricultural use, had the slowest recovery. These findings highlight the need for better drought management, including improved water use strategies and drought-resistant crops, to help vulnerable areas cope with future flash droughts.

Observational Analysis of Long-term Streamflow Response to Flash Drought in the Mississippi River Basin

Weather and Climate Extremes

June 1, 2025

This study looks at how sudden, intense droughts, called flash droughts, affect river flow in the Mississippi River Basin (MRB) from 1980 to 2022. Using a drought index called SAPEI, researchers identified over 1,000 flash droughts and found regional differences in how they occur. The eastern MRB has frequent but short droughts, the northwest has fewer but longer ones, and the southern MRB experiences the most severe droughts, influenced by upstream water use. A strong link was found between drought conditions and lower river flow, showing that SAPEI is a useful tool for tracking these impacts. This research helps improve water management and prepares for future droughts.

A Stand-Alone Framework for Predicting Spatiotemporal Errors in Satellite-Based Soil Moisture Using Tree-Based Models and Deep Neural Networks

GIScience & Remote Sensing

May 1, 2025

Soil moisture (SM) is a critical climate variable, and assimilating satellite SM into land surface models via land data assimilation (LDA) enhances continuous SM modeling and climate extreme monitoring. However, LDA often assumes static SM error dynamics, limiting accuracy. This study introduces a novel framework integrating triple collocation analysis (TCA) with machine learning (ML), including Light Gradient Boosting Machine (LGBM) and Deep Neural Networks (DNN), to quantify spatially and temporally continuous satellite-based SM errors globally.

Using only Soil Moisture Active Passive (SMAP) retrieval data, our TCA-based time-variant error prediction models successfully recover error information in regions where SMAP SM error data were previously unreliable. The SMAP-based model outperforms models relying on external datasets like the Global Land Data Assimilation System (GLDAS), offering a robust approach for improving LDA in data-scarce areas. This method also extends to other satellite-based geophysical datasets, broadening its applicability beyond SMAP.

From Theory to Hydrological Practice: Leveraging CYGNSS Data Over Seven Years for Advanced Soil Moisture Monitoring

Remote Sensing of Environment

January 31, 2025

This study explores the potential of the Cyclone Global Navigation Satellite System (CYGNSS) mission for soil moisture (SM) monitoring, highlighting its ability to capture fine-scale SM variability through high revisit frequencies at sub-daily intervals. While CYGNSS was originally designed for tropical cyclone monitoring, its seven-year data record demonstrates significant promise in reliably monitoring diurnal SM dynamics using spaceborne L-band bistatic radar and GNSS-Reflectometry (GNSS-R) technology.

Despite this potential, SM retrieval from CYGNSS remains limited by knowledge gaps and unique challenges tied to its technical design. This study addresses these gaps by analyzing CYGNSS real-world data, synthesizing recent advancements in mitigating external uncertainties, and improving SM inversion techniques. Notably, algorithm-related challenges include accurate partitioning of coherent and incoherent signal components and correcting attenuation effects caused by vegetation and surface roughness. Data-related challenges involve variations in CYGNSS spatial footprint, temporal frequency, signal penetration depth, and incidence angle changes, as well as excessive reliance on reference SM datasets for model calibration, validation, and training. The computational demands of processing CYGNSS’s rapid multi-sampling data further complicate its operational use.

Future research directions identified in this study focus on leveraging machine learning and deep learning approaches to improve CYGNSS SM data quality and quantity. Additionally, assimilating CYGNSS-derived SM data into physical models offers promising opportunities to enhance predictions of hydroclimatic variables and extreme climate events, addressing key challenges in water resource monitoring and climate resilience.

Over 60% precipitation transformed into terrestrial water storage in global river basins from 2002 to 2021

Communications Earth & Environment

January 31, 2025

This paper provides a novel quantitative assessment of the transformation of daily precipitation into terrestrial water storage across 121 global river basins over a two-decade period. The study introduces the average daily fraction of precipitation transformed into terrestrial water storage, leveraging enhanced terrestrial water storage statistical reconstruction and water storage data from the Gravity Recovery and Climate Experiment (GRACE) and its follow-on mission. We reveal that approximately 64% of land precipitation contributes to terrestrial water storage, with notable variations across different climatic and geographical regions. These findings offer critical insights into the interactions between precipitation, land surface processes, and climate change, providing valuable implications for hydrological modeling and future water resource management.

Improving Weather Forecast Skill of the Korean Integrated Model (KIM) by Assimilating SMAP Soil Moisture Anomalies

Quarterly Journal of the Royal Meteorological Society

November 1, 2024

This study examines the impact of assimilating Soil Moisture Active Passive (SMAP) data into the Korean Integrated Model (KIM) to improve global soil moisture estimates and weather forecasts. SMAP soil moisture retrievals are integrated into the Noah land surface model using the ensemble Kalman filter through NASA’s Land Information System. Experiments from March to July 2022 show that assimilating SMAP data, particularly with anomaly-based bias correction, significantly enhances soil moisture estimates and improves weather forecasts, especially in northern Africa and West

* = mentored by Dr. Kim

Changes in the Speed of the Global Terrestrial Water Cycle Due To Human Interventions

Hyunglok Kim, Wade T. Crow, and Venkataraman Lakshmi
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Under Preperation

Developing Independent CYGNSS Soil Moisture Retrieval Algorithm with Mitigated Vegetation Effects: Incorporating a Two-Step and Relative SNR Approaches

Ziyue Zhu, Hyunglok Kim*, Venkataraman Lakshmi
Jounar of Remote Sensing
minor revision

A Novel Soil Moisture Validation Method Utilizing Brightness Temperature

Ziyue Zhu, Runze Zhang, Bin Fang, Hyunglok Kim, Venkataraman Lakshmi
-
Major Revision

Towards Self-calibration of Rainfall Estimation through Soil Dynamics and its Signals Using Supervised and Unsupervised Machine Learning Clustering Methods over CONUS

Mohammad Saeedi, Hyunglok Kim, and Venkataraman Lakshmi
-
under review

Enhancing Detection of Flood-Inundated Areas using Novel Hybrid PoLSAR- Metaheuristic-Deep Learning Models

Fatima et al.
-
Under Preperation

Synergistic impact of simultaneously assimilating radar- and radiometer-based soil moisture retrievals on the performance of numerical weather prediction systems

Kwon et al.
Hydrology and Earth System Sciences
Under Review

Flood inundation mapping with CYGNSS over CONUS: a two-step machine- learning-based framework

Wang et al.
Journal of Hydrology
under review

Simultaneous Estimation of Soil Moisture and Soil Organic Matter from Dielectric Measurements - Part 1: Optimal Estimation Strategy

Park et al.
Agricultural and Forest Meteorology
under review

Simultaneous Estimation of Soil Moisture and Soil Organic Matter from in situ Dielectric - Part 2: Application of Optimal Estimation and Machine Learning Approaches

Park et al.
Agricultural and Forest Meteorology
under review

Systematic Modeling Errors Strongly Undermine The Value Of Land Data Assimilation Systems And Microwave Remote Sensing For Water Flux Estimation

W. T. Crow, H. Kim , S. Kumar
IEEE International Geoscience and Remote Sensing Symposium
July 21, 2023

Utilizing Bayesian Machine Learning for Analyzing Error Patterns in Global-Scale Soil Moisture Data

H. Kim , W. T. Crow, W. Wagner, X. Li, V. Lakshmi
Hydrology Machine Learning (HydroML) Symposium, Phase 2 at Berkeley Lab
May 1, 2023

Hydrological flash drought forecasting using meteorological flash drought indices and machine learning approaches – A case study in the Mississippi River Basin

S. Bakar, D. Quintero, M. Le, H. Kim, S. S. Adams, P. Beling, V. Lakshmi
American Geophysical Union, Fall Meeting
December 1, 2022

Reconstruction of the SMAP-based 12-hourly soil moisture product over the CONUS through water balance budgeting

R. Zhang, S. Kim, H. Kim, B. Fang, A. Sharma, V. Lakshmi
American Geophysical Union, Fall Meeting
December 1, 2022

Changes in Extreme Precipitation Patterns in the Meuse River Basin as a Driver of the July 2021 Flooding

B. Goffin, P. Kansara, H. Kim, V. Lakshmi
American Geophysical Union, Fall Meeting
December 1, 2022

Retrieving Runoff in Ungauged Basins using Satellite Observations of Rainfall and Soil Moisture

H. Kim, W. Crow
American Geophysical Union, Fall Meeting
December 1, 2022

Uncertainty Analysis Framework in the Water Balance Equation Using Bayesian Statistical Modeling Approach

H. Kim, W. Crow
American Geophysical Union, Fall Meeting
December 1, 2022

Global downscaling and assimilation of soil moisture

V. Lakshmi, B. Fang, H Kim
IAHS2022
March 1, 2022

Deep Learning and Bayesian Inference via Samplings and Variational Approximations to Characterize Spatially Continuous Global-scale Satellite-based Soil Moisture Error Patterns

H. Kim, V. Lakshmi
American Geophysical Union, Fall Meeting
December 1, 2021

Water Cycle in Different Time Scales: Analyzing the Impact of Human-driven Changes in Land Cover using Bayesian Inferences and Data Assimilation Methods

H. Kim, V. Lakshmi
American Geophysical Union, Fall Meeting
December 1, 2021

Contact me

If you have a keen interest in the intersection of climate change and its impact on hydrological research fields, I encourage you to consider pursuing a Master's, PhD, or postdoctoral position. By delving deeper into this critical area of study, you can play an essential role in addressing the world's most pressing environmental challenges and help safeguard our water resources, ecosystems, and communities. Your dedication and expertise can significantly contribute to the development of sustainable solutions and innovative approaches to hydrological research. Embark on this exciting journey and become part of the passionate community of scientists working towards a more resilient and environmentally responsible future.