All Stories

  1. Disentangling climate-driven and vegetation-mediated impacts on water resources in the upper Yellow River Basin
  2. A multi-chain surrogate-assisted hybrid optimization framework for joint identification of groundwater contaminant sources and hydrogeological parameters
  3. SWAT-UQ: A platform for uncertainty analysis, calibration and optimization of SWAT models
  4. Expressing Gratitude to Reviewers: A Message From the Editors of Reviews of Geophysics for 2025
  5. Evaluating microphysics scheme impacts on summer precipitation in Northwestern China using a convection permitting WRF model
  6. Integrated multi-dimensional framework for water conservation capacity evaluation and attribution in the Yellow River water conservation area
  7. Precipitation observing network gaps limit climate change impact assessment
  8. Statistical Postprocessing of Subseasonal Cumulative Precipitation Forecasts Using a Spatial Heterogeneity-aware U-Net
  9. Integrating geospatial intelligence and machine learning for flood susceptibility mapping
  10. An Efficient Global Automatic Threshold Detection Algorithm for Large‐Scale Flood Distribution Analysis
  11. The effects of teleconnections and monsoon on karst spring discharge based on wavelet phase difference and partial wavelet coherence
  12. Added Value of Convection‐Permitting WRF Modeling for Daily and Sub‐Daily Summer Precipitation Over Northwestern China
  13. A Review of the Past Half Century of Geostationary Satellite Thermal Observations for Global Precipitation Estimation: Developments, achievements, and future prospects
  14. Scale Effect on Evapotranspiration: Predicting the Continental and Global Scale Water Balance Based on Percolation Theory and Optimality Principle
  15. Estimating multi-source input uncertainties and their propagation for integrated water quantity and quality simulations
  16. Latent diffusion model for quantitative precipitation estimation and forecast at km scale
  17. Warming climate and water withdrawals threaten river flow connectivity in China
  18. Vegetation and wind speed dominate precipitation-evaporation recycling processes during 1980–2021
  19. Advancing the Reliability of Future Hydrological Projections in a Snow‐Dominated Alpine Watershed: Integrating Uncertainty Decomposition and CycleGAN Bias Correction
  20. Anatomy and assessment of surface water and energy balance components simulated by CMIP6 models in Pan Third Pole
  21. The Needs, Challenges, and Priorities for Advancing Global Flood Research
  22. Quantifying the contributions of hydrological pre-processor, post-processor, and data assimilator to ensemble streamflow prediction skill
  23. Commitment to Advance Excellence and Inclusion in the Earth and Space Sciences Scholarly Publications
  24. Coupled hydrologic and hydraulic modeling for a lowland river basin in China
  25. Expressing Gratitude to Reviewers: A Message From the Editors of Reviews of Geophysics for 2024
  26. A combined wavelet analysis-quantile mapping (WA-QM) method for bias correction: capturing the intra-annual temporal patterns in climate model precipitation simulations and projections
  27. Surrogate modelling-based multi-objective optimization for best management practices of nonpoint source pollution
  28. Advancing river flood forecasting with a collaborative integrated modeling method
  29. Impacts of Different Satellite‐Based Precipitation Signature Errors on Hydrological Modeling Performance Across China
  30. Attribution of Vegetation Dynamics in the Yellow River Water Conservation Area Based on the Deep ConvLSTM Model
  31. Unsupervised deep learning bias correction of CMIP6 global ensemble precipitation predictions with cycle generative adversarial network
  32. Learning Distributed Parameters of Land Surface Hydrologic Models Using a Generative Adversarial Network
  33. Observed precipitation underestimates reality across the Asian water tower
  34. Expressing Gratitude to Reviewers: A Message From the Editors of Reviews of Geophysics for 2023
  35. An open online simulation strategy for hydrological ensemble forecasting
  36. Land Data Assimilation: Harmonizing Theory and Data in Land Surface Process Studies
  37. Satellite-based precipitation error propagation in the hydrological modeling chain across China
  38. Reliable precipitation nowcasting using probabilistic diffusion models
  39. Investigating the Performance of CMIP6 Seasonal Precipitation Predictions and a Grid Based Model Heterogeneity Oriented Deep Learning Bias Correction Framework
  40. Knee Point‐Based Multiobjective Optimization for the Numerical Weather Prediction Model in the Greater Beijing Area
  41. Spatiotemporal Variations in Precipitation Forecasting Skill of Three Global Subseasonal Prediction Products over China
  42. Uncertainty Quantification for the Noah‐MP Land Surface Model: A Case Study in a Grassland and Sandy Soil Region
  43. A surrogate modeling method for distributed land surface hydrological models based on deep learning
  44. Reliability of Ensemble Climatological Forecasts
  45. Hydrological Modeling in the Upper Lancang-Mekong River Basin Using Global and Regional Gridded Meteorological Re-Analyses
  46. Hydrological Modeling in the Upper Lancang-Mekong River Basin Using Global and Regional Gridded Meteorological Reanalyses
  47. Hydrological response to climate change and human activities in the Three-River Source Region
  48. A brief review of the coupled human-Earth system modeling: Current state and challenges
  49. The First 30 Years of GEWEX
  50. The performance of CMIP6 models in simulating surface energy fluxes over global continents
  51. Seasonality and Impact Factor Analysis of Streamflow Sensitivity to Climate Change Across China
  52. Towards reliable uncertainty quantification for hydrologic predictions, Part I: Development of a particle copula Metropolis Hastings method
  53. Towards reliable uncertainty quantification for hydrologic predictions, part II: Characterizing impacts of uncertain factors through an iterative factorial data assimilation framework
  54. Evaluation and Statistical Post‐Processing of Two Precipitation Reforecast Products During Summer in the Mainland of China
  55. Bayesian retro- and prospective assessment of CMIP6 climatology in Pan Third Pole region
  56. Hydroclimatic extremes and impacts in a changing environment: Observations, mechanisms, and projections
  57. Thank You to Our 2021 Peer Reviewers
  58. High-quality reconstruction of China’s natural streamflow
  59. Xiaolangdi Dam: A valve for streamflow extremes on the lower Yellow River
  60. A multi-objective adaptive surrogate modelling-based optimization algorithm for constrained hybrid problems
  61. Convolutional neural network-based statistical post-processing of ensemble precipitation forecasts
  62. Differences in parameter estimates derived from various methods for the ORYZA (v3) Model
  63. Tracing Uncertainty Contributors in the Multi‐Hazard Risk Analysis for Compound Extremes
  64. A Combined Optimization‐Assimilation Framework to Enhance the Predictive Skill of Community Land Model
  65. Effect of sensitivity analysis on parameter optimization: Case study based on streamflow simulations using the SWAT model in China
  66. Evaluation of historical CMIP6 model simulations and future projections of temperature over the Pan-Third Pole region
  67. Numerical Investigation and Uncertainty Analysis of Eastern China’s Large-Scale Urbanization Effect on Regional Climate
  68. Quantifying physical parameterization uncertainties associated with land-atmosphere interactions in the WRF model over Amazon
  69. Corrigendum
  70. Multi‐Objective Adaptive Surrogate Modeling‐Based Optimization for Distributed Environmental Models Based on Grid Sampling
  71. Changes in Unevenness of Wet‐Day Precipitation Over China During 1961–2020
  72. Future Climate Change Hotspots Under Different 21st Century Warming Scenarios
  73. Sub-regional groundwater storage recovery in North China Plain after the South-to-North water diversion project
  74. Open Science: Open Data, Open Models, …and Open Publications?
  75. High-Resolution SMAP Satellite Soil Moisture Product: Exploring the Opportunities
  76. Understanding the spatial patterns of evapotranspiration estimates from land surface models over China
  77. Thank You to Our Peer Reviewers for 2020
  78. The Future of Sensitivity Analysis: An essential discipline for systems modeling and policy support
  79. Bias Correction and Ensemble Projections of Temperature Changes over Ten Subregions in CORDEX East Asia
  80. Global surface air temperatures in CMIP6: historical performance and future changes
  81. The Effectiveness of the South‐to‐North Water Diversion Middle Route Project on Water Delivery and Groundwater Recovery in North China Plain
  82. The Performance of CMIP6 Versus CMIP5 in Simulating Temperature Extremes Over the Global Land Surface
  83. Advancing Precipitation Estimation, Prediction, and Impact Studies
  84. Evaluation and projection of daily maximum and minimum temperatures over China using the high-resolution NEX-GDDP dataset
  85. Quantifying Water Scarcity in Northern China Within the Context of Climatic and Societal Changes and South‐to‐North Water Diversion
  86. Do CFSv2 Seasonal Forecasts Help Improve the Forecast of Meteorological Drought over Mainland China?
  87. Impact of rural depopulation and climate change on vegetation, runoff and sediment load in the Gan River basin, China
  88. An Objective Approach to Generating Multi-Physics Ensemble Precipitation Forecasts Based on the WRF Model
  89. The Changing Relationship Between Rainfall and Surface Runoff on the Loess Plateau, China
  90. Assessing the sensitivity of land-atmosphere coupling strength to boundary and surface layer parameters in the WRF model over Amazon
  91. Possible Increased Frequency of ENSO-Related Dry and Wet Conditions over Some Major Watersheds in a Warming Climate
  92. Variations in start date, end date, frequency and intensity of yearly temperature extremes across China during the period 1961–2017
  93. Ensemble flood forecasting: Current status and future opportunities
  94. Improved Land Evapotranspiration Simulation of the Community Land Model Using a Surrogate-Based Automatic Parameter Optimization Method
  95. Evaluation of parameter interaction effect of hydrological models using the sparse polynomial chaos (SPC) method
  96. Thank You to Our Peer Reviewers for 2019
  97. The energy and water cycles under climate change
  98. Improving WRF Typhoon Precipitation and Intensity Simulation Using a Surrogate-Based Automatic Parameter Optimization Method
  99. Sensitivity Analysis‐Based Automatic Parameter Calibration of the VIC Model for Streamflow Simulations Over China
  100. A Variable-Correlation Model to Characterize Asymmetric Dependence for Postprocessing Short-Term Precipitation Forecasts
  101. Factors Influencing the Performance of Regression-Based Statistical Postprocessing Models for Short-Term Precipitation Forecasts
  102. Improving WRF model turbine-height wind-speed forecasting using a surrogate- based automatic optimization method
  103. Parameter Sensitivity Analysis for Computationally Intensive Spatially Distributed Dynamical Environmental Systems Models
  104. An improved meta-Gaussian distribution model for post-processing of precipitation forecasts by censored maximum likelihood estimation
  105. Assessment and Reduction of the Physical Parameterization Uncertainty for Noah‐MP Land Surface Model
  106. Global heat stress on health, wildfires, and agricultural crops under different levels of climate warming
  107. How parameter specification of an Earth system model of intermediate complexity influences its climate simulations
  108. Multiple‐Wavelet Coherence of World's Large Rivers With Meteorological Factors and Ocean Signals
  109. Combinatorial Optimization for WRF Physical Parameterization Schemes: A Case Study of Three-Day Typhoon Simulations over the Northwest Pacific Ocean
  110. Dynamics and Attributions of Baseflow in the Semiarid Loess Plateau
  111. Comparison of the Generalized Likelihood Uncertainty Estimation and Markov Chain Monte Carlo Methods for Uncertainty Analysis of the ORYZA_V3 Model
  112. Non-uniform changes in different categories of precipitation intensity across China and the associated large-scale circulations
  113. Drought Characteristics and Propagation in the Semiarid Heihe River Basin in Northwestern China
  114. Assessment of Parametric Sensitivity Analysis Methods Based on A Quasi Two-Dimensional Groundwater Model
  115. Seasonal drought predictability and forecast skill in the semi-arid endorheic Heihe River basin in northwestern China
  116. Analysis of precipitation characteristics on the loess plateau between 1965 and 2014, based on high-density gauge observations
  117. Meteorological and Hydrological Drought on the Loess Plateau, China: Evolutionary Characteristics, Impact, and Propagation
  118. Vegetation-Climate Interactions on the Loess Plateau: A Nonlinear Granger Causality Analysis
  119. A systematic assessment and reduction of parametric uncertainties for a distributed hydrological model
  120. An estimate of human and natural contributions to changes in water resources in the upper reaches of the Minjiang River
  121. Long-term trends in global river flow and the causal relationships between river flow and ocean signals
  122. Spatiotemporal Changes in Extreme Temperature and Precipitation Events in the Three-Rivers Headwater Region, China
  123. 2015–16 floods and droughts in China, and its response to the strong El Niño
  124. Shuffled Complex-Self Adaptive Hybrid EvoLution (SC-SAHEL) optimization framework
  125. Dynamic Manning's roughness coefficients for hydrological modelling in basins
  126. A nonparametric standardized runoff index for characterizing hydrological drought on the Loess Plateau, China
  127. Parameter optimization for carbon and water fluxes in two global land surface models based on surrogate modelling
  128. A Review of Global Precipitation Data Sets: Data Sources, Estimation, and Intercomparisons
  129. Changes in the Spatial Heterogeneity and Annual Distribution of Observed Precipitation across China
  130. Drainage network extraction from a high-resolution DEM using parallel programming in the .NET Framework
  131. A review on statistical postprocessing methods for hydrometeorological ensemble forecasting
  132. An adaptive surrogate modeling-based sampling strategy for parameter optimization and distribution estimation (ASMO-PODE)
  133. Improvement of rank histograms for verifying the reliability of extreme event ensemble forecasts
  134. Bayesian multi-model projections of extreme hydroclimatic events under RCPs scenarios
  135. Unraveling anthropogenic influence on the changing risk of heat waves in China
  136. Assessing the applicability of WRF optimal parameters under the different precipitation simulations in the Greater Beijing Area
  137. Detecting the quantitative hydrological response to changes in climate and human activities
  138. Wavelet-based variability of Yellow River discharge at 500-, 100-, and 50-year timescales
  139. Automatic Model Calibration: A New Way to Improve Numerical Weather Forecasting
  140. Climate Change and Water: Case Study on Eastern Monsoon Region of China
  141. Integrating weather and climate predictions for seamless hydrologic ensemble forecasting: A case study in the Yalong River basin
  142. Parametric sensitivity analysis of precipitation and temperature based on multi-uncertainty quantification methods in the Weather Research and Forecasting model
  143. Seasonal drought ensemble predictions based on multiple climate models in the upper Han River Basin, China
  144. The Art and Science of Climate Model Tuning
  145. The nonstationary impact of local temperature changes and ENSO on extreme precipitation at the global scale
  146. Contribution analysis of the long-term changes in seasonal runoff on the Loess Plateau, China, using eight Budyko-based methods
  147. A Comprehensive Evaluation of Microwave Emissivity and Brightness Temperature Sensitivities to Soil Parameters Using Qualitative and Quantitative Sensitivity Analyses
  148. Modeling streamflow and sediment responses to climate change and human activities in the Yanhe River, China
  149. Handbook of Hydrometeorological Ensemble Forecasting
  150. Assessing the weighted multi-objective adaptive surrogate model optimization to derive large-scale reservoir operating rules with sensitivity analysis
  151. Record-Breaking Heat in Northwest China in July 2015: Analysis of the Severity and Underlying Causes
  152. Environmental impact assessments of the Xiaolangdi Reservoir on the most hyperconcentrated laden river, Yellow River, China
  153. An evaluation of parametric sensitivities of different meteorological variables simulated by the WRF model
  154. Quantification and attribution of errors in the simulated annual gross primary production and latent heat fluxes by two global land surface models
  155. Hydrological monitoring and seasonal forecasting: Progress and perspectives
  156. Century-scale causal relationships between global dry/wet conditions and the state of the Pacific and Atlantic Oceans
  157. Impact assessment of climate change and human activities on net runoff in the Yellow River Basin from 1951 to 2012
  158. A nonstationary bias-correction technique to remove bias in GCM simulations
  159. A statistical model for karst spring discharge estimation under extensive groundwater development and extreme climate change
  160. Uncertainty Quantification in Climate Modeling and Projection
  161. Functional degradation of the water–sediment regulation scheme in the lower Yellow River: Spatial and temporal analyses
  162. Linkage Between Hourly Precipitation Events and Atmospheric Temperature Changes over China during the Warm Season
  163. Multiobjective adaptive surrogate modeling-based optimization for parameter estimation of large, complex geophysical models
  164. Joint analysis of changes in temperature and precipitation on the Loess Plateau during the period 1961–2011
  165. A GUI platform for uncertainty quantification of complex dynamical models
  166. Linkages between Large-Scale Climate Patterns and Karst Spring Discharge in Northern China
  167. Bi-objective analysis of water–sediment regulation for channel scouring and delta maintenance: A study of the lower Yellow River
  168. Temperature and precipitation changes over the Loess Plateau between 1961 and 2011, based on high-density gauge observations
  169. Evaluation of the PERSIANN-CDR Daily Rainfall Estimates in Capturing the Behavior of Extreme Precipitation Events over China
  170. The hydro-environmental response on the lower Yellow River to the water–sediment regulation scheme
  171. Comparative analysis of CMIP3 and CMIP5 global climate models for simulating the daily mean, maximum, and minimum temperatures and daily precipitation over China
  172. Multi-objective parameter optimization of common land model using adaptive surrogate modeling
  173. Stepwise sensitivity analysis from qualitative to quantitative: Application to the terrestrial hydrological modeling of a Conjunctive Surface-Subsurface Process (CSSP) land surface model
  174. Evaluating the skill of NMME seasonal precipitation ensemble predictions for 17 hydroclimatic regions in continental China
  175. Extreme climate events and agricultural climate indices in China: CMIP5 model evaluation and projections
  176. Assessing WRF model parameter sensitivity: A case study with 5 day summer precipitation forecasting in the Greater Beijing Area
  177. Development of a large-sample watershed-scale hydrometeorological data set for the contiguous USA: data set characteristics and assessment of regional variability in hydrologic model performance
  178. The Gravity Environment of Zhouqu Debris Flow of August 2010 and Its Implication for Future Recurrence
  179. Evolution of the Yellow River Delta and its relationship with runoff and sediment load from 1983 to 2011
  180. An Intercomparison of Sampling Methods for Uncertainty Quantification of Environmental Dynamic Models
  181. Post-processing of ensemble forecasts in low-flow period
  182. An evaluation of post-processed TIGGE multimodel ensemble precipitation forecast in the Huai river basin
  183. An evaluation of adaptive surrogate modeling based optimization with two benchmark problems
  184. Evaluating Skill of Seasonal Precipitation and Temperature Predictions of NCEP CFSv2 Forecasts over 17 Hydroclimatic Regions in China
  185. A Bayesian analysis of nonstationary generalized extreme value distribution of annual spring discharge minima
  186. Projected changes in temperature and precipitation in ten river basins over China in 21st century
  187. Variations in global temperature and precipitation for the period of 1948 to 2010
  188. Assessment of CMIP5 climate models and projected temperature changes over Northern Eurasia
  189. A global soil data set for earth system modeling
  190. Hydrologic post-processing of MOPEX streamflow simulations
  191. Would the ‘real’ observed dataset stand up? A critical examination of eight observed gridded climate datasets for China
  192. A comprehensive evaluation of various sensitivity analysis methods: A case study with a hydrological model
  193. The impact of the South-North Water Transfer Project (CTP)'s central route on groundwater table in the Hai River basin, North China
  194. Assessing parameter importance of the Common Land Model based on qualitative and quantitative sensitivity analysis
  195. Evaluation and application of Bayesian multi-model estimation in temperature simulations
  196. Development of a China Dataset of Soil Hydraulic Parameters Using Pedotransfer Functions for Land Surface Modeling
  197. A China data set of soil properties for land surface modeling
  198. WHY WAS THE AUGUST 2010 ZHOUQU LANDSLIDE SO POWERFUL?
  199. Improving kinematic wave routing scheme in Community Land Model
  200. On the Applicability of Temperature and Precipitation Data from CMIP3 for China
  201. Evaluating the predictive skill of post‐processed NCEP GFS ensemble precipitation forecasts in China's Huai river basin
  202. Developed and developing world responsibilities for historical climate change and CO 2 mitigation
  203. Landslides Caused Deforestation
  204. Continental-scale water and energy flux analysis and validation for North American Land Data Assimilation System project phase 2 (NLDAS-2): 2. Validation of model-simulated streamflow
  205. Continental-scale water and energy flux analysis and validation for the North American Land Data Assimilation System project phase 2 (NLDAS-2): 1. Intercomparison and application of model products
  206. Combining Simulation and Emulation for Calibrating Sequentially Reactive Transport Systems
  207. A hydrologic post-processor for ensemble streamflow predictions
  208. Bayesian estimation of local signal and noise in multimodel simulations of climate change
  209. Reply to Comment by B. Renard et al. on “An integrated hydrologic Bayesian multimodel combination framework: Confronting input, parameter, and model structural uncertainty in hydrologic prediction”
  210. Comment on “Dynamically dimensioned search algorithm for computationally efficient watershed model calibration” by Bryan A. Tolson and Christine A. Shoemaker
  211. Regional Parameter Estimation of the VIC Land Surface Model: Methodology and Application to River Basins in China
  212. Multi-model ensemble hydrologic prediction using Bayesian model averaging
  213. An integrated hydrologic Bayesian multimodel combination framework: Confronting input, parameter, and model structural uncertainty in hydrologic prediction
  214. Multi-objective calibration of forecast ensembles using Bayesian model averaging
  215. Multimodel Combination Techniques for Analysis of Hydrological Simulations: Application to Distributed Model Intercomparison Project Results
  216. The model parameter estimation experiment (MOPEX)
  217. Model Parameter Estimation Experiment (MOPEX): An overview of science strategy and major results from the second and third workshops
  218. Western Pacific geophysics meeting in Beijing, China
  219. NOAA'S Advanced Hydrologic Prediction Service: Building Pathways for Better Science in Water Forecasting
  220. The distributed model intercomparison project (DMIP): motivation and experiment design
  221. Model parameter experiment begins new phase
  222. An intercomparison of soil moisture fields in the North American Land Data Assimilation System (NLDAS)
  223. Streamflow and water balance intercomparisons of four land surface models in the North American Land Data Assimilation System project
  224. The multi-institution North American Land Data Assimilation System (NLDAS): Utilizing multiple GCIP products and partners in a continental distributed hydrological modeling system
  225. Snow process modeling in the North American Land Data Assimilation System (NLDAS): 2. Evaluation of model simulated snow water equivalent
  226. Land surface model spin‐up behavior in the North American Land Data Assimilation System (NLDAS)
  227. Snow process modeling in the North American Land Data Assimilation System (NLDAS): 1. Evaluation of model‐simulated snow cover extent
  228. Surface radiation budgets in support of the GEWEX Continental‐Scale International Project (GCIP) and the GEWEX Americas Prediction Project (GAPP), including the North American Land Data Assimilation System (NLDAS) project
  229. Total water storage in the Arkansas‐Red River basin
  230. Evaluation of the North American Land Data Assimilation System over the southern Great Plains during the warm season
  231. Validation of the North American Land Data Assimilation System (NLDAS) retrospective forcing over the southern Great Plains
  232. Real‐time and retrospective forcing in the North American Land Data Assimilation System (NLDAS) project
  233. Effects of Frozen Soil on Soil Temperature, Spring Infiltration, and Runoff: Results from the PILPS 2(d) Experiment at Valdai, Russia
  234. Global optimization for watershed model calibration
  235. Preface
  236. Use of a priori parameter estimates in the derivation of spatially consistent parameter sets of rainfall-runoff models
  237. The Representation of Snow in Land Surface Schemes: Results from PILPS 2(d)
  238. A Priori estimation of land surface model parameters
  239. Simulations of a Boreal Grassland Hydrology at Valdai, Russia: PILPS Phase 2(d)
  240. A parameterization of snowpack and frozen ground intended for NCEP weather and climate models
  241. Scale dependencies of hydrologic models to spatial variability of precipitation
  242. The Project for Intercomparison of Land-surface Parameterization Schemes (PILPS) phase 2(c) Red–Arkansas River basin experiment:
  243. The Project for Intercomparison of Land-surface Parameterization Schemes (PILPS) Phase 2(c) Red–Arkansas River basin experiment:
  244. The Project for Intercomparison of Land-surface Parameterization Schemes (PILPS) phase 2(c) Red-Arkansas River basin experiment:
  245. Correction to “FIFE 1987 water budget analysis” by Q. Y. Duan, J. C. Schaake, and V. I. Koren
  246. FIFE 1987 water budget analysis
  247. Simple water balance model for estimating runoff at different spatial and temporal scales
  248. Modeling of land surface evaporation by four schemes and comparison with FIFE observations
  249. Optimal use of the SCE-UA global optimization method for calibrating watershed models
  250. Calibration of rainfall-runoff models: Application of global optimization to the Sacramento Soil Moisture Accounting Model
  251. Shuffled complex evolution approach for effective and efficient global minimization
  252. Effective and efficient global optimization for conceptual rainfall-runoff models
  253. A maximum likelihood criterion for use with data collected at unequal time intervals