Papers by Konstantine Georgakakos
Eos, Transactions American Geophysical Union, 2005
Fig. 3. (a) EW9605 Hydrosweep bathymetry (200-m grid interval, 100-m contour interval) of the Mon... more Fig. 3. (a) EW9605 Hydrosweep bathymetry (200-m grid interval, 100-m contour interval) of the Mona Rift region with structural interpretation based on HAWAII MR 1 sidescan sonar imagery and singlechannel seismic data. The epicenter of the 1918 magnitude 7.3 event is shown by the black star. The location of the seismic profile in Figure 3c is shown by solid white line. ESFZ indicates East Septentri onal Fault Zone. Mapped mass-wasting slumps are outlined by dashed white lines, (b) HMR1 sidescan imagery (17-m grid interval) clearly showing large crescentic cracks in carbonate platform, (c) Single-channel seismic profile across the Mona Rift and one of the multiple landslide deposits mapped. The 1918 tsunami could have been triggered by sea floor rupture along the Mona Canyon Fault and an associated submarine landslide. Original color image appears at back of this volume.
Water Resources Research, 1985
A method of generating synthetic radar-rainfall data is described. The data are generated by impo... more A method of generating synthetic radar-rainfall data is described. The data are generated by imposing random noise on a given, high-quality radar-rainfall field. Certain conditions are imposed on the resultant rainfall field so that the noise parameters are prespecified. The conditions pertain to the second order statistics of the generated rainfall fields: the mean, the variance, the correlation, and the variance of the logarithmic ratio of the resultant field to the original field. Accuracy of the generation method is evaluated from implementing a test case using Global Atmospheric Research Program Atlantic Tropical Experiment radar data. The method can be used in a number of different, mainly hydrologic, applications. These include validation of radar and rain gage data merging procedures, testing of various methods for computation of mean areal precipitation, and sensitivity analysis of rainfall-runoff models. !.
Water Resources Research, 1990
In this paper a state‐space formulation of the Muskingum‐Cunge routing scheme is proposed. The st... more In this paper a state‐space formulation of the Muskingum‐Cunge routing scheme is proposed. The state‐space formulation utilizes real‐time discharge measurements, accounts for modeling and observation errors, and allows real‐time updating through a Kalman filter estimator. The new model is tested in two different geotechnical conditions to forecast six‐hour discharge values in hypothetical channels. For realism, the geomorphologic characteristics of these case studies are determined on the basis of the regime theory. DWOPER, a field‐tested numerical dynamic routing model, was used to provide ground truth data for the validation of the proposed model.
Water Resources Research, 1984
A one‐dimensional, physically based precipitation model is formulated. Particular emphasis is pla... more A one‐dimensional, physically based precipitation model is formulated. Particular emphasis is placed on its utility for real‐time river flow forecasting. Thus, the model is in state‐space form, suitable for use with modern estimation theory techniques, and it uses only operationally readily available, meteorological variables as its input. Parameterization of the model components is based on well‐established observations and theories on water vapor condensation, precipitation mechanisms, and subcloud evaporation of falling hydrometeors. Model input consists of ground level station temperature, pressure, and dewpoint temperature observations. The model predicts the spatially averaged, ground surface precipitation rate in the characteristic area determined by the spatial scales of the input. The water equivalent mass condensed in a cloud column defined by the characteristic area is the model state. Key physical parameters in the formulation are the pressure at the cloud top, the heigh...
Journal of the Atmospheric Sciences, 1990
Journal of Hydrology, 1999
Threshold runoff is the amount of excess rainfall accumulated during a given time period over a b... more Threshold runoff is the amount of excess rainfall accumulated during a given time period over a basin that is just enough to cause flooding at the outlet of the draining stream. Threshold runoff estimates are indicators of maximal sustainable surface runoff for a given catchment, and are thus an essential component of flash flood warning systems. Used in conjunction with soil moisture accounting models and areal rainfall data, they form the basis of the US National Weather Service (NWS) flash flood watch/warning program. As part of their modernization and enhancement effort, the NWS determined that improved flash flood guidance and thus improved threshold runoff estimation is needed across the United States, with spatial resolution commensurate to that afforded by the WSR-88D (NEXRAD) radars. In this work, Geographic Information Systems (GIS) and digital terrain elevation databases have been used to develop a national system for determining threshold runoff. Estimates of threshold runoff are presented for several locations in the United States, including large portions of the states of Iowa, Oklahoma, and California, and using several options in computing threshold runoff. Analysis of the results indicates the importance of channel geometry in flash flood applications. Larger threshold runoff estimates were computed in Oklahoma (average value of 34 mm) than in Iowa (14 mm) or California (9.5 mm). Comparisons of the threshold runoff estimates produced by the GIS procedure with those based on manually computed unit hydrographs for the selected catchments are shown as a preliminary measure of the accuracy of the procedure. Differences of up to about 15 mm for hourly rainfall durations were obtained for basins larger than 50 km 2 .
Journal of Hydrology, 1999
The feasibility of using a bi-spectral frequency analysis method to estimate daily mean areal pre... more The feasibility of using a bi-spectral frequency analysis method to estimate daily mean areal precipitation (MAP) from 3hourly METEOSAT visible (VIS) and infrared (IR) images over the Blue Nile river catchment (about 35-40Њ East Longitude and 8-12Њ North Latitude) in support of hydrologic studies is explored. The data record used spans the period 14 July 14 August 1995. At first, the study addresses the spatiotemporal variability of the satellite images, the determination of the relationship between topography and satellite data and the inference of MAP from satellite images using the bi-spectral method and a multivariate regression. Validation of the estimated MAPs is carried out with data from a sparse raingauge network in the region. These estimates are next used in a sensitivity study to determine the dependence of the Blue Nile region hydrologic response on the type of precipitation forcing (raingauge-based vs. satellite-based estimates). A conceptual semi-distributed hydrologic model is used to simulate hydrologic processes pertaining to soil water and channel routing with a 1 × 1Њ resolution. Principal conclusions of this initial sensitivity study are: (a) use of the bi-spectral method, complemented with an appropriate multivariate regression formulation, improves MAP estimates during the aforementioned time interval substantially, for daily rainfall rates greater than about 7 mm day Ϫ1 ; (b) aggregate hydrologic response of the Blue Nile region is very sensitive to the type of precipitation forcing used; and (c) substantial spatial variability of the sensitivity of hydrologic response to the type of precipitation forcing exists in the region. The use of satellite-derived MAP estimates is recommended together with recalibration of hydrologic models using spatially variable parameter values, and analysis of uncertainty propagation through model components and for various sub-catchments.
Journal of Geophysical Research: Atmospheres, 1996
In this study we investigate the ability of satellite visible and infrared data to produce reliab... more In this study we investigate the ability of satellite visible and infrared data to produce reliable rainfall amount estimates that could be used by hydrological models to predict streamflow for large basins. Rainfall estimates are obtained by (1) classification of clouds to raining and nonraining clouds and (2) applying a multivariate statistical model between rainfall and indices derived from the satellite observations. Satellite data corresponding to 180 randomly selected days in the period May‐September 1982–1988 are used in this study that focuses on the estimation of daily rainfall. The Des Moines River basin in the midwestern United States is the application area. The correlation coefficient between model‐predicted and rain gauge‐observed mean areal precipitation over areas of order 10,000 km2 is found to be about 0.85. In an example application the satellite rainfall estimates are used to force the operational National Weather Service hydrologic forecast model for a subbasin ...
Geomorphology, 1992
Disequilibrium is a common if not dominant condition in geomorphology and other environmental sci... more Disequilibrium is a common if not dominant condition in geomorphology and other environmental sciences. Because disequilibrium in a system implies a time lag, time steps and time delays in simple deterministic equations may reflect process, and such equations have been shown to result in chaos. After examining some models of hillslope and catchment development, we present an oversimplified model of a hillslope in which we calculate the elevation of a point at time t + 1 as a function (we consider both linear and nonlinear terms) of the difference in elevation between it and the next uphill and downhill points at time t. We show how this simple hillslope model will not produce chaos under one set of limiting conditions that do not include nonlinearity, but, without proof, we note that in most conditions the hillslope mode is so similar to other models in which chaos arises because of disequilibrium that chaotic behavior is a likely possibility. A formulation wherein the time step is in a weathering and/or erosion coefficient rather than the elevation is also proposed.
A modeling framework is formulated and applied to assess the sensitivity of the hydrological regi... more A modeling framework is formulated and applied to assess the sensitivity of the hydrological regime of two catchments in a convective rainfall environment with respect to projected climate change. The study uses likely rainfall scenarios with high spatiotemporal resolution that are dependent on projected changes in the driving regional meteorological synoptic systems. The framework was applied to a case study in two medium-sized Mediterranean catchments in Israel, affected by convective rainfall, by combining the HiReS-WG rainfall generator and the SAC-SMA hydrological model. The projected climate change impact on the hydrological regime was examined for the RCP4.5 and RCP8.5 emission scenarios, comparing the historical (beginning of the 21st century) and future (mid-21st-century) periods from three General Circulation Models simulations available from CMIP5. Focusing on changes in the occurrence frequency of regional synoptic systems and their impact on rainfall and streamflow patterns, we find that the mean annual rainfall over the catchments is projected to be reduced by 15 % (range 2-23 %) and 18 % (7-25 %) for the RCP4.5 sand RCP8.5 emission scenarios, respectively. The mean annual streamflow volumes are projected to be reduced by 45 % (10-60 %) and 47 % (16-66 %). The average events' streamflow volumes for a given event rainfall depth are projected to be lower by a factor of 1.4-2.1. Moreover, the streamflow season in these ephemeral streams is projected to be shorter by 22 % and 26-28 % for the RCP4.5 and RCP8.5, respectively. The amplification in HESSD 11, 2014 Modeling convective rainfall sensitivity to climate change N.
Water Resources Research, 1991
A Monte Carlo study of a physically based distributed-parameter hydrologic model is described. Th... more A Monte Carlo study of a physically based distributed-parameter hydrologic model is described. The catchment model simulates overland flow and streamflow, and it is based on the kinematic wave concept. Soil Conservation Service curves are used to model rainfall excess within the basin. The model was applied to the Ralston Creek watershed, a small (7.5 km 2) rural catchment in eastern Iowa. Sensitivity of the model response with respect to rainfall-input spatial and temporal sampling density was investigated. The input data were generated by a space-time stochastic model of rainfall. The generated rainfall fields were sampled by the varied-density synthetic rain gauge networks. The basin response, based on 5-min increment input data from a network of high density with about 1 gauge per 0.1 km 2, was assumed to be the "ground truth," and other results were compared against it. Included in the study was also a simple lumped parameter model based on the unit hydrograph concept. Results were interpreted in terms of hydrograph characteristics such as peak magnitude, time-to-peak, and total runoff volume. The results indicate higher sensitivity of basin response with respect to the temporal resolution than to the spatial resolution of the rainfall data. Also, the frequency analysis of the flood peaks shows severe underestimation by the lumped model. This may have implications for the design of hydraulic structu-es. l. INTRODUCTION Recent technological advances in remote sensing, geographic information systems, and computers make the use of distributed hydrologic models an attractive alternative for flow simulation and flood prediction. The major areas of application of distributed models are, according to Beven and O'Connell [1982], forecasting effects of land use change, forecasting the effects of spatially variable inputs and outputs, forecasting movements of pollutants and sediments, and forecasting the hydrological response of an ungaged catchment. The main advantages possessed by a distributed model rest on the spatially distributed nature of its inputs and its use of physically based parameter values measurable in the field; with a distributed model one can measure the effect of changes in the physical parameters on the hydrological response on the whole or part of the catchment [Beven, 1985]. The traditional approach of lumped models, until recently the only one feasible to use in an operational environment, suffers from the inability to properly account for inhomogeneities in basin characteristics and model inputs. The distributed approach can, by definition, represent the basins and the hydrologic processes following the variability of the relevant parameters at a subbasin scale. The distributed models are still in very limited use mainly due to their demands for high computational power and long time required for their setup for a particular basin. Both problems are becoming less and less constraining as new generations of inexpensive but powerful workstations are more and more popular, and the high-resolution data of topography, geology, land cover, and land use are easily manageable using the sophisticated data bases and geographic information systems. Also, the problem of low spatial and temporal resolution of rainfall input is, or will be soon, much im-
Journal of Hydrometeorology, May 1, 2023
The enhancement of precipitation over the mountain regions of Southern California, in conjunction... more The enhancement of precipitation over the mountain regions of Southern California, in conjunction with mesoscale and synoptic-scale forcings, can result in high-intensity, short-duration extreme precipitation events (EPEs) that are often associated with hazardous impacts. In this study, candidate upstream atmospheric precursors at relevant spatiotemporal scales to such hazards are explored using a WRF mesoscale model with 5-km grid spacing and an hourly temporal resolution. This high-resolution model, once validated against observations, is used to discern statistically significant physicsbased signals between hypothetical mesoscale forcings and the modeled precipitation response. Specifically, the role of upstream instability in modeled EPEs is indexed by convective available potential energy (CAPE) and is examined for two mountainous regions of Southern California at several lag times. A Monte Carlo approach reveals statistically significant differences between the relationship of CAPE associated with EPEs in comparison to the analogous relationship for any precipitation event. These findings hold even with accounting for the well-established role of favorably oriented low-level moisture flux in orographic precipitation. This could indicate that atmospheric instability plays a role in providing additional lifting mechanisms, in conjunction with orographic and synoptic-scale forcings, to facilitate the high short-duration precipitation intensities that have been observed in the region. This diagnostic exploratory study provides additional candidate indicators of predictability of such EPEs at higher spatiotemporal scales than previous work, based on mesoscale model physics. Further analysis should examine the identified precursors using observational data with adequate resolution.
Hydrology and Earth System Sciences, Jan 29, 2015
A modeling framework is formulated and applied to assess the sensitivity of the hydrological regi... more A modeling framework is formulated and applied to assess the sensitivity of the hydrological regime of two catchments in a convective rainfall environment with respect to projected climate change. The study uses likely rainfall scenarios with high spatiotemporal resolution that are dependent on projected changes in the driving regional meteorological synoptic systems. The framework was applied to a case study in two medium-sized Mediterranean catchments in Israel, affected by convective rainfall, by combining the HiReS-WG rainfall generator and the SAC-SMA hydrological model. The projected climate change impact on the hydrological regime was examined for the RCP4.5 and RCP8.5 emission scenarios, comparing the historical (beginning of the 21st century) and future (mid-21st-century) periods from three general circulation model simulations available from CMIP5. Focusing on changes in the occurrence frequency of regional synoptic systems and their impact on rainfall and streamflow patterns, we find that the mean annual rainfall over the catchments is projected to be reduced by 15 % (outer range 2-23 %) and 18 % (7-25 %) for the RCP4.5 sand RCP8.5 emission scenarios, respectively. The mean annual streamflow volumes are projected to be reduced by 45 % (10-60 %) and 47 % (16-66 %). The average events' streamflow volumes for a given event rainfall depth are projected to be lower by a factor of 1.4-2.1. Moreover, the streamflow season in these ephemeral streams is projected to be shorter by 22 % and 26-28 % for the RCP4.5 and RCP8.5, respectively. The amplification in reduction of streamflow volumes relative to rainfall amounts is related to the projected reduc-tion in soil moisture, as a result of fewer rainfall events and longer dry spells between rainfall events during the wet season. The dominant factors for the projected reduction in rainfall amount were the reduction in occurrence of wet synoptic systems and the shortening of the wet synoptic systems durations. Changes in the occurrence frequency of the two dominant types of the regional wet synoptic systems (active Red Sea trough and Mediterranean low) were found to have a minor impact on the total rainfall.
Bulletin of the American Meteorological Society, Nov 1, 1984
Quantitative hydrologic forecasting usually requires knowledge of the spatial and temporal distri... more Quantitative hydrologic forecasting usually requires knowledge of the spatial and temporal distribution of precipitation. First, it is important to accurately measure the precipitation falling over a particular watershed of interest. Second, especially for small watersheds and/or for longer forecast lead times, forecasts of precipitation are critical to the achievement of the greatest possible hydrologic forecast accuracy and longest possible lead time. This paper describes the current hydrologic forecasting program of the U.S. National Weather Service (NWS) and highlights the relevance of Quantitative Precipitation Forecasting (QPF) products to real-time hydrologic forecasting. Specific requirements for QPF products in support of hydrologic forecasting applications are defined and current operational QPF procedures are reviewed to determine to what extent they meet these requirements. It is concluded that no known QPF procedures capable of fulfilling all desired requirements are currently available operationally, although much valuable QPF information is available to meet parts of these requirements. Some recent advances in mesoscale QPF research are examined and these techniques are treated in two categories: those uncoupled dynamically from and those dynamically coupled to hydrologic forecasting procedures. Finally, a summary of possible future directions toward achieving improved use of QPF information in hydrologic forecasting applications is presented.
Water, Dec 13, 2016
A high-resolution 3-D orographic precipitation model (OPM) forced by Climate Forecast System (CFS... more A high-resolution 3-D orographic precipitation model (OPM) forced by Climate Forecast System (CFS) reanalysis fields was developed for the Lake Kinneret watershed (Israel-Syria-Lebanon territories). The OPM was tuned to represent the interaction between the advected and stratiform rainfall, and the local orographic enhancement. The OPM evaluation was focused on the densely instrumented lower part of the watershed. To evaluate the ungauged upper-elevation, bias-adjusted precipitation estimates from the Global-Hydro-Estimator were used. The OPM simulates higher rainfall amounts in the upper-elevation watershed compared to currently used rainfall estimates from an elevation dependent regression. The larger differences are during rain events with southwesterly wind flow and high moisture flux. These conditions, according to the OPM, are conducive to enhanced orographic lifting in the Hermon Mountain. A sensitivity analysis indicated that the higher wind speeds for southwesterly-northwesterly trajectories generate significant orographic lifting and increase the precipitation differences between the lower and upper elevations.
EGU General Assembly Conference Abstracts, Apr 1, 2015
Nonlinear Processes in Geophysics, Feb 2, 2005
Reconstructing the dynamics of nonlinear systems from observations requires the complete knowledg... more Reconstructing the dynamics of nonlinear systems from observations requires the complete knowledge of its state space. In most cases, this is either impossible or at best very difficult. Here, by using a toy model, we investigate the possibility of deriving useful insights about the variability of the system from only a part of the complete state vector. We show that while some of the details of the variability might be lost, other details, especially extreme events, are successfully recovered. We then apply these ideas to the problem of rainfall estimation from satellite imagery. We show that, while reducing the number of observables reduces the correlation between actual and inferred precipitation amounts, good estimates for extreme events are still recoverable.
Flash Flood Guidance consists of indices that estimate the amount of rain of a certain duration t... more Flash Flood Guidance consists of indices that estimate the amount of rain of a certain duration that is needed over a given small basin in order to cause minor flooding. Backwater catchment inundation from swollen rivers or regional groundwater inputs are not significant over the spatial and temporal scales for the majority of upland flash flood prone basins, as such, these effects are not considered. However, some lowland areas and flat terrain near large rivers experience standing water long after local precipitation has ceased. NASA is producing an experimental product from the MODIS that detects standing water. These observations were assimilated into the hydrologic model in order to more accurately represent soil moisture conditions within basins, from sources of water from outside of the basin. Based on the upper soil water content, relations are used to derive an error estimate for the modeled soil saturation fraction; whereby, the soil saturation fraction model state can be updated given the availability of satellite observed inundation. Model error estimates were used in a Monte Carlo ensemble forecast of soil water and flash flood potential. Numerical experiments with six months of data (July 2011-December 2011) showed that MODIS inundation data, when assimilated to correct soil moisture estimates, increased the likelihood that bankfull flow would occur, over non-assimilated modeling, at catchment outlets for approximately 44% of basin-days during the study time period. While this is a much more realistic representation of conditions, no actual events occurred allowing for validation during the time period.
Eos, Transactions American Geophysical Union, 1985
For the past few years, several sectors of the U.S. meteorological community have been actively p... more For the past few years, several sectors of the U.S. meteorological community have been actively planning and organizing the National STORM Program. STORM is an acronym for Stormscale Operational and Research Meteorology, and its first major project, called STORM‐Central, will focus on the central third of the United States [Interagency Team for STORM‐Central, 1984]. The overall objective of STORM is to obtain knowledge and techniques that will be useful in forecasting mid‐latitude mesoscale convective systems and their attendant weather. Such systems cause most of the beneficial precipitation and almost all of the threatening events, such as violent thunderstorms, flash floods, hail, high winds, and tornados. The experimental phase of STORM‐Central is now scheduled to take place during the period April–July of 1988 and/or 1989. An upper‐air sounding system with a resolution of 200 km will be deployed over an area from the Gulf Coast of Canada and from the Rocky Mountains to the cent...
WMO has long regarded weather radar as an advanced and invaluable observing system supporting the... more WMO has long regarded weather radar as an advanced and invaluable observing system supporting the World Weather Watch Programme and a multitude of meteorological and other applications protecting lives and property affected by local severe weather phenomena. Answering an increasingly urgent and emerging need to address the requirement for access to radar data and products by Numerical Weather Prediction (NWP), hydrological and climate applications, the WMO organized a workshop dedicated to the regional and global exchange of weather radar data. The workshop was arranged with the approval and support of the WMO Commission for Basic Systems (CBS), Open Programme Area Group on Integrated Observing Systems (OPAG-IOS) and took place over 24-26 April in Exeter, United Kingdom, with 18 participants from around the world representing radar operators, data users and data exchange experts. The workshop considered current and future requirements of weather radar data by reviewing the requireme...
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Papers by Konstantine Georgakakos