Hatim O. Sharif

On the calibration and verification of two-dimensional, distributed, Hortonian, continuous watershed models.

Physically based, two-dimensional, distributed parameter Hortonian hydrologic models are sensitive to a number of spatially varied parameters and inputs and are particularly sensitive to the initial soil moisture field. However, soil moisture data are generally unavailable for most catchments. Given an erroneous initial soil moisture field, single-event calibrations are easily achieved using different combinations of model parameters, including physically unrealistic values. Verification of single-event calibrations is very difficult for models of this type because of parameter estimation errors that arise from initial soil moisture field uncertainty. The purpose of this study is to determine if the likelihood of obtaining a verifiable calibration increases when a continuous flow record, consisting of multiple runoff producing events is used for model calibration. The physically based, two-dimensional, distributed, Hortonian hydrologic model CASC2D [Julien et al., 1995] is converted to a continuous formulation that simulates the temporal evolution of soil moisture between rainfall events. Calibration is performed using 6 weeks of record from the 21.3 km 2 Goodwin Creek Experimental Watershed, located in northern Mississippi. Model parameters are assigned based on soil textures, land use/land cover maps, and a combination of both. The sensitivity of the new model formulation to parameter variation is evaluated. Calibration is performed using the shuffled complex evolution method [Duan et al., 1991]. Three different tests are conducted to evaluate model performance based on continuous calibration. Results show that calibration on a continuous basis significantly improves model performance for periods, or subcatchments, not used in calibration and the likelihood of obtaining realistic simulations of spatially varied catchment dynamics. The automated calibration reveals that the parameter assignment methodology used in this study results in overparameterization. Additional research is needed in spatially distributed hydrologic model parameter assignment methodologies for hydrologic forecasting.

The Use of an Automated Nowcasting System to Forecast Flash Floods in an Urban Watershed

Abstract Flash flooding represents a significant hazard to human safety and a threat to property. Simulation and prediction of floods in complex urban settings requires high-resolution precipitation estimates and distributed hydrologic modeling. The need for reliable flash flood forecasting has increased in recent years, especially in urban communities, because of the high costs associated with flood occurrences. Several storm nowcast systems use radar to provide quantitative precipitation forecasts that can potentially afford great benefits to flood warning and short-term forecasting in urban settings. In this paper, the potential benefits of high-resolution weather radar data, physically based distributed hydrologic modeling, and quantitative precipitation nowcasting for urban hydrology and flash flood prediction were demonstrated by forcing a physically based distributed hydrologic model with precipitation forecasts made by a convective storm nowcast system to predict flash floods in a small, highly ur...

Statistical Analysis of Radar Rainfall Error Propagation

Abstract The prediction uncertainty of a hydrologic model is closely related to model formulation and the uncertainties in model parameters and inputs. Currently, the foremost challenges concern not only whether hydrologic model outputs match observations, but also whether or not model predictions are meaningful and useful in the contexts of land use and climate change. The latter is difficult to determine given that model inputs, such as rainfall, have errors and uncertainties that cannot be entirely eliminated. In this paper the physically based simulation methodology developed by Sharif et al. is used to expand this investigation of the propagation of radar rainfall estimation errors in hydrologic simulations. The methodology includes a physics-based mesoscale atmospheric model, a three-dimensional radar simulator, and a two-dimensional infiltration-excess hydrologic model. A time series of simulated three-dimensional precipitation fields over a large domain and a small study watershed are used, which ...

Application of a distributed hydrologic model to the november 17, 2004, flood of bull creek watershed, Austin, Texas

This study presents a hydrologic analysis of a flood event that took place over a small urbanizing watershed in Austin, Texas. The physically based, distributed-parameter gridded surface subsurface hydrologic analysis (GSSHA) hydrologic model was used to simulate the watershed response to a very high-intensity rain event. The hydrologic model was forced by both gauge-observed and multisensor precipitation estimator (MPE) rainfall input. Observed discharge was compared to GSSHA-generated hydrograph under various degrees of representation of the watershed physiography. In addition, simulation hydrographs by GSSHA using five different model grid sizes were compared in order to evaluate the effect of grid size on model predictions. The simulation hydrograph for the model using a 30-m grid cell generally compared well to the observed flow data once the effects of storm water detention were simulated. The comparison of simulation results from models using 30, 60, 90, 120, and 150 m grid size highlighted the los...

Analysis of flood fatalities in texas

AbstractFloods are the leading cause of fatalities connected with natural disasters in Texas. A combination of physiography and precipitation often result in extreme hydrologic conditions that cause floods in the state. This paper reviews flood-related fatalities in Texas between 1959 and 2008. Information on flood-fatality victims and the flood-causing events was obtained from the National Climatic Data Center. The data collected included the date, time, location, and weather conditions and the gender and age of the flood victims. Comparison with other states reveals that the size of the population of Texas is a major factor in the increased number of fatalities. The data also suggest that driving or walking into floodwaters may be responsible for more than 93% of flood fatalities in Texas. Although most high-fatality counties are located in the Texas “Flash Flood Alley” that includes major urban centers, normalization of fatality data shows that the flood-fatality risk is actually higher in other areas ...

Flood hazards in an urbanizing watershed in Riyadh, Saudi Arabia

Riyadh, the capital of the Kingdom of Saudi Arabia, has experienced unusual levels of urbanization in the past few decades, making it one of the fastest growing cities in the world. This paper examines flood hazards in the rapidly urbanizing catchment of Al-Aysen in Riyadh. Remote sensing and geographic information system techniques were employed to obtain and prepare input data for hydrologic and hydraulic models, with the former based on the very popular curve number approach. Due to the limited nature of the rainfall data, observations from two rain gauges in the vicinity of the catchment were used to estimate design storms. The hydrologic model was run in a semi-distributed mode by dividing the catchment into many sub-catchments. The impact of urbanization on run-off volume and peak discharge resulting from different storms was investigated, with various urbanization scenarios simulated. Flood hazard zones and affected streets were also identified through hydrologic/hydraulic model simulation. The mismatch between administrative and catchment boundaries can create problems in flood risk management for similar cities since hydrologic processes and flood hazards are based on the hydrologic connectivity. Since flooding events impact the road network and create driving hazards, governmental decision-makers must take the necessary precautions to protect drivers in these situations.

Person-place-time analysis of vehicle fatalities caused by flash floods in Texas

A significant number of crashes on roads are caused by adverse weather conditions. Among the most serious consequences of rainfall and flooding in regard to road safety are the motor vehicle-related flash flood fatalities. These fatalities are of particular concern in Texas. Information on motor vehicle fatalities caused by flash floods was extracted from the National Climatic Data Center Storm Data reports. Review of reports on flash flood fatalities in general, where the death circumstances are provided, reveals that most fatalities are motor vehicle-related (80%). Moreover, data from the reports indicate that from 1959 to 2009 the number of motor vehicle-related flash flood fatalities in Texas exceeds by far the total number of flood fatalities in any other state. Demographic analysis of the flash flood fatalities indicates that, in Texas, all ages are at risk and that males are much more affected than females. Spatial analysis indicates that the highest numbers of fatalities occur in counties having major urban areas. The increase in the frequency and intensity of storms and floods that is projected to result from climate changes and variability, and the rapid urbanization in the region may considerably increase the impacts of weather on road safety in Texas.

Multidecadal High-Resolution Hydrologic Modeling of the Arkansas–Red River Basin

Abstract Land surface heterogeneity and its effects on surface processes have been a concern to hydrologists and climate scientists for the past several decades. The contrast between the fine spatial scales at which heterogeneity is significant (1 km and finer) and the coarser scales at which most climate simulations with land surface models are generated (hundreds of kilometers) remains a challenge, especially when incorporating land surface and subsurface lateral fluxes of mass. In this study, long-term observational land surface forcings and derived solar radiation were used to force high-resolution land surface model simulations over the Arkansas–Red River basin in the Southern Great Plains region of the United States. The most unique aspect of these simulations is the fine space (1 km2) and time (hourly) resolutions within the model relative to the total simulation period (51 yr) and domain size (575 000 km2). Runoff simulations were validated at the subbasin scale (600–10 000 km2) and were found to ...

Application of validation data for assessing spatial interpolation methods for 8-h ozone or other sparsely monitored constituents

The adverse health effects of high concentrations of ground-level ozone are well-known, but estimating exposure is difficult due to the sparseness of urban monitoring networks. This sparseness discourages the reservation of a portion of the monitoring stations for validation of interpolation techniques precisely when the risk of overfitting is greatest. In this study, we test a variety of simple spatial interpolation techniques for 8-h ozone with thousands of randomly selected subsets of data from two urban areas with monitoring stations sufficiently numerous to allow for true validation. Results indicate that ordinary kriging with only the range parameter calibrated in an exponential variogram is the generally superior method, and yields reliable confidence intervals. Sparse data sets may contain sufficient information for calibration of the range parameter even if the Moran I p-value is close to unity. R script is made available to apply the methodology to other sparsely monitored constituents.

Motor Vehicle-Related Flood Fatalities in Texas, 1959–2008

The number of related fatalities is one of the most essential socioeconomic characteristics of a natural disaster as death is the most serious and irreversible consequence of a disaster. Texas leads the nation in flash flood fatalities. From 1959 through 2008 the flood fatalities in Texas (839) were more than three times the fatalities in the next leading state, Pennsylvania (265). Flood fatalities in Texas represent a serious public health problem. Flood fatality statistics were extracted from National Climatic Data Center Storm Data publications. Review of the flood fatalities, where the death circumstances are provided, reveals that most fatalities are motor vehicle related (77%). Data analysis indicates that, in Texas, males are much more likely to be the victims of motor vehicle–related flood accidents than are females. Most motor vehicle–related fatalities happened at night (56%). Most fatalities resulted from flash floods. Spatial analysis indicates that the highest numbers of fatalities occur in counties having major urban areas. Flood fatalities in Texas can be reduced through a combination of improved hydrometeorological forecasting, educational programs aimed at enhancing the public awareness of flood risk and the seriousness of flood warnings, and timely and appropriate action by local emergency and safety authorities.