PRISM (Precipitation-elevation Regressions on Independent Slopes Model) is an analytical model that uses point data and a digital elevation model (DEM) to generate gridded estimates of monthly and annual precipitation. PRISM is uniquely suited to regions with mountainous terrain because it incorporates a conceptual framework that addresses the spatial scale and pattern of orographic precipitation. This paper presents a brief overview and evaluation of PRISM as applied to the entire United States. The development of an updated isohyetal analysis for the state of Oregon is discussed. Using precipitation data from the National Climatic Data Center and the Soil Conservation Service, PRISM generated estimates of precipitation values for each grid cell. The gridded output was then imported into a geographic information system (GIS) for plotting and map creation. Data were made available to interested users in both hardcopy (map) and digital (numerical matrix) format. The updated isohyetal analysis is thought to be of quality and accuracy comparable to that of traditional labor-intensive methods of a fraction of the cost.
The peaks over threshold (POT) series model has been used in hydrology to analyse extreme phenomena such as floods or precipitation amounts exceeding a certain threshold level. The results of a study treating two important aspects of the application of the POT model in hydrology are presented. This paper concentrates on the use of the POT model to study the seasonal variations in flood data in the Provinces of Quebec and New Brunswick (Canada). POT models are capable of taking seasonal variations in river-flow processes into account by considering exceedances from different seasons to be non-identically distributed. The study deals specifically with the choice of seasons to be entered into the model, emphasizing the importance of determining these seasons on the basis of the data on hand instead of taking the four usual seasons winter, spring, summer and fall, which might not be necessarily applicable to the specific region under study. A graphical procedure is proposed for the determination of seasons at some gaging sites in the Provinces of Quebec and New Brunswick. An appropriate partitioning of the year into seasons is obtained for different parts of these two Provinces. In addition, the transferability of information from a gaged basin to an ungaged basin is undertaken by applying multiple regression to the estimation of the threshold level from the physical characteristics of the basin.
The author describes three types of technology transfer programs that have been found to be effective in hydraulic engineering and other engineering disciplines and cites examples of how such programs have been used by a major American engineering company in developing and under-developed countries.
The Orange River drains the largest basin in Africa south of the Zambezi with a total catchment area of approximately 1 million km 2 . Due to the increasing water demands in the basin and in some adjacent catchments, considerable development of the Orange River has taken place. Further major developments are already being implemented, including the first phase of the Lesotho Highlands Water Project at a cost of more than $3 000 million. In view of the importance of the water resources of the Orange River basin to the Southern African economy, a major resource analysis was initiated in 1988, the results of which are presented in the paper.
The problem of flood frequency estimation in ungauged catchments is approached by using statistical procedures jointly with derived distribution techniques. The application of this methodology to a large region in Northern Italy is presented. It is shown that the conjunctive use of statistical and deterministic concepts can improve the reliability of flood predictions.
A 'total fit' method is suggested for computing Nash's instantaneous unit hydrograph (IUH) which minimizes the errors of all ordinates between the observed and generated hydrographs. Previous methods have used the method of moments or minimized peak flow errors. The results of the total fit method are compared with the above two methods using 34 storms from three different river basins. In each case, the total fit criterion produced superior results. A regionalization procedure is also presented and applied to a region in Colombia using nine watersheds and 523 effective rainfall-direct runoff events. The results shows that the proposed regional model gives better results than other methodologies.
Field infiltration experiments at several locations in two sand dune areas in Saudi Arabia were conducted. Results of these field tests are compared with predictions of the Green and Ampt approach. Two sets of infiltration tests in the two areas that were performed under different initial moisture contents are also presented. Infiltration characteristics of the sand dunes in the two regions are also discussed.
Increasing urbanisation and the subsequent decrease in natural storage and infiltration potential in a catchment, cause increased runoff peaks and volumes. Existing stormwater drainage systems become inadequate to handle increased runoffs, causing flooding to occur more frequently, in turn damaging property and endangering life. A unique damage rating system, which takes into account applicable stormwater system design standards, has been developed to identify, quantity and rank areas according to the flooding hazard. From the damage rating system an urban catchment can be sub-divided into high, medium and low damage potential zones. Should no stormwater system design standards be available, a method to select the most cost-beneficial design standards using a Benefit Factor, which relates the risk of flooding to the capital cost of the drainage system, has been developed. Remedial measures are then identified to reduce the flooding potential. A phased remedial works programme is compiled, in which the required remedial work is prioritised to reduce the flood damage potential in the shortest possible time. In this paper the above concepts are defined and illustrated by abstracting relevant information from a Stormwater Management Project undertaken for the Welkom City Council.
Determination of minimum streamflows to be left in the stream channel for the sustenance of the downstream habitats becomes a difficult task when quantitative requirements of such habitats are unknown as in the case of a tropical riverine forest habitat. A release pattern that preserves the more deterministic and seasonal features of the streamflow has been adopted in this case study.
The State of New Jersey, due to large population density and intensive development, has become a frontier in regulating the stormwater runoff. This paper presents an overview of current Stormwater Management Practices in that State. The paper also includes the author's suggestions for improving the current practices.
Results of a numerical optimization model capable of designing containment systems for sites with three-dimensional groundwater flow indicate that site hydrogeology and economic considerations are both important factors in deciding whether slurry wall installation is advantageous. The optimization model selects the lowest cost combination of slurry walls and groundwater pumping. The analysis includes consideration of optimal well location and pumping rate as well as installation and operating costs for wells and slurry walls. The optimization system was applied to a test site with relatively shallow head gradients and soils of low conductivity. For this case, the optimal design called for a small number of pumping wells and no slurry walls. A sensitivity analysis was then performed to investigate the tradeoffs between slurry walls and groundwater pumping. In general, for low groundwater flow velocities, the optimal solution includes only pumping. As groundwater flow velocities increase, the most efficient solution includes both pumping and slurry walls.
As an integral part of a study to develop a combined wastewater treatment system for petrochemical facilities, comprehensive hydrological studies were performed prior to the detailed design of the wastewater treatment system to accurately define the flow rates, retention basin operations and flood control alternatives. The runoff of a design storm for the facilities was segregated into potentially contaminated stormwater and clean stormwater. This was achieved by curbing the potentially oily and chemically contaminated areas and providing adequate pumping capacity at the lift station. The existing oxidation pond was used as a detention basin rather than as a treatment facility. The wastewater in the detention basin is gradually released to the treatment facility and, after treatment, the effluent is discharged to the permitted outfalls at a rate allowed by the existing permit. Operating procedures for the detention basin and flood control alternatives for the facilities were determined by the hydrological routings and storm runoff modeling.
Wellhead protection is an emerging issue for water suppliers based largely on the 1986 Amendments to the Federal Safe Drinking Water Act (SDWA). State governments are developing wellhead protection programs to meet the SDWA requirements and to provide local governments and water suppliers with guidance. Data requirements to support a wellhead protection program can become unwidely without the benefit of a Geographic Information System (GIS). An Environmental Landscape Classification System (ELCS) can be created and managed using a GIS to provide spatial relationships between geology, glacial deposits, topography, soils, land use, pollution activities and groundwater resources. These spatial relationships, when combined with appropriate analytical techniques, may be used to define critical areas surrounding well sites in need of special protection. Further, the ELCS can assist in the assessment of pollution vulnerability of individual well sites based on known locations of potential contaminant activities. An Environmental Landscape Classification System was created for Lehigh and Northampton counties, located in eastern Pennsylvania, as part of a project to develop a regional wellhead protection strategy.
The probabilistic calculation of design floods was initiated in the 60s to overcome statistical calculation uncertainties. SPEED takes up and expands this original concept in the form of a coherent system of probabilistic studies of rainfall (regional analysis) and rainfall-runoff relations.
Cloud seeding has been conducted for water years 1990-91 and 1991-92 in Monterey County, California in response to the extended drought. The Monterey County Water Resources Agency weather modification program included both airborne and ground based cloud seeding operations and was designed to increase rainfall and subsequent runoff in the watershed drainages of Arroyo Seco River, and San Antonio and Nacimiento reservoirs. This paper describes the seeding methods, results, and program cost.
For a period of over two years, HDR Engineering, Inc. (HDR) and HYDMET have been working with the City and County of Sacramento to develop a Drainage Manual. This Manual contains seven sections including goals and policies, hydrology, street drainage and storm inlets, storm sewers and appurtenances, culvert and bridge hydraulic design, open-channels, and storage. The draft Manual is currently being reviewed by the City and County and should be finalized sometime this year. This paper will address the hydrologic procedures incorporated in the Draft Drainage Manual, the basis for their development, and difficulties encountered during the development phase of the Manual. A second paper to be presented at this symposium (Hall, Wegener, and Crouch) provides an overview by the County of Sacramento of subsequent refinements to the hydrologic procedures in the draft Manual which are now being proposed as interim procedures until additional field information is collected during the next 5 years.
In the Western United States approximately 75 percent of the annual runoff results from snowmelt. Observations of the snow cover provide an important source of information for forecasting seasonal water supply months in advance. The National Weather Service River Forecast System (NWSRFS) snow model relies on estimates of mean areal precipitation and mean areal temperature to compute estimates of current snow cover conditions. Because of the difficulty in accurately estimating precipitation in the mountains, it is essential that snow water equivalent observations be used to update model simulated snow cover conditions to ensure that forecasts of future water supply are accurate. This paper describes a software system that interpolates point observations to produce gridded estimates of snow water equivalent. The system uses the GRASS Geographical Information System (GIS) to store, analyze, and display point, line and gridded data. The GIS permits the analysis of multiple data layers such as elevation, seasonal precipitation, and derived data layers, e.g. slope and aspect. The outputs of the system include gridded estimates of snow water equivalent, as well as estimates of the areal snow cover conditions needed by the snow accumulation and ablation model that is part of NWSRFS. Updating the simulated snow conditions has been demonstrated to provide significant improvements in streamflow forecasting in areas with significant snow cover.
The dependence of extracted drainage network properties on the spatial resolution of DEMs is investigated for a study basin. A series of raster DEMs of increasing cell size is generated by grid cell aggregation. Several drainage network properties are extracted from these DEMs and changes in these properties with increasing DEM cell size are examined. For grid coefficients less than 0.05 the extracted properties generally remain within about 10% of those extracted from the finest DEM. For networks with sinuous channel patterns and for properties related to channel length or to small drainage features, the grid dependency can start at grid coefficients smaller than 0.05. The dependency is introduced by the inability of a DEM to accurately reproduce such drainage features as their size approaches the spatial resolution of the DEM.
Dynamic flood routing model based on the four-point implicit finite-difference solution of the complete one-dimensional Saint-Venant equations of unsteady flow are inherently deterministic. Such a model (FLDWAV) developed by the National Weather Service for real-time flood forecasting has been enhanced with a stochastic estimator based on an extended Kalman filter to provide optimal updating capabilities utilizing real-time observed river stages. The stochastic enhancement is described, and selected applications of the enhanced model spanning a wide range of unsteady flows are presented.
