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| ==Required Data== | | ==Required Data== |
| '''Watershed Delineation'''
| | * [[Watershed Delineation]] |
| | * [[Precipitation]] |
| | * [[Rainfall Losses]] |
| | * [[Unit Hydrograph]] |
| | * [[Reach Routing]] |
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| :“For most hydrologic studies, it is essential that good topographic maps be used. It is important that the maps contain contours of ground-surface elevation, so that drainage basins can be delineated and important features such as slopes, exposure, and stream patterns can be measured”.<ref name="EM 1110-2-1420">[[Hydrologic Engineering Requirements for Reservoirs (EM 1110-2-1420) | EM 1110-2-1420 Hydrologic Engineering Requirements for Reservoirs, USACE, 1997]]</ref>
| | ==Best Practices Resources== |
| | | {{Document Icon}} [[Hydrologic Engineering Requirements for Reservoirs (EM 1110-2-1420)|Hydrologic Engineering Requirements for Reservoirs (EM 1110-2-1420) (U.S. Army Corps of Engineers)]] |
| '''Precipitation'''
| | {{Document Icon}} [[Engineering Guidelines for the Evaluation of Hydropower Projects: Chapter 8- Determination of the Probable Maximum Flood|Engineering Guidelines for the Evaluation of Hydropower Projects: Chapter 8- Determination of the Probable Maximum Flood (Federal Energy Regulatory Commission)]] |
| | | {{Document Icon}} [[Hydrology National Engineering Handbook: Chapter 4- Storm Rainfall Depth and Distribution|Hydrology National Engineering Handbook: Chapter 4- Storm Rainfall Depth and Distribution (United States Geological Survey)]] |
| :“In areas of relatively uniform terrain and little spatial variability or precipitation, classical textbook procedures, such as Thiessen polygons or the isohyetal method, can be used and are generally adequate. These procedures are simple methods of developing spatial averages from point measurements but are inadequate to describe the orthographic or other spatially variable behavior of any appreciable complexity. In these cases, such as in mountainous areas, more comprehensive algorithms are needed to develop spatial averages from point measurements that describe the elevational (vertical) and horizontal variability. For time series at the watershed scale, the algorithm based on detrended kriging developed by Garen, Johnson, and Hanson (1994) is an example. (Further information on this procedure is available from the NRCS National Water and Climate Center in Portland, Oregon.) For annual or monthly averages or monthly time series at somewhat larger spatial scales (watershed to regional), the best method is to use the PRISM maps and GIS layers, as mentioned previously. This would be the recommended procedure in most watershed yield analyses”.<ref name="NEH210-630-20">[[National Engineering Handbook 210 Part 630 Hydrology: Chapter 20 Watershed Yield | National Engineering Handbook 210 Part 630 Hydrology: Chapter 20 Watershed Yield, NRCS, 2009]]</ref>
| | {{Document Icon}} [[Flood Hydrology Manual|Flood Hydrology Manual (Bureau of Reclamation)]] |
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| '''Rainfall Losses'''
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| :“Evaporation data is usually required for reservoir studies, particularly for low-flow analysis. Reservoir evaporation is typically estimated by measuring pan evaporation or computing potential evaporation”.<ref name="EM 1110-2-1420" />
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| :“Evapotranspiration (ET) is difficult to estimate because it is a complex process. It is determined by the atmospheric demand for water vapor (potential ET) and the availability of water to be evaporated. ET is a sum of pure evaporation from free water surfaces, such as wet vegetation, puddles, and lakes, and the transfer of soil moisture through plants and out their leaves (transpiration). The former process depends only on the atmospheric conditions (temperature, humidity, wind), whereas the latter also depends on plant characteristics (stomatal resistance) and on soil moisture availability”.<ref name="NEH210-630-20" />
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| :“Many models are available for estimating potential evapotranspiration from meteorological data (Jensen, Burman, and Allen 1990; ASCE 1996). They vary in their assumptions, the processes described, the input data required, and the temporal scale for which they are appropriate. Potential ET can also be estimated from pan evaporation data if suitable pan coefficients are available” (National Engineering Handbook 210 Part 630 Hydrology: Chapter 20 Watershed Yield, NRCS, 2009).<ref name="NEH210-630-20" />
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| :“Even if potential ET is adequately estimated, the actual ET is less than or equal to this amount and depends primarily on soil moisture availability. Because of this interplay between the atmospheric demand and the soil moisture, determining the actual ET is problematic without a detailed hydrologic model operated at a short time step (i.e., a day or less). If adequate assumptions can be made, however, reasonable estimates of actual ET as a fraction of potential ET are possible”.<ref name="NEH210-630-20" />
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| '''Unit Hydrograph'''
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| :“In the 1930’s, L.K. Sherman (Sherman 1932, 1940) advanced the theory of the unit hydrograph, or unit graph. The unit hydrograph procedure assumes that discharge at any time is proportional to the volume of runoff and that time factors affecting hydrograph shape are constant”.<ref name="NEH210-630-16">[[National Engineering Handbook 210 Part 630 Hydrology: Chapter 16 Hydrographs | National Engineering Handbook 210 Part 630 Hydrology: Chapter 16 Hydrographs, NRCS, 2007]]</ref>
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| :“Field data and laboratory tests have shown that the assumption of a linear relationship among watershed components is not strictly true. The nonlinear relationships have not been investigated sufficiently to ascertain their effects on a synthetic hydrograph. Until more information is available, the procedures of this chapter will be based on the unit hydrograph theory”.<ref name="NEH210-630-16" />
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| :“Many variables are integrated into the shape of a unit hydrograph. Since a dimensionless unit hydrograph is used and the only parameters readily available from field data are drainage area and time of concentration, consideration should be given to dividing the watershed into hydrologic units of uniformly shaped areas. These subareas, it at all possible, should have a homogeneous land use and approximately the same size. To assure that all contributing subareas are adequately represented, it is suggested that no subarea exceed 20 square miles in area and that the ratio of the largest to smallest drainage area not exceed 10”.<ref name="NEH210-630-16" />
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| '''Reach Routing'''
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| :“Two important judgements are needed by the engineer when developing data for channel reach routing. One is the selection of a representative cross section. This cross section should represent an average flow velocity through the reach. If there are several cross sections available for selection, the engineer should select the section most typical of the reach conditions. The second important judgement is the selection of a reach length. To properly represent reach storage characteristics, the reach length should be an average length of the routing reach. HEC-RAS allows for left overbank, right overbank, and channel reach lengths. WinTR-20 allows for only floodplain length and channel length. WinTR-55 is limited to only a single channel length”.<ref name="NEH210-630-17">[[National Engineering Handbook 210 Part 630 Hydrology: Chapter 17 Flood Routing | National Engineering Handbook 210 Part 630 Hydrology: Chapter 17 Flood Routing, NRCS, 2014]]</ref>
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| :“Storage in a reach is often underestimated because backwater storage in tributaries is usually not considered in developing water surface profiles using HEC-RAS. If this type of storage is significant, it should be estimated. The simplest way to account for this would be to increase the floodplain length. A more complex analysis would involve adjusting HEC-RAS cross sections to include an ineffective flow area for tributary backwater” (National Engineering Handbook 210 Part 630 Hydrology: Chapter 17 Flood Routing).
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| ==Examples==
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| {{Website Icon}}
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| ==Best Practices Resources==
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| {{Document Icon}} [[Hydrologic Engineering Requirements for Reservoirs (EM 1110-2-1420)]]
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| {{Document Icon}} [[National Engineering Handbook 210 Part 630 Hydrology: Chapter 20 Watershed Yield]]
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| {{Document Icon}} [[National Engineering Handbook 210 Part 630 Hydrology: Chapter 16 Hydrographs]]
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| {{Document Icon}} [[National Engineering Handbook 210 Part 630 Hydrology: Chapter 17 Flood Routing]]
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| ==Trainings== | | ==Trainings== |
| {{Video Icon}} | | {{Video Icon}} [[On-Demand Webinar: Introduction to Hydrologic Modeling Using Geospatial Information]] |
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Hydrologic Engineering Requirements for Reservoirs (EM 1110-2-1420) (U.S. Army Corps of Engineers)
On-Demand Webinar: Introduction to Hydrologic Modeling Using Geospatial Information