Hydrologic Objects for Modeling: One Viewpoint

Thomas A. EvansUS Army Corps of EngineersHydrologic Engineering Center

What is HEC?

The Hydrologic Engineering Center (HEC) is an office of the US Army Corps of Engineers established to support the nation in its water resources management responsibilities by increasing the Corps’s technical capability in hydrologic engineering and water resources planning and management.

By means of programs in research, training, planning analysis, and technical assistance, HEC incorporates state-of-the-art procedures and techniques into manuals and comprehensive computer programs. The products are developed for the Corps; however, they are available to the public.

Hydrologic and Hydraulic Models from HEC HEC-1 Flood Hydrograph model–now

superceded by HEC-HMS (Hydrologic Modeling System)

HEC-2 River Hydraulics model–now superceded by HEC-RAS (River Analysis System)

Watershed Analysis in HEC-HMS

HEC-HMS Basin Element Class Element ID & Description (strings) Location (2D coordinate pair) Contributing Area (real number) Downstream Element (basin element) Upstream Elements (list of basin elements) Observed Hydrograph (time series) Output Hydrograph (time series) others...

Basin Element Subclasses

Sub-basin– Rainfall/Runoff method– Loss Method– Baseflow Method

Routing reach– Downstream Location – Routing method

Basin Element Subclasses

Reservoir– Storage/Discharge method

Diversion– Second Downstream element

Junction Source Sink

HEC-HMS Basin Model

What do basin elements do? [Combine inputs to single Time Series] Transform input Time Series into output Time

Series (two output TS for diversion) Example: a subbasin convolves input

hyetograph with unit hydrograph to produce direct runoff hydrograph

Geographic data represented as connections and coefficients in transformation equations

Rainfall/Runoff Transformation

Why is HEC Interested in GIS? Description of the physical environment is

primary source of simulation parameters GIS and CADD systems are best methods for

storage, distribution, and transformation of environmental data

GIS is a powerful tool for communication GIS is a step toward more sophisticated

modeling methods

Pre-Existing Complications

Roots of HEC models (e.g. HEC-1, HEC-2) predate current GIS technology

HEC needs to support non-GIS users HEC will not create dependencies on

commercial and proprietary products (except MS Windows ) in its core products

Pre-Processor/Post-Processor Approach Keep models and GIS separate

– Avoid dependencies on proprietary products– Permit use of models without GIS

Identify model parameters and data with GIS sources

Define and publish data import and export methods for models

Develop example GIS applications

Abstraction of Watershed Data Description

– Directly observable: elevation, soil characteristics land cover, drainage elements

Aggregation– Basin, reach definitions: characteristics still tied

to observable phenomena Parameterization

– Mathematical description of limited behavior: coefficients in equations tied to specific methods

HEC-HMS and GIS

Hydrologic Model Parameters

GIS Hydrologic Model

GeoHMS: Basin Definition

HEC-HMS

DEM, NHD, HUC,

STATSGO, Land Use Rainfall Grids or

Time Series

Basin Components Watersheds

Stream Network

Raw GIS Data

Watershed & River Topology

GeoHMS: Model Parameterization

or

Possible Relationships between Arc Hydrology Objects & Basin Elements Basin Element represented as component of

corresponding Hydrology Object Basin Elements and Hydrology Objects

represent each other as interfaces Transformation between Hydrology Objects

and Basin Elements handled by processes external to both