Geospatial Stream Flow Model(GeoSFM)

USGS FEWS NETEROS Data Center

Sioux Falls, SD 57198

U.S. Department of the InteriorU.S. Geological Survey

Objectives

To develop a model is a wide-area flood risk monitoring using existing datasets

To use the model to routinely monitor flood risk across Africa and provide early warning to decision makers

GIS IN FLOOD MONITORING The Mid-West Floods of 1993

Creation of Global Elevation Datasets for hydrologic modeling in 1997

Initiation of GIS-based distributed flood modeling at the USGS in the late 1990’s;

Now being applied in Southern Africa, East African and the Mekong River Basin in Vietnam

Model Overview Leverage the vast geospatial data archived at EDC

• Initial parameters derived from existing datasets• Input data generated daily from available datasets

Catchment scale modeling framework • Semi-distributed hydrologic model• Inputs aggregated to the catchment level

GIS based Modeling • Takes advantage of existing spatial analysis

algorithms• Includes integration with external routing codes

FEWS Flood Risk Monitoring System Flow Diagram

Flood Inundation Mapping

GIS PostprocessingGIS Preprocessing

SatelliteRainfall Estimates

GDAS PET Fields

FAO Soil Data

Land Use/ Land Cover

Elevation Data

Rainfall Forecasts

Stream Flow Model

Water Balance

Lumped Routing

Dist. Routing

Stage Forecasting

http:/www.fews.net

Geospatial Stream Flow Model, An ArcView 3.2 Extension

Using Menus,Message Boxes and Tools

Hydrographplotting tool

Tool for Dam Insertion

Model Components Terrain Analysis Module Parameter Estimation Module Data Preprocessing Module Water Balance Module Flow routing Module Post-processing Module

Terrain Analysis Module

The goal of Terrain Analysis to divide the study area into smaller subbasin,

rivers

to establish the connectivity between these elements

to compute topography dependent parameters

Using ArcView’s Terrain Analysis Functions with USGS 1 km DEM

Flow Direction

Flow Accumulation

FlowLength

Hill Length

Slope

Downstream Subbasin

Subbasins

Key Lessons from Terrain Analysis

Procedures for Terrain Analysis have been refined over the last decade, and they work very well

USGS 1km DEM (Hydro1k) is sufficient for delineation in most basins; it is currently being refined for trouble areas

Parameter Estimation Module

The goal of Parameter Estimation

to estimate surface runoff parameters in subbasins

to estimate flow velocity and attenuation parameters

to summarize parameters for each subbasin

Estimating Surface Runoff Characteristics

Initially computed on a cell by cell basis

Now moving towards generalizing land cover and soil class over subbasin first

(Maidment (Ed.), 1993, Handbook of Hydrology) (Chow et al, 1988, Applied Hydrology)

Overland Velocity with Manning’s Equation

Initially computed on a cell by cell basis

Now moving towards generalizing land cover and slopes class over subbasin first

V = (1/n) * R2/3 * S1/2

Weighted flow length and aggregationalgorithm to create Unit Hydrographs

Overland Velocity, Flow Time

Flow Path, Flow Length

Aggregate cells at basin outlet During each routing interval

n

i i

i

vl

t 1

Key Lessons in Parameterization While GIS routines work well, existing parameter

tables in hydrology textbooks are only of limited utility

There is no on-going effort to document parameters from previous studies though these are often extremely useful

Uniform parameter estimates are often at least as good spatially distributed parameters; simpler is better

Field observations and local estimates are invaluable

Data Preprocessing Module

The goal of Data Processing

to convert available station & satellite rainfall estimates into a common format

to set up ascii files for water balance and flow routing models to ingest

Interpolation routines to grid point rainfall data

Daily GridsGage Data

Grids adhere to a namingconvention which allowsfor subsequent automation

Zonal algorithms to compute subbasin mean values and export to an ASCII files

Rain / Evap Grid Output toASCII File

Subbasins

Key Lessons in Data Preprocessing

Using a single rainfall value for each subbasin is consistent with the resolution/precision of the satellite rainfall estimates

Saving data values in ASCII files (instead of directly assessing the grids) speeds up subsequent flow routing computations considerably

Water Balance Module

The goal of Water Balance

to separate input rainfall into evapotranspiration, surface, interflow, baseflow and ground water components

to maintain an accounting of water in storage (soil moisture content) at the end of each simulation time step

Conceptual Model of Water Balance

Two Water Balance Options Single layered soil with

• Hortonian with partial contributing areas• Same subsurface reservoir but multiple

residence times for interflow and baseflow

Two layered soil with• SCS Curve Number Method• Separate reservoirs and residence times for

interflow and baseflow

Soil layer

Ground Water

Saturated Hydraulic Conductivity

Hortonian with Partial Contributing Areas

Rainfall

Partitioning Fluxes in single layered model

Ground Water

Soil layer Interflow Linear Reservoir

+Baseflow Linear Reservoir

Unit Hydrograph

Rainfall

Surface Runoff

Transferring Fluxes in single layered model

Upper layer

Lower layer

Ground Water

Green – Ampt Based Parameterization

SCS Curve Number Method

Rainfall

Partitioning Fluxes in two layered model

Upper layer

Ground Water

Interflow

Baseflow

Lower layer

Conceptual Linear Reservoir

Unit Hydrograph

Rainfall

Surface Runoff

Conceptual Linear Reservoir

Transferring Fluxes in two layered model

Key Lessons in Water Balance SCS Curve number classes don’t match up very

well with land cover / vegetation classes

Hortonian with partial areas performs at least as well and is easier to parameterize than SCS method for runoff generation

Recession portion of the hydrograph has been the most difficult to model correctly

Flow routing Module

The goal of Flow Routing

to aggregate the runoff contributions of each subbasin at the subbasin outlet

to move the runoff from one subbasin to the next, through the river network to the basin outlet

Routing Overview

Outlet

Sub-basin 3

Main channel

Sub-basin 2

Sub-basin 4

+

+

+

Sub-basin 1

Main channel

Within subbasin routingApply unit hydrograph to excess runoff to obtain runoff at subbasin outlet

Water Balance

Runoff

Unit Hydrograph

Three Flow Routing Options

Pure Translation Routing Diffusion Analog Routing Muskingum Cunge Routing

Pure Translation RoutingFl

o w

Time

InputFl

ow

Time

Output

• Only parameter required is lag time or celerity• Simple but surprising effective in large basins

Diffusion Analog RoutingLinear routing method

Requires two parameters• Velocity for translation• Diffusion coefficient for attenuation

Flo w

Time

InputFl

ow

Time

Output

Muskingum-Cunge RoutingFl

ow D

e pth

Distance along river reach

River reach

Conceptual reach sections with time varying storage

Non-Linear, Variable Parameter routing method Accounts for both translation and dispersion

Key Lessons in Flow Routing The less parameters you have to estimate, the

easier it is to obtain a representative model

The ease of developing a representative model (not precision of the model) determines whether or not end users adopt the model

I highly recommend the diffusion analog model for large scale applications; it achieves a reasonable balance between simplicity and process representation

Post-processing Module

The goal of Postprocessing to compute flow statistics (max, min, mean,

25, 75, 33, 66 and 50 percentile flow)

to rank and display current flows relative to percentile flows (high, low, medium)

to perform preliminary inundation mapping (based on uniform flow depths within each reach)

to display hydrographs where needed

Characterizing Flood RiskGenerate Daily

Historical Rainfall (1961-90) by reanalysis

Produce a synthetic

streamflow record

Compute Bankfull storage

Determine locations where bankfull storage

Is exceeded

Colour coded maps to indicate level of risk

Hydrographs with their historical context

Nzoia Basin, Kenya

Nzoia Basin, Modeled vs Observed Streamflow

Limpopo River Basin

Olifants, Kruger National Park - Mamba

Key Lessons in Postprocessing The importance of hydrographs to decision

makers is highly overrated

The most important questions decision makers want answered are how many people were/will be affected, and where are they?

Risk maps and flood maps are far better methods of commuting to decision makers than hydrographs

Estimates of affected/at risk populations and their locations are the most useful outputs of the hydrologic analysis

Conclusions The Geospatial Stream Flow Model (GeoSFM) is a semi-

distributed hydrologic model for wide-area hydrologic analysis

It uses globally available terrain, soil and land cover data, and satellite derived estimates of daily rainfall and PET

The model outputs include stream flow and flood hazard maps

Preliminary results of model validation in the Nzoia and Limpopo river basins were satisfactory

The model continues to evolve in response to field applications