Geospatial Stream Flow Model Geo SFM USGS FEWS

Geospatial Stream Flow Model (Geo. SFM) USGS FEWS NET EROS Data Center Sioux Falls, SD 57198 U. S. Department of the Interior U. 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 GIS Preprocessing Satellite Rainfall Estimates Stream Flow Model GIS Postprocessing Water Balance Rainfall Forecasts GDAS PET Fields Lumped Routing http: /www. fews. net FAO Soil Data Land Use/ Land Cover Elevation Data Dist. Routing Stage Forecasting Flood Inundation Mapping

Geospatial Stream Flow Model, An Arc. View 3. 2 Extension

Using Menus, Message Boxes and Tools Hydrograph plotting 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 Arc. View’s Terrain Analysis Functions with USGS 1 km DEM Flow Direction Flow Accumulation Flow Length Hill Length Slope Subbasins Downstream Subbasin

Key Lessons from Terrain Analysis § Procedures for Terrain Analysis have been refined over the last decade, and they work very well § USGS 1 km DEM (Hydro 1 k) 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) * R 2/3 * S 1/2

Weighted flow length and aggregation algorithm to create Unit Hydrographs Overland Velocity, Flow Time Aggregate cells at basin outlet During each routing interval Flow Path, Flow Length

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 Grids adhere to a naming convention which allows for subsequent automation Gage Data Daily Grids

Zonal algorithms to compute subbasin mean values and export to an ASCII files Rain / Evap Grid Subbasins Output to ASCII File

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

Partitioning Fluxes in single layered model Rainfall Soil layer Ground Water Hortonian with Partial Contributing Areas Saturated Hydraulic Conductivity

Transferring Fluxes in single layered model Rainfall Surface Runoff Unit Hydrograph Soil layer Ground Water Interflow Linear Reservoir + Baseflow Linear Reservoir

Partitioning Fluxes in two layered model Rainfall Upper layer Lower layer Ground Water SCS Curve Number Method Green – Ampt Based Parameterization

Transferring Fluxes in two layered model Rainfall Surface Runoff Unit Hydrograph Upper layer Interflow Conceptual Linear Reservoir Lower layer Baseflow Conceptual Linear Reservoir Ground Water

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 Sub-basin 1 Sub-basin 2 + Main channel Sub-basin 3 + Sub-basin 4 Main channel + Outlet

Within subbasin routing Apply unit hydrograph to excess runoff to obtain runoff at subbasin outlet Runoff Water Balance Unit Hydrograph

Three Flow Routing Options § Pure Translation Routing § Diffusion Analog Routing § Muskingum Cunge Routing

Pure Translation Routing • Only parameter required is lag time or celerity • Simple but surprising effective in large basins Output Flow Input Time

Diffusion Analog Routing Linear routing method Requires two parameters • Velocity for translation • Diffusion coefficient for attenuation Output Flow Input Time

Muskingum-Cunge Routing Non-Linear, Variable Parameter routing method Accounts for both translation and dispersion ch r rea Rive Flow Depth Conceptual reach sections with time varying storage Distance along river reach

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 Risk Generate Daily Historical Rainfall (1961 -90) by reanalysis Produce a synthetic streamflow record Determine locations where bankfull storage Is exceeded Compute Bankfull storage

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 (Geo. SFM) is a semidistributed 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