Approaches to Continental Scale River Flow Routing by Kwabena Oduro Asante Dr David Maidment Dr James Famiglietti Dr Francisco Olivera Dr Randall Charbeneau Dr Daene Mc. Kinney
Acknowledgements Dissertation Committee National Science Foundation EROS Data Center of the USGS GIS Hydro Research Group Global Hydrology Group
Dissertation Outline • Chapter 1: General Introduction • Chapter 2: Literature Review • Chapter 3: Data Development • Chapter 4: STS and HMS Methodology • Chapter 5: Model Applications • Chapter 6: Conclusions and Recommendations
Motivation • The changing scope of hydrologic problems • Local scale to global scale • Single phase to full hydrologic cycle • Spatially lumped to spatially distributed • The limitations of current routing models • Local scale models untested at global scale • Lack of integration of hydrologic cycle phases • Scale dependence of existing large scale models
Objectives • To develop a database of hydrologic parameters to support continental scale runoff routing • To implement a continental scale runoff routing system for transferring water balance model outputs to ocean models • To examine the robustness of the modeling approach implemented as compared to a watershed based approach
Chapter 2: Literature Review
Conceptual Models of a River Basin Cell-to-Cell CTC Watershed Based HMS Source-to-Sink STS
Methods of Characterizing Flow Translation with Incidental Dispersion Example: Linear Reservoir Routing I Q Q S t Translation with Surrogate Dispersion Example: Cascade of Linear Reservoirs I s s s S = KQ s Q Q = I * f(k, n) I Q Translation with Physically Based Dispersion Example: Diffusion Wave Routing Q = I * f(v, D) I Q
Chapter 3: Data Development Study Objective 1 “to develop a GIS database to support large-scale surface water routing globally”
Lower Datum and Project DEM Identify Inland Catchments and insert in projected DEM Terrain Analysis Delineate Drainage Basins Compute Flow Accumulation Fill DEM and Compute Flow Direction Compute Flow Length
27 Major Inland Catchments
1500 Major Drainage Basins
GIS Hydro ‘ 99: Digital Atlas of the World Water Balance www. crwr. utexas. edu
2 1 Define Sinks along continental margin and within Inland Catchments Delineate Drainage Basins from Sink Locations Preprocessing for the Source to sink (STS) model 4 Determine routing parameters for each Source from Flow length and other Spatially distributed data 3 Define Sources while preserving basin boundaries as well as Ocean and Atmospheric modeling units
Linking to Ocean and Water Balance Models STS Modeling Units
1 Delineate Stream Network from Flow Accumulation Grid and define outlets at stream intersections 2 Delineate Watersheds from outlet grid and Flow direction grid and convert to a vector coverage Preprocessing for the Hydrologic Modeling System (HMS) 4 Create HMS basin file detailing element properties and connectivity 3 Compute stream and watershed parameters and connectivity
Chapter 4: Methodology Study Objective 2 “to implement a modeling framework which incorporates basin boundaries in a grid based model while maintaining computational efficiency by only performing routing at desired locations”
STS Modeling Assumptions • The control volume is the flow path from a given source to its sink • The transfer of flow along the flow path is a linear process • The parameters of the transfer function are time invariant
STS Model Components Listing of source properties and their connectivity to sinks and to other models Sinks listed by sink id Parameters such as no. of events, outlets, sources and routing interval FORTRAN code for Routing with and without dispersion Input runoff files and output files containing the results of simulation runs
Diffusion Wave IRF V = velocity in m/s D = disp. coef. in m 2/s P = 3. 14159…. . x = distance in m t = time in s
Impulse Distribution about Pure Translation Lag Time
Input Runoff from GCM (generated by Branstetter, M. )
Discharge at Continental Margin
HMS Modeling Assumptions • Each hydrologic element has a unique control volume linked to the next downstream element • The transfer of flow along the flow path may be linear or non-linear • The parameters of the transfer function are time invariant
HMS Model Components Listing of properties of hydrologic elements and their connectivity Description of input runoff and relation to basin elements Simulation parameters such as start and end time and interval Routing Codes for methods assigned in the basin file Data Storage System including input runoff and routed flow data
HMS Flow Routing Subbasin Response by SCS Unit Hydrograph with lag_time = max { (0. 6 maxlagtime in minutes), 3. 5 interval} River Reach Response by Muskingum Routing with Numerical stability n = int (2 x K / 60) + 1 Pure Translation
Chapter 5: Model Applications Study Objective 3 “to examine of the robustness of the source to sink approach as compared to the watershed based approach in continental scale applications”
The Application Basins The Congo Basin Area = 3. 78 million km 2 Mean flow = 45 000 m 3/s The Nile Basin Area =3. 25 million km 2 Mean flow = 2, 500 m 3/s
STS Model Runs STS Model of the Nile Basin STS Model of the Congo Basin
Longitudinal Decomposability in STS 2000 km 1200 km 800 km
Longitudinal Decomposability in STS Cell 4 Cell 3 Cell 2 Cell 1 ü !
Effect of STS Modeling Unit Size Source size = 30’ (60 x 60 km) Source size = 10’ (20 x 20 km) Source size = 5’ (10 x 10 km)
Effect of Spatial Resolution STS basin response for the Congo ü !
Effect of Temporal Resolution STS basin response for the Congo ü !
Effect of Spatial Distribution of V and D on STS basin response for the Nile
Effect of Spatial Distribution of Velocity STS basin response for the Nile Distributed V is important !
Effect of Spatial Distribution of Dispersion STS basin response for the Nile Distributed D is not critical !
Combined Effect of Velocity and Dispersion STS basin response for the Nile Distributed V and D is best !
HMS Model Runs The Nile Basin The Congo Basin
Longitudinal Decomposability in HMS reach length = 162, 000 m flow velocity = 0. 3 m/s muskingum K = 0. 3 n=4 n=5 n=6 n=7 n=8
Longitudinal Decomposability in HMS higher n = less dispersion !
Effect of HMS Modeling Unit Size Stream Delineation Threshold of 10, 000 km 2 Stream Delineation Threshold of 1, 000 km 2
Effect of Spatial Resolution HMS Basin Response for the Congo HMS is spatially scale dependent !
Effect of Temporal Resolution on HMS Congo Basin Response Higher routing interval = more dispersion
Comparing STS and HMS Basin Responses STS Model of the Congo Basin HMS Model of the Congo Basin
Comparing STS and HMS Basin Responses Congo Basin, 1000 km 2 threshold responses almost identical !
Comparing STS and HMS Basin Responses Congo Basin, 10000 km 2 threshold responses at higher threshold not identical !
Comparing STS and HMS Basin Responses Nile Basin, Spatially Distributed V and D
Comparing STS and HMS Basin Responses for Non-uniform Velocity Case, Nile Basin Similar responses result from a common grid of V and D !
Comparing Simulated Flows with Observed Data
Observed Flows after deducting Baseflow
Parameters Obtained by Method of Moments Period 1 Velocity, V in m Disp. Coef. , D in m 2/s Period 2 Period 3 0. 107 0. 110 0. 114 0. 110 3924 7302 6010 5746 Mean
STS Simulated and Observed flows
Routing Input with Mean Parameters (V = 0. 110 m/s, D = 5746 m 2/s) The Method of Moments is suitable for estimating V and D !
Comparing Observed flows with HMS Routed flows
HMS Simulated and Observed flows HMS can be used to describe the hydrology of a large basin !
Chapter 6: Conclusions and Recommendations
Conclusions • GTOPO 30 DEMs are sufficient for the delineation and parameterization continental scale hydrologic models but not for the determination of hydraulic parameters (V, D) • • STS models are suitable for continental scale routing and parameter determination because spatial and temporal scale have minimal effect on their response. • Watershed models are scale dependent with respect to both temporal and spatial scale and are therefore not suitable for global parameterization. However, they can sufficiently represent the hydrology of a large basin. • The method of moments is suitable for the determination of hydraulic routing parameters (V, D) from observed flow data.
Recommendations • Implement automated network calibration of Velocity and Dispersion coefficient for global parameter calibration • Undertake further testing of diffusion type cell to cell routing with a view to replacing the linear reservoir equation • Examine the effects of time varying velocities and dispersion coefficients • Implement nested source to sink models for reservoirs • Implement diffusion wave routing in watershed models
Summary • Processed a Terrain Database to support Hydrologic Modeling globally • Implemented a routing system for global runoff, allowing for interactions with land-atmospheric and ocean models • Developed a continental scale HMS model • Developed a global STS model of the entire earth • Examined the robustness of the STS and HMS models to changes in temporal and spatial modeling scale • Compared both the STS and HMS models to observed flows
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