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 McKinney

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 Q t S = KQ Example: Linear Reservoir Routing I Q S Translation with Surrogate Dispersion Example: Cascade of Linear Reservoirs Q I Q = I * f(k,n) s I Q Translation with Physically Based Dispersion Example: Diffusion Wave Routing Q I 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”

Terrain Analysis Identify Inland Catchments and insert in projected DEM Fill DEM and Compute Flow Direction Lower Datum and Project DEM Terrain Analysis Delineate Drainage Basins Compute Flow Length Compute Flow Accumulation

27 Major Inland Catchments

1500 Major Drainage Basins

GIS Hydro ‘99: Digital Atlas Digital Atlas of the World Water Balance www.crwr.utexas.edu

Preprocessing for the Source to sink (STS) model 2 1 Delineate Drainage Basins from Sink Locations Define Sinks along continental margin and within Inland Catchments Preprocessing for the Source to sink (STS) model 3 4 Define Sources while preserving basin boundaries as well as Ocean and Atmospheric modeling units Determine routing parameters for each Source from Flow length and other Spatially distributed data

Linking to Ocean and Water Balance Models STS Modeling Units

Preprocessing for the Hydrologic Modeling System (HMS) 2 1 Delineate Watersheds from outlet grid and Flow direction grid and convert to a vector coverage Delineate Stream Network from Flow Accumulation Grid and define outlets at stream intersections Preprocessing for the Hydrologic Modeling System (HMS) 3 4 Create HMS basin file detailing element properties and connectivity 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 m2/s x = distance in m t = time in s

Impulse Distribution about Pure Translation Lag Time

(generated by Branstetter,M.) 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 n = int (2 x K / 60) + 1 Numerical stability 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 km2 Mean flow = 45 000 m3/s The Nile Basin Area =3.25 million km2 Mean flow = 2,500 m3/s

STS Model Runs STS Model of the Nile Basin STS Model of the Congo Basin

Longitudinal Decomposability in STS 1000 km 1200 km 800 km 2000 km

Longitudinal Decomposability in STS ! Longitudinal Decomposability in STS Cell 4 Cell 3 Cell 2 Cell 1

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 !

Effect of Temporal Resolution STS basin response for the Congo !

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 km2 Stream Delineation Threshold of 1,000 km2

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 km2 threshold responses almost identical !

responses at higher threshold not identical ! Comparing STS and HMS Basin Responses Congo Basin, 10000 km2 threshold responses at higher threshold not identical !

Comparing STS and HMS Basin Responses Nile Basin, Spatially Distributed V and D

Similar responses result 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 2 3 Mean Velocity, V in m 0.107 0.110 0.114 Disp. Coef., D in m2/s 3924 7302 6010 5746

STS Simulated and Observed flows

Routing Input with Mean Parameters is suitable for estimating (V = 0.110 m/s, D = 5746 m2/s) The Method of Moments is suitable for estimating V and D !

Comparing Observed flows with HMS Routed flows

HMS can be used to describe the hydrology of a large basin ! Simulated and Observed flows HMS can be used to describe the hydrology of a large basin !

Chapter 6: Conclusions and Recommendations

Conclusions GTOPO30 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