Integrating GIS into Distributed Modeling in Hydrology

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Lecture Outline • • Structure of a watershed Watershed and stream network delineation Integration of TOPMODEL and GIS Simulation results from a case study

Watershed — the area enclosed within a drainage boundary Drainage divide — a line defined topographically which separates distinct areas of land drainage. Drainage boundary — a closed line drawn along drainage divides

Subwatershed — a subdrainage area within a watershed Outlet — a location on the flowline, upstream of which a drainage area is defined.

A grid defines geographic space as a matrix of identically-sized square cells. Each cell holds a numeric value that measures a geographic attribute (like elevation) for that unit of space.

Reach — a length of channel considered as a single hydrologic entity. • Example: a length of river between two tributaries

Waterbody — a volume of water having a horizontal water surface, which is defined within a specific area. • Width is significant when compared to the length. • Examples: lake, pond, reservoir, swamp, marsh, bay.

Flow Network — a set of connected flowlines through channel reaches and water bodies Also called River Network, Stream Network.

Reach catchment — the drainage area locally defined around a particular channel reach. The drainage water from the reach catchment area flows to this channel reach before encountering any other downstream channel reaches or waterbodies.

Watershed Delineation by Hand 20 ft contour Watershed divide 100 ft contour Stream Center Line Drainage direction Outlet

Digital Elevation Grid — a grid of cells (square or rectangular) in some coordinate system having land surface elevation as the value stored in each cell. Square Digital Elevation Grid — a common special case of the digital elevation grid

Direction of Steepest Descent 30 30 67 49 67 56 49 52 48 37 58 Slope: 56 55 22 58 55 22

Eight Direction Pour Point Model D 8 4 3 1 5 6 2 7 8

Grid Network

Contributing Area Grid 1 1 4 1 1 1 1 3 12 2 3 6 1 1 1 3 1 1 4 3 3 1 2 1 1 1 1 1 2 16 1 1 3 6 25 2 16 25 2

Contributing Area > 10 Cell Threshold 1 1 1 4 3 3 1 12 1 1 2 16 1 1 3 6 25 2

Watershed Draining to This Outlet

Watershed and Drainage Paths Delineated from 30 m DEM Automated method is more consistent than hand delineation

Hillslope Modelling Precip’n E T Precipitation & ET dry Lateral subsurface flow wet Streamflow

TOPMODEL • The most popular distributed hydrological model. • Two major advantages: Simplicity and the possibility of visualizing the predictions of the model in a spatial context.

Assumptions • The dynamics of the water table can be approximated by uniform subsurface runoff production per unit area over the area, A, draining through a point, and • The hydraulic gradient of the saturated zone can be approximated by the local surface topographic slope, tan B

Topmodel (Based on Beven and Kirkby, 1979 and later) Assumption 1. Hydraulic conductivity decreasing with depth - sensitivity parameter f Assumption 2. Saturated lateral flow driven by topographic gradient and controlled by depth to water table (soil moisture deficit). Assumption 3. Steady state. Saturated lateral flow related to equilibrium recharge rate. Determines depth to water table and saturation excess runoff generation when z < 0

• Wetness Index – wetness index = ln(As / tan. B), where: • As = Contributing Catchment Area in meters squared • B = Slope of cell measured in degrees