415w03_9_drainbasin.docx

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GEOG415

Lecture 9A: Drainage Basins

Drainage basin (watershed, catchment) - Drains surface water to a common outlet Drainage divide - how is it defined?

Scale effects? - Represents a hydrologic cycle

- Open system Exceptions?

“Watershed hydrology” e.g. logging impacts

Dunne and Leopold (1978, Fig. 14-1)

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Typical requirement for a watershed-hydrology project - Stream gauging station at the outlet Objectives?

Hydrologic response unit Collection of areas having similar response to rain, snow, etc.

What controls the response? Scales: current challenge in hydrology Hydrological process studies are conducted at scales of 1100 m. How do we integrate this information into a model of watershed (1-10 km)? → up-scaling Outputs of global climate models are given at scales of several hundred kilometers. How do we feed this information into a watershed model? → down-scaling

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Drainage-basin form Basin form and channel patterns affect the hydrologic response of the basin. Palmate channel network (Fig. 14a and b) Stream discharge gradually increases downstream.

Pinnate channel network (Fig. 14c) Stream discharge suddenly increases at point C.

C

Dunne and Leopold (1978, Fig. 14-2)

Basin size and discharge Average basin discharge (flow rate) is proportional to basin area, if the topography, geology, and climate are the same.

Why?

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Stream orders How do we classify the rivers from a hydrological view point? e.g. Bow River vs Nose Creek

This diagram shows stream orders according to the Strahler system. How does it work?

Dunne and Leopold (1978, Fig. 14-4)

Scale effects Order of a particular stream assigned on a 1:25,000 map sheet and on a 1:250,000 map sheet. Are they same or different?

“Zero-th” order channels They do not show on map sheets as water courses, but becomes channels during storms. Importance?

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Drainage density = Total channel length (km) / Basin area 2

(km ) How does this affect the hydrologic response?

What controls drainage density?

Scale effects? Area 2

(km ) Fig. 14-2(a) 3.4 Fig. 14-2(b) 32.6 Fig. 14-2(c) 245.2

Channel length Drainage Density (km) 11.1 99.8 121.9

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(km km ) 3.27 3.06 2.01

Relief ratio = elevation difference, highest - lowest length of the basin, parallel to the main stream

Relief ratio indicates the average slope of the basin.

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discharge / basin area (mm/d)

Example: Blackstone Creek and Jean-Marie Creek, NWT

Drain. Dens. -2 Blackstone 0.524 km km

3

Jean-Marie 0.237 km km

Relief Ratio 0.0055 -2

0.0034

2 1 0 4/1

5/1

6/1

7/1

8/1

9/1

10/1 11/1

1999

Why is blackstone hydrograph more peaky? Why does Jean-Marie has a slower response?

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9-7

GEOG415

Lecture 9B: Water Balance

Water balance of a drainage basin ∆SM + ∆GWS = P - I - AET - OF - GWR ∆SM: soil moisture storage change ∆GWS: groundwater storage change P: precipitation I: interception AET: evapotranspiration OF: Overland flow GWR: groundwater runoff → baseflow

Dunne and Leopold (1978, Fig. 8-1)

Water-balance evaluation provides essential information for: -

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Some parameters are relatively easy to measure.

Others are almost impossible to measure.

Hydrologic models are commonly used to estimate water balance. Dunne and Leopold (1978, p.239-244) presents an example using a “double-tank” model. P Model inputs are precipitation (P) and AET potential evaporation (PET), and available water capacity (AWC) of the soil. Actual evaporation (AET) is calculated from PET and AWC. When P > PET, AET is assumed equal to PET. Rationales? Moisture surplus (S) occurs when P - AET exceeds the amount required to fill up the upper tank. Half of moisture surplus is detained in the lower tank, and the rest is drained as runoff (RO). Rationales?

SM S Detention RO

Difference between PET and AET (soil moisture deficit) is an indicator of the stress on plants and crops.

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Results of model simulation (DL, Table 8-1) P, PET (mm/month)

300

Model Inputs

P PET

200 100

moisture deficit (mm) soil moisture (mm)

0 200 100 0 30 20 10 0

runoff (mm)

100 50 0 Jan Feb Mar

Apr May Jun Jul Aug Sep Oct Nov Dec Jan

This model is a useful tool for demonstrating effects of climate. However, it is too crude for accurately simulating the water balance of a specific site. Hydrologic models all have usefulness and limitation.

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Model calibration To be used as quantitative tools, models need to be calibrated. What is calibration?

stream discharge (m3/s)

How is it done? model simulation

actual observation

time (day)

Lumped hydrologic model The hydrologic model described above is called a ‘lumped’ model, which integrates all hydrological processes in the basin in a ‘black box’. The calibrated model may produce sufficiently accurate results for practical purposes, but offers little insight into physical processes. Problems: - Transferability - Landuse change - Spatial variability

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Distributed hydrologic model - A basin is divided into a number of grid cells. → spatial information represented by GIS - Different types of geology, topography, vegetation, etc. within a basin → explicitly represented in the model.

- Remote sensing techniques for parameter estimation. - Lateral transfer of water between grid cells. - Physically-based calculation of evaporation and runoff. → energy balance for evaporation Darcy’s law for infiltration

With all these improvements, distributed hydrologic models are far from being reliable tools. Challenges are: - misrepresentation of physical processes (simply wrong!) - large uncertainties of parameters (soils, vegetation, etc.) - scaling issues - lack of data to calibrate the models against.