Watershed Delineation

1
Watershed Delineation

• Procedure
• Data Sources
• Relationship to Groundwater Protection Strategies

2
Why delineate?

• Establish boundaries and internal characteristics
  of the control volume
• Focus attention on components of the hydrologic
  system that are important with respect to a
  management or policy issues
• Identify parts of the hydrologic system that are
  of greatest importance and significance

3
What can be delineated

• Surface areas that represent the part of a
  landscape that contributes flow to a stream
• Surface areas that may influence groundwater
  recharge or groundwater quality
• Either can be delineated at any scale (for
  example, the watershed for the Mississippi River,
  or for a drainage ditch in a development)

4
Techniques for watershed delineation

• Watershed (catchment or drainage basin) area
  contributing flow to a specified outlet (e.g.
  stream gage)
• Defined by topography
• Defined by artificial structures (e.g. roads,
  railway lines, gutters)
• Represents a control volume for inputs and
  outputs that influence a specific hydrologic
  process (streamflow)

5
Topographic maps characteristics

• Present information at various scales
• Represent distribution of elevations of the land
  surface
• Scaled representations
• Scale reflects ratio of depicted length to actual
  length

6
Topographic maps

• Scales include large (lt124,000) and small
  (?124,000)
• Small scale data (large area) are usually
  collected and mapped by federal agencies (e.g.
  U.S. Geological Survey)
• Large scale data are usually collected as part of
  special studies
• Common scales for watershed work include 124,000
  (1 inch 2000 feet),

7
Common methods for collecting topographic
information

• Total station surveys
• Cross sectional surveys
• Stereo aerophotography
• Satellite imagery

8
Levels of accuracy expected

• Accuracy determined by two factors accuracy in
  map development and accuracy in map use
• Map development is technique art
• Map use has a lower bound for information,
  related to measurement accuracy
• Generally, maps can only be read to 1-2 of
  scale (e.g. 1/100th 2/100th of an inch)

9
Implications

• At lower bound and scale of 124,000, level of
  resolution for linear features is 20 feet.
• Errors related to length are compounded (e.g.
  area is a multiplicative function of length (L2))
  
• The smaller the scale, the less accurate the
  inferences drawn from map use.

10
Sensitivity to Area (Example)

• Extremely important system characteristic
• Expressed in units of km2, mi2
• Outcomes of analysis extremely sensitive to
  estimation of A
• Rational Method
• Peak discharge determined by multiplicative
  function of ratio of runoff to rainfall, rainfall
  intensity, area
• QFCiA (F?units conversion constant)
• Used to estimate Qp for small to medium sized
  basins (esp. urban drainage)

11
i4 mm/hr
C.6 (Residential, multiple units detached)
A? 1 km2 (2 of area, given map use accuracy)
Margin of error (upper lower bound map
accuracy) .02 m3/sec 1 ft3/sec Error 120
of the total flow in Galena Creek at USGS gaging
station on Mt. Rose highway on 8/28/03 (.84 cfs)
http//nv.usgs.gov/
12
Delineation Principles

• Water flows downhill
• Exit point for watershed serves as a beginning
  for delineation

13
Procedure (by hand)

• Use a topographic map or survey data

Contour lines
Contour interval 20 units
Contour interval 40 ft typical of 124,000
scale, 50 m, 1100,000 scale
14
Water flows downhill
Watershed boundaries delineate the control volume
of the system under consideration Identify
divides boundaries that enclose areas that
contribute to drainage at another point in the
watershed
Divide crosses contours at right angle
15

• Contours represent topography
• Topography is partly developed by erosive forces,
  such as water movement
• Contours point back into upstream areas

16

• Outlet serves as a first point of reference
• Working upstream from outlet, identify
  topographic maxima adjacent to mapped water
  course
• Refine with field information as needed

17
Commonly used characteristics

• Area
• Stream profile
• Width
• Length
• Average slope
• Stream channel length
• Ratios

18
Estimating area from maps

• Variety of techniques
• Cut-out and weigh
• Planimeter
• Count squaresGIS techniques
• DEMs and delineation algorithms

19
Example Steamboat Creek

• Watershed characteristics
• Drainage area 244 mi2
• Source Washoe Lake
• Course 18 miles
• Enters Truckee River
• Primary source recharge from eastern side of
  Sierra Mountains
• Tributaries Galena Creek, Whites Creek, return
  flows from several irrigation ditches, Steamboat
  Springs

20
Reno
Sierra Mountains
Virginia City
Carson City
21
Implications
Snow fall on this side of the Sierra Mountains
acts as recharge
Contaminant sources on this side of the divide
may affect Steamboat Creek and the Truckee River
Snow fall on this side recharges Lake Tahoe,
Truckee River
Contaminant sources on this side may affect the
Carson River
22
Stream Profile

• Developed by plotting elevation of the stream
  channel (y axis) against distance from a
  reference point (such as the source or outlet)
• Reflects long term effects of climate and
  landforms, especially resistance to erosion.

23
Example James River, Virginia
24
Relationship to Groundwater

• In general, watershed delineation also delineates
  boundaries of groundwater flow system
• Exceptions deep regional flow systems that do
  not have important local recharge from within the
  watershed

25
Groundwater delineation techniques

• Based on simple to complex methods well cover
  these in the groundwater section
• Simplest is a fixed radius circle (1000 feet
  from wellhead)
• Complex involve data acquisition and modeling
• Usually represents an area within a watershed,
  such that wellhead protection area is smaller
  than watershed

26
Wellhead protection areas

• Represent control volumes for groundwater
  recharge zones
• Primarily developed to identify potential sources
  of contamination that could affect groundwater
  quality
• Partly based on risks posed by potential sources
  of contamination (mobility and stability of
  chemicals in the subsurface, toxicity)

27
Example Wellhead Protection
28
Implications
The boundaries of the control volume represent
distances from public water supply wells and
estimated travel times
Travel times are used as measures of risk and are
related to potential chemical breakdown times in
the subsurface
29
Summary

• Watershed delineation is a technique for
  identifying a control volume, which focuses
  attention on processes within an area that will
  affect surface water and ground water quantity
  and quality
• Wellhead protection zones are control volumes
  that represent risk of contamination and reflect
  the slow travel times of subsurface water

30
Next time

• Global water and energy movement
• Readings pp 39-41, 51-52