Biol 333: Lecture 8: Geologic Setting

What two factors result in a wetland hydroperiod?

1.  Surplus of water

• Excess water

• Water balance

• Dynamic!!

2. Capacity for water storage near the surface

• Geologic setting

• Landscape position

• A capacity to hold or bring up water near the surface; often dependent on soil properties

• Static!!

How does geologic setting influence hydroperiod?

1. Total water storage (TS) is fixed (Geologic Setting)

= storage capacity when dry determines maximum ΔS

2. Antecedent storage moisture = currently how full or wet

3. Potential / available storage = what the wetland can take at this time before overflowing

= TS - antecedent

What is a depression in a wetland?

an impermeable basin; low to zero groundwater recharge

• Low to moderate storage capacity

• < 2m deep when full

What is detention in a wetland?

maintains near surface water by resistance to outflow; can be found in flat land or on slopes

• Low to moderate storage capacity

• Soil slows down water

What size of storage is ideal for wetlands?

Small storage is ideal; requires lots of water to fill

What does total storage size determine?

• Volume inputs needed to fill storage

• Frequency and duration of saturated soils, standing water

• Runoff generations

• Memory or time to respond following extended droughts

What does hydroperiod result from?

Δ volume of water (balance of inflows + outflows); temporally dynamic. = ΔVol

Divided by:

Geologic setting // water storage: non dynamic = A(WL)

I. Surface contour of basin / depression

II. Subsurface of soil, groundwater conditions (SY)

What is the formula for hydroperiod?

ΔWL = ΔVol / [A(WL) x SY]

How can steep gradient edge vs low graident edge influence hydroperiod?

• Low gradient edge = large changes in area with small changes in WL

• Steep gradient edge = small changes in area, large changes in WL

What are the two kinds of soil in wetlands?

1. Mineral Soils = Sand, Silt and Clay

2. Organic Soils = Fibric peat, Mesic peat and Humic Peat

• Properties change with depth and decomposition

Does pore size equal total porosity?

No

What are the two aspects of total porosity?

1. Specific Yield: SY = the volume of water that drains freely out of a soil

2. Holding Capacity HC = the volume of water held in soil after drainage

SY + HC = Total porosity

How is porosity ranked for wetland soils?

Organic soils:  clay > silt > sand

Mineral Soils: fibric > mesic > humic

How is SY ranked for wetland soils?

Organic soils: sand > silt > clay

Mineral Soils: fibric > mesic > humic

How is HC ranked for wetland soils?

Organic soils: clay > silt > sand

Mineral Soils: humic > mesic > fibric

What soils are best for wetland?

Soils with small pores, low hydraulic conductivity and low specific yield

= Hard to drain, easy to rewet

→ Maintain WT near surface

How can we compare wetlands for water balance and similar storage?

Different water balance with similar storage

• Different hydroperiod

• Different wetland type and function

Different water balance with different storage

• Can have similar hydroperiod
  
  

Similar water balance with different storage

• Different hydroperiod

• Different wetland type and function

What are three important things to consider for constructing wetlands?

• Water budget

• Basin shape

• Soil

What is a diplodemic soil structure?

Near surface = hydrological active layer = acrotelm

• Fibric peat and mesic peat

• Lets go of water; High k

• Harder to wet up; Low SY

Deeper layers = hydrological inactive layer = catotelm

• Humic peat

• Holds on to water; low k

• Easy to wet up; low SY

→ Peatands are hard to dry out and easy to wet up – but not flood

Explain wetland distribution based on water balance and geomorphology in Coastal BC and Quebec?

• Not extensive distribution

• P » PET

• Hilly to rugged

Explain wetland distribution based on water balance and geomorphology in the prairies?

• Extensive

• P < PET

• Flat; depressions and poor drainage

Explain wetland distribution based on water balance and geomorphology in the Hudson Bay Lowlands?

• Extensive

• P > PET

• Flat

Explain wetland distribution based on water balance and geomorphology in northern Alberta?

• Extensive peatlands

• P < PET

• Flat to undulating

When were the major glaciation events in Canada?

• Canada has glaciated several times during the Quaternary 

• Last major climate cycle began 125,000 years ago with alternating glacial and non-glacial periods

• Most recent event occurred 25,000 - 10,000 years ago

Where did some of the maximum glacial extents occur?

• Lots of glacial lakes and seas in areas that are now wetlands!

• Maximum glacial lake extend in AB!

• Maximum glacial seas extend in Hudson bay

What does the water balance of peatlands look like?

• AET < PET due to reduced evaporation if the water table drops below 25 cm

• Precipitation peaks during summer at the same time as PET ; which reduces periods of high run off generation and no period with substantial water table draw down

What does the geomorphology of peatlands look like?

• Drained pro-glacial lakes; flat with poor drainage = detention storage

• Diplotelmic peat profile; peatlands are hard to dry out and easy to wet up

When did peatlands likelu form? and why?

• ~5,000 - 10,000 years ago

• Possibly when P was greater or PET was lower → more favourable climate

• Once peat forms it creates conditions that hold on to water

• Peatlands can remain in climates which are not favourable for their initial development

Why is it unusual that there are extensive wetlands in northern Alberta?

It is an area with a moisture deficit

Even including areas of large bogs which rely only on P for moisture