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Watersheds and the Hydrologic Cycle
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The Global Hydrologic Cycle
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Water Cycle in Florida
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Florida Water Facts • Surface Area = 170,452 km2
• Average Rainfall = 140 cm (55”) – Total Annual Rain = 238 billion m3 (62.6
trillion gallons)
• River and Stream = 51,858 miles
• Largest River = Apalachicola (Q = 25,000 ft3/s = 22 km3/yr)
• Lake Area = 12,100 km2
• Springs ~ 8 billion gallons/ day
• Groundwater supplies 95% of water – 7.2 billion gallons/day; 80% to S. Florida
• Low relief – travel time for water (rainfall to sea) can be decades
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The Hydrologic Check-Book
• Mass Balance – Water mass is conserved
– Therefore: Water In = Water Out
• Sources – Rain, Snow, Groundwater, Human Effluent
• Sinks – Ground, River, Atmosphere, Humans
• Stores – Wetlands, Lakes, Rivers, Soil, Aquifers
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The Water Budget (Exam 1)
P= Q + ET + G + ΔS
Precipitation
Surface runoff
Evapotranspiration
Groundwater
Storage
INFLOW OUTFLOW
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Rainfall (~ 140 cm)
Infiltration to Deep Aquifer (~5 cm, though upto ~ 40 cm)
Subsurface Runoff (~ 32 cm)
Interception (~ 30 cm)
Transpiration (~ 70 cm)
Annual Water Budget - Flatwoods
Surface Runoff (~ 3 cm)
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Rainfall (~ 140 cm)
Infiltration to Deep Aquifer (~5 cm, in areas much higher)
Interception (~ 15 cm)
Transpiration (~ 80 cm)
Annual Water Budget – Ag Land
Surface Runoff (~ 20 cm)
Subsurface Runoff (~ 20 cm)
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Rainfall (~ 140 cm)
Infiltration to Deep Aquifer (~ 2 cm)
Interception (~ 20 cm)
Transpiration (~ 50 cm)
Annual Water Budget – Urban Land
Surface Runoff (~ 60 cm)
Subsurface Runoff (~ 5 cm)
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In > Out ?
Out > in ?
In = Out ?
What if Inmeasured > Outmeasured
AND water level is falling?
Changes in Internal Stores • ΔStorage = Inputs – Outputs
– Usually easy to measure (e.g., lake volume)
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Volume = Depth x Area
Depth = Volume / Area
Rules of Water Balances • Water balance terms must be in common units
– (usually meters depth over the watershed area).
• Precipitation and ET measured in depth (m/yr)
• Water flows measured as volumes (m3/yr).
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Catchment Water Balance
• Area = 100 ha (10,000 m2 per ha)
• Annual Measurements:
– Rainfall = 2 m (tipping-bucket rain gage)
– Surface outflow (Q) = 2,000,000 m3 (weir)
– ET = 1.5 m (evaporation pan)
– Groundwater = 1,000,000 m3 (shallow wells)
– Assume ΔS=0
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Budget: P= Q + ET + G + ΔS • Area = 100 ha; 1 ha = 10,000 m2
• P = 2.0 m
• Q = 2,000,000 m3/(100 ha * 10,000 m2/ha)
= 0.2 m
• ET = 1.5 m
• G = 1,000,000 m3/(100 ha * 10,000 m2/ha)
= 0.1 m
• ∆S = 0
• 2.0 = 0.2 + 1.5 + 0.1 + 0 (?!)
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Watersheds
• A land area from which all rainfall drains to the same point.
– The “watershed” is technically the divide between two such areas (called basins)
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Delineating Watersheds 1. Identify
outlet point
2. Identify high points
3. Link high pts crossing contour lines at right-angles
1
2
3
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High-Relief Watersheds
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Why Watersheds?
• Control boundaries
– Water that falls either comes out the bottom or is abstracted
• Imagine a water budget if you weren’t sure where the water was going…
P + Qin1 + Qin2 = Qout1 + Qout2 + ET + G1 + G1
+ ΔS
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Experimentation – Paired Watersheds
• What is the effect of forest management on: – Water yield
– Sediment yield
– Nutrient export
– Water temperature
– Time of transport
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Paired Watershed Response
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Paired Watershed Study of Forest Harvest Effects on Peakflows
Beschta et al. 2000 – H.J. Andrews Experimental Forest (OR)
LOGGING ROAD BUILDING
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Does watershed delineation work in Florida?
• Low-relief – Delineation of boundaries
hard
– In parts of Florida, water flows depend on:
• where it rained,
• where the wind is blowing and
• where people put the canals and roads
• National contour maps too coarse (5 ft)
• Roads act as flow delineators
????
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“Delineating” Watersheds in
Florida
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Paired Watersheds in Florida
• Shown that (Riekerk 1983):
– High intensity logging increased water yields by 250%
– Low intensity logging increased water yields 150%
– Clear cutting altered nutrient export rates
– All forest operations (fire?) were less than urbanization (much more on this later)
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Stream Patterns
• Dendritic patterns follow Strahler Order
– 1st order is unbranched
– 2nd order occurs when two 1st order reaches converge
– Increased order requires convergence of two reaches of the same order
– What are springs?
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Network Forms
• Regional landform dictates shape
• Regional geology dictates drainage density
• Regional climate (rainfall) dictates density and maturation rate
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Stream Order Distribution
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Watershed Networks
• Mass transport is optimal at minimum work – Reach and network scales at the
SAME time
• Necessary conditions for dendritic drainage – Minimal energy expenditure at
any link in the network
– Equal Energy Expenditure per unit area
– Minimum Energy Expenditure in the Network as a whole
Rodriguez-Iturbe et al. (1992) - WRR
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Water Convergence Across Scales
- St Johns River Basin
- Ocklawaha River Basin
- Orange Creek Sub-Basin
- Newnans Lake Watershed
- Hatchett Creek Watershed
- ACMF “Hillslope”
- Lake Mize catchment
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Lake Mize hillslope Lake Mize
Conference Center
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Austin Cary Forest
Hatchett Creek
Lake Mize
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Hatchet Creek Catchment
Hatchett Creek
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Newnans Lake
Watershed Newnans
Lake
Gainesville
Santa Fe Swamp
Hatchett Creek
Lake Forest Creek
Prairie Creek
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Orange Creek Basin
Newnans Lake
Lochloosa Lake
Orange Lake
Paynes Prairie
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Ocklawaha River Basin
Orange Creek Basin
Lake Apopka
Rodman Reservoir
Ocklawaha River
Silver Springs
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St. Johns River Basin
(and Water Management
District) St. Johns River
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Next Time…
• Precipitation
– Where it falls
– When it falls
– How it’s made
– How we measure it