Hydrology
Hydrology • Watershed – Area that contributes water to a point along a stream • Scale is user-defined • Other names: – Drainage basin – Catchment
• Hydrologic balance in watershed (Fig. 1.1)
– Inputs • Precipitation (Rain, Snow)
– Outputs • ET, streamflow
– Pathway to stream channel • Overland (FAST) • Groundwater (SLOW)
• Hydrograph -- time series
record of streamflow: discharge plotted against time
• 3 terms – Discharge = streamflow = volumetric flow rate • Units: Q = L3/T – (e.g., m3/sec, gallons/day, acre-feet/yr)
– Current velocity = linear flow rate • Units: U (or V) = L/T – (e.g., m/s)
– “Flow” = Q or U
• Hydrograph response to rainstorm (Fig. 1.5) – Precipitation (ppt) occurs – Runoff to stream (pathway) – Rising limb – Peak flow, with some lag – Recession limb – Baseflow
• Factors influencing shape of a hydrograph: • (1) lithology and soils -- infiltration vs. overland flow • (2) topography -- how fast overland flow occurs • (3) vegetative cover (type & extent) • (4) type of precipitation (rain vs. snow) • (5) stream size • Small stream -- generally a relatively fast response to precipitation compared to • Large river -- lots of small tributaries -- integrates small tributary responses
• These factors vary at a variety of spatial scales – Within watershed (e.g., elevation and type of precipitation, geology and soils) – Geographic scales (e.g., volume and timing of precipitation, vegetation cover, topography)
• The shape of the hydrograph tells us a lot about watershed characteristics and biological potential of the river. (Fig. 2) •
• •
Flashy stream -- one whose hydrograph responds quickly to changes in precipitation (little groundwater) – Many extremes, both high and low Stable stream -- one with more sluggish response (more groundwater flow) – Less variable environment • temperature • nutrients • physical disturbance
Q
yr
day
Poff & Ward 1990)
• How do we measure Q? • • • • •
1) Time to fill a bucket 2) Use equation Q = A * U m3/s m2 m/s So, for a river cross section:
– Q=W*D*U (where is U measured??) •
If river has irregular bottom . . . Divide river into “cells” (USGS figure)
• Consider relationship between U and Q •
If Q increases, what happens to U?
•
If the cross-sectional area of stream changes, how does that change U? Consider 3 X-sec’s. Let Q = 10 m3/s. We know … continuity of flow: Q1 = Q2 = Q3
• •
X-sec 1
A1 = 10 m2 U1 = ___ U1 = 10 m3/s / 10 m2 = 1 m/s
X-sec 2
A2 = 50 U2 = ___
m2
U2 = 10 m3/s / 50 m2 = 0.2 m/s
X-sec 3
A 3 = 5 m2 U3 = ___ U3 = 10 m3/s / 5 m2 = 2 m/s
Riffle
Pool
Chute
(wide, shallow, fast)
(wide, deep, slow)
(narrow, deep, fast)
• Why is current velocity (U) important ecologically? – 1) erosional force on organisms – 2) erosional force on sediment (i.e., habitat) • (faster flows --> remove smaller particles) – 3) delivery of nutrients, gases, food; removal of wastes (“physiological enrichment”)
• How does current vary in space and time? – At a point location • Water moves in 3 dimensions – downstream, horizontal, vertical
• Velocity changes over time – Turbulence (velocity fluctuations for a fixed discharge) – Change in discharge
– For whole stream • Different places have different velocity (e.g., riffles vs. pools)
• Is current velocity same everywhere in stream? (Top? Bottom? Middle?) • Why? – As water encounters solid surface . . . Friction! – [deck of cards - how far do cards move as a function of distance from solid boundary?] – No-slip condition (velocity = 0 at boundary)
• Where is there friction in stream?
Water surface
max
Height from Bottom (cm)
– bed – banks – atmosphere [Figure 4.1] • What would a Vertical Velocity Profile look like for Fig. B?
? no slip 0
1
U (m/sec)
• Hydrologic balance in watershed (Fig. 1.1)
– Inputs • Precipitation (Rain, Snow)
– Outputs • ET, streamflow
– Pathway to stream channel • Overland (FAST) • Groundwater (SLOW)
• Hydrograph -- time series
record of streamflow: discharge plotted against time
• 3 terms – Discharge = streamflow = volumetric flow rate • Units: Q = L3/T – (e.g., m3/sec, gallons/day, acre-feet/yr)
– Current velocity = linear flow rate • Units: U (or V) = L/T – (e.g., m/s)
– “Flow” = Q or U
• Hydrograph response to rainstorm (Fig. 1.5) – Precipitation (ppt) occurs – Runoff to stream (pathway) – Rising limb – Peak flow, with some lag – Recession limb – Baseflow
• Factors influencing shape of a hydrograph: • (1) lithology and soils -- infiltration vs. overland flow • (2) topography -- how fast overland flow occurs • (3) vegetative cover (type & extent) • (4) type of precipitation (rain vs. snow) • (5) stream size • Small stream -- generally a relatively fast response to precipitation compared to • Large river -- lots of small tributaries -- integrates small tributary responses
• These factors vary at a variety of spatial scales – Within watershed (e.g., elevation and type of precipitation, geology and soils) – Geographic scales (e.g., volume and timing of precipitation, vegetation cover, topography)
• The shape of the hydrograph tells us a lot about watershed characteristics and biological potential of the river. (Fig. 2) •
• •
Flashy stream -- one whose hydrograph responds quickly to changes in precipitation (little groundwater) – Many extremes, both high and low Stable stream -- one with more sluggish response (more groundwater flow) – Less variable environment • temperature • nutrients • physical disturbance
Q
yr
day
Poff & Ward 1990)
• How do we measure Q? • • • • •
1) Time to fill a bucket 2) Use equation Q = A * U m3/s m2 m/s So, for a river cross section:
– Q=W*D*U (where is U measured??) •
If river has irregular bottom . . . Divide river into “cells” (USGS figure)
• Consider relationship between U and Q •
If Q increases, what happens to U?
•
If the cross-sectional area of stream changes, how does that change U? Consider 3 X-sec’s. Let Q = 10 m3/s. We know … continuity of flow: Q1 = Q2 = Q3
• •
X-sec 1
A1 = 10 m2 U1 = ___ U1 = 10 m3/s / 10 m2 = 1 m/s
X-sec 2
A2 = 50 U2 = ___
m2
U2 = 10 m3/s / 50 m2 = 0.2 m/s
X-sec 3
A 3 = 5 m2 U3 = ___ U3 = 10 m3/s / 5 m2 = 2 m/s
Riffle
Pool
Chute
(wide, shallow, fast)
(wide, deep, slow)
(narrow, deep, fast)
• Why is current velocity (U) important ecologically? – 1) erosional force on organisms – 2) erosional force on sediment (i.e., habitat) • (faster flows --> remove smaller particles) – 3) delivery of nutrients, gases, food; removal of wastes (“physiological enrichment”)
• How does current vary in space and time? – At a point location • Water moves in 3 dimensions – downstream, horizontal, vertical
• Velocity changes over time – Turbulence (velocity fluctuations for a fixed discharge) – Change in discharge
– For whole stream • Different places have different velocity (e.g., riffles vs. pools)
• Is current velocity same everywhere in stream? (Top? Bottom? Middle?) • Why? – As water encounters solid surface . . . Friction! – [deck of cards - how far do cards move as a function of distance from solid boundary?] – No-slip condition (velocity = 0 at boundary)
• Where is there friction in stream?
Water surface
max
Height from Bottom (cm)
– bed – banks – atmosphere [Figure 4.1] • What would a Vertical Velocity Profile look like for Fig. B?
? no slip 0
1
U (m/sec)
