Hydrology and Water Supply

Where is earth’s water

96.5% in ocean

2.5% freshwater

Within freshwater, what is the distribution of water?

69% glaciers + icecaps

30% groundwater

1% freshwater + surface water

Within freshwater, what is the distribution of water?

ground ice and permafrost

lakes

soil, swamps, atm

rivers, living things

What is a drainage area?

watershed/drainage basin

Area from ridge to ridge that
contributes to the water supply of
the stream/river

Drainages are nested hierarchically

stream order

Perennial streams without
tributaries = 1st order

When two-streams of equal order
come together, the downstream
reach is increased one order

What types of use for groundwater?

agricultural

industry

household

consequences of groundwater depletion

• Land subsidence: Permanent aquifer damage, infrastructure impacts

• Saltwater intrusion: Coastal aquifer contamination

• Economic disruption: Agricultural and municipal water supply threats

3 water pathways in streams

Overland flow

Groundwater flow

Through flow

Overland flow

surface runoff

Water running over a land surface and directly into stream channels
Occurs when precipitation rate > soil infiltration rate

Groundwater flow

Release of water from
underground aquifers into
stream channels

Throughflow

Lateral movement of water through the soil and into stream channels
Follows soil saturation (gravity accelerates process)

Drainage Lake

• Surface inflow and outflow

• Water derived from streams

• High turbidity

• High organic matter inputs

Seepage lake

• Groundwater input

• Low turbidity

• Strong rock influence

• Typically: high nitrate (NO3), low P

Residence Time

Average time one water molecule
spends in a body of water

Gaining reach

base flow increases going downstream (from
groundwater), even without tributary inputs

groundwater recharges stream

Losing reach

the water table is below the bottom of the stream
channel; reach loses water into the ground

stream recharges groundwater

How are velocity and depth related?

Velocity declines exponentially with depth
Mean velocity at about 60% of depth!

How to measure discharge using dilution gaging

• Set a reach (mixing length = 15-20x stream widths)

• Add a known amount of salt to the stream in a single slug

• Track the slug at the end of the reach using a meter or logger

• Use the breakthrough curve to calculate Q (area under the curve)

Modern approaches to measuring discharge?

• Acoustic Doppler Current Profilers: sound waves measure water velocity

• Remote Sensing

Rating curves

• relationship between discharge and river stage

• Assumes that bed and channel banks remain stable

• calc discharge

The hydrograph

continuous record of Q over time

• Base flow: streamflow that is not a direct result of surface runoff

• Rising (ascending) limb

• Peakflow: highest instantaneous
  flow during a time period

• Falling (receding/descending) limb

Substantial overland flow causes rapid increases in Q → flashy peaks

Flood frequency curves

distribution of flows and their recurrence interval

Land use alters streamflow via

• Changing balance of ET vs. runoff

• Altering runoff pathways

• Could alter precipitation patterns (e.g., massive deforestation in Amazon)

How does deforestation impact stream flow?

increases streamflow

• Agricultural cropping = increases mean flow (e.g., tile drainage)

• smaller storms = flashier response

• Urbanization increase impervious surface, increasing rapid runoff

How does climate change impact on temperature change hydrology?

• Increased evapotranspiration: Higher temperatures = greater water demand

• Snowpack dynamics: Reduced snow storage, earlier melt timing

• Growing season extension: Longer periods of plant water demand

How does climate change impact on precipitation pattern change hydrology?

• Timing shifts: Earlier snowmelt, altered seasonal precipitation

• Geographic redistribution: Wet areas getting wetter, dry areas drier

• Intensity changes: More extreme precipitation events, longer droughts

How does climate change impact on extreme events change hydrology?

• Increased flood frequency: More intense storms overwhelm infrastructure

• Prolonged droughts: Multi-year dry periods becoming more common

• Flash flood risk: Urbanized areas face greater runoff challenges