Geo lecture #16

Hydrologic cycle 

• Residence time= average length of time that water spends in a reservoir=reservoir size/yearly flux

• Average residence times:

-oceans= 4,000 years

-glaciers= 20-1,000 years

-seasonal snow cover=2-6 

-soil moisture=1-12 months

-groundwater 100 to 10,000

-lakes 50-100 years

-rivers 2-6 months 

-atmosphere 10 days

Rainfall patterns 

• Controlled by large-scale atmospheric circulation 

Atmospheric circulation 

• Low pressure at the surface where air rises- moisture condenses as air rises and cools and rainfall occurs

• High pressure where air sinks-air is dry and deserts form 

Monsoon

• Seasonal sea breeze significant for precipitation 

El Nino/La nina 

• Interannual changes in ocean and atmospheric circulation across the tropical pacific

• Impacts rainfall patterns

Current el nino/la nina conditions 

• Current la nina conditions returning to enso-neutral 

Climate change and precipitation 

• Increases and drought event 

Drought

• Deficiency of precipitation over a prolonged period of time, usually at least one season 

• Reduced surface and groundwater levels 

• Ca experienced a 3 year record drought 2013-2015

Flooding in california 

• Major floods in winter 2023-2024 ca has a history of major floods but exacerbated by climate change 

Groundwater

• Water present in the subsurface including soil moisture as well as water below the water table 

-water table= upper surface in the earths below which all pore spaces in sediments and rocks are filled with water

• Water can exist in aquifers= rock and sediment units below the surface that can hold and transport water

Aquifers in california 

• During an average year, 515 groundwater basins contribute about 38% of the states total water supply, but this increases to about 46% in drought years 

Overdraft 

• If supply of water to a water-table aquifer is less than its discharge, the water table will lower, decreasing the size of the aquifer 

Rives 

• Two sources: runoff and baseflow (water below the water table)

• Drainage basin is the land surface from which all the surface runoff drains: also called a watershed

Glaciers 

• Glaciers are large accumulations of ice that exist year-round on the land surface 

• Supply many rivers (Himalayan glaciers, “The third Pole” supply brahmaputra, ganges, indus , mekong, and yangtze rivers

Climate change and melt water 

• Higher temperatures leading to greater summer melt 

• Smaller area of ice and snow cover means less melt water with time

• Peal discharge is shifting to late spring/early summer

Climate change and sea level 

• Sea level rise is due to thermal expansion of seawater and melting of glaciers 

• Rising sea levels causing seawater invasion of coastal freshwater aquifers