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Infiltration:
The flow of water from the ground surface into the soil
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Groundwater zone:
Also called the saturated zone, where water completely fills up the pore spaces
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The water table:
The top of the saturated/groundwater zone
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Soil water:
Occurring in the unsaturated zone, where some pores contain water and others, air
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Unsaturated zone:
Where soil is only partially infiltrated with water
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How does water travel in the saturated zone?
Motion along the path of decreasing gravitational potential energy
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What factors affect infiltration rates?
- Water viscosity (the greater, the lesser the infiltration)
- Soil permeability (the greater, the greater the infiltration)
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What affects water viscosity?
Increases with decrease in temperature
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What affects soil permeability?
- Soil type: Fine-grained vs. coarse-grained
- Rainfall intensity: The heavier the rain, the lesser the rate of infiltration (due to the compactment of the soil increasing water being run off
- Water quality: Calcium ions in water makes clay flocculate (the combination of clay particles to form crumbs), increasing permeability, whereas sodium ions cause deflocculation
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How to measure infiltration?
- It would be inaccurate, but our best estimate would be to determine the average depth of rainfall over a drainage basin and subtract from that the measurement of streamflow out of the basin (runoff; this method does ignore evaporation)
- Another way is to map all the soil types in the basin and make infiltrometer measurements on each soil type, similar to the Theissen method of finding total precipitation over a basin.
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Soil water:
Subsurface water in the unsaturated zone
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Porosity (Soil Water; what it is and what it depends on):
- Volume of pore spaces/total volume
- Depends on grain shape, arrangement, and degree of assortment
- (Size not that important)
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Field capacity (Soil Water):
Volume of capillary water/total volume; quantity of water held against gravity
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Gravity water:
Pore volume-capillary water
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Specific yield (Soil Water):
- Volume of gravity water/total volume; after the saturation point
- (Higher with higher permeability)
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How to measure soil in water:
- Gravimetric method: Weigh, dry, reweigh (disadvantage is the removal of study soil)
- Tensiometer: measures capillary “suction” force of soil pores (H2O from tube with porous staff is sucked out as soil dries, creating a measurable vacuum; the relationship between the soil water tension and the particular soil can approximate the soil water content; may be expensive, hoever)
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Irrigation methods:
Gravity (Furrows, Wild Floods), Sprinklers, Drip Irrigation
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Gravity Irrigation:
- Furrows: For “row” crops, from higher to lower elevation
- (Challenges: adequate steepness, erosion, evaporation, infiltration {bad water quality})
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Wild Floods: Surrounding land with embankments to form basin and then flooding it with water
- (Challenges: Uniform distribution, erosion, evaporation, infiltration…30-50% efficiency)
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Sprinklers:
- Used when there is groundwater or soil permeability is too high
- Less water used, higher efficiency, , low risk of erosion/infiltration, but higher energy need
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Drip Irrigation:
- Direct to root irrigation
- Highest efficiency, but high cost, chance of clogging, possible salt deposits
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Most irrigated states:
- (dry west (of 100th meridian), humid east)
- California, Nebraska, Texas, Arkansas, Colorado. Kansas, Idaho, Florida, Washington, Oregon
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Comparison of irrigation between states:
- Irrigation techniques depend of the type of crop and the water source.
- To compare, the west uses =< 20 in. rain/year (generally melted snow)…our gw is >100 ft deep, expensive to pump; East uses ~30 in. rain/year (generally gw)
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How much of the population uses groundwater?
50%
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Groundwater comprises how much of our freshwater supply?
25%
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Zone of saturation:
Groundwater reservoir
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Aquifer:
A geologic formation that is both porous and permeable, storing, trasmiting and yielding significant amounts of water
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What makes a good aquifer?
High porosity, high permeability
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Best aquifers?
Unconsolidated sand/gravel, coarse-grained
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Consolidated Sediments serving as aquifers
- Volcanic rock
- Sandstone
- Limestone
- Fractured grantie
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Unconfined aquifer:
Unconfined beneath the water table (watertable aquifers
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Confined aquifer:
Confined underneath an impermeable boundary
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Perched aquifer:
Groundwater body perched on an impervious formation above the regional water table
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Porosity (Ground Water):
- Ratio of space available to water to volume of aquifer
- Primary: upon first formation of aquifer
- Secondary: after cracking or dissolution has occurred
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Permeability:
- A porous mediums ability to transmit fluid
- Determined by size and degree of interconnected ness between pore spaces
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Darcy’s law
- Q = K A ((h1-h2)/L)
- Where Q = quantity of discharge (Volume/Time)K = coefficient of
- permeability = hydraulic conductivity (property of fluid + material;
- distance/time)Q/A=KiI= (h1-h2)/ L =hydraulic gradient
- Real velocity = Vx =V/n where n=porosity
- V= Ki Apparent Velocity
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Specific retention:
- Volume of capillary water/total volume;
- Capillary water held after a particular drainage time
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