The flow of water from the ground surface into the soil
Also called the saturated zone, where water completely fills up the pore spaces
The water table:
The top of the saturated/groundwater zone
Occurring in the unsaturated zone, where some pores contain water and others, air
Where soil is only partially infiltrated with water
How does water travel in the saturated zone?
Motion along the path of decreasing gravitational potential energy
What factors affect infiltration rates?
Water viscosity (the greater, the lesser the infiltration)
Soil permeability (the greater, the greater the infiltration)
What affects water viscosity?
Increases with decrease in temperature
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
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.
Subsurface water in the unsaturated zone
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)
Field capacity (Soil Water):
Volume of capillary water/total volume; quantity of water held against gravity
Pore volume-capillary water
Specific yield (Soil Water):
Volume of gravity water/total volume; after the saturation point
(Higher with higher permeability)
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)