Watershed Quiz 5 Flashcards
Evaporation
Net loss of water from a surface due to
1.water going from liquid to vapor
2.net transfer or water vapir to atmosphere
Conditions necessary for evaporation
Water + energy = evaporation
And must coincide at the same time
Evaporation dominant controls
-High VPD = faster water vapor moves from evaporating surface
-Distance from ea to es drives evaporation
-wind drives evaporation rates
-atmospheric demand = (es-ea)
Evaporation fomula
E=Cu(es-ea)
U= wind
C=constant
VPD on windy day
Windy day keeps VPD large because wind keeps ea low
Soil water evaporation
Saturated= similar rates to free water
Dry=rates slow but don’t stop
Unsaturated soils mean less evaporation
Why do unsaturated soils have less evaporation
-disconnection of soil water from atmosphere
-adhesion of water to soil
-build-up of water vapor in pore spaces
Do finer textured soils or coarse soils have faster evaporation rates?
Finer soils because soils can cling to surface
What is evaporation like for soils under a canopy?
E is minor because
-interception from canopy
-understory and litter insulate
-reduced wind speed
-transportation of canopy increases ea
Transpiration
Evaporation from within the leaves of plants
Aka the soil plant atmosphere continium = pathway soil water takes through a plant back to the atmosphere
Stomata transpirstion
Inside stomata opening VPD is high outside is low VPD this causes transpiration. VPD and wind still relevant to transpiration
Plant physiology for T
-hydrualic conductivity
-xylem dysfunction
-stomatal control
Hydraulic conductivity
The ease at which water moves through porouos medians, it’s greater in hardwoods due to vessel elements
Xylem
Transport tissue of vascular plants
Tracheids and vessel elements
Tracheid = conifer + hardwood
Vessel element = hardwood only, more susceptible to xylem dysfunction
Water potential
-Quantifies the forces causing water to move in plants
- units PA, force per unit area
-water moves from high to low water potential
-defined as a measure of the potential energy of water due to its position, configuration, or quantity
Water potential - pressure potential
A measure of the outward pressure of water on a cell walls , thus usually positive in living cells
-negative in xylem because water column is under tension
-pull tension (neg)
-push tension (pos)
Xylem dysfunction
Cavitation or embolism(gas bubble)
-decreases hydraulic conductivity increasing stress
Stomatal control
-High VPD but low ground water can lead to xylem dysfunction
-dried out soil leads to change of tension in the water columns
-this xylem dysfunction can be avoided with stomatal closure
Measurements of ET
-Evaporation pan
have to use a coefficient because pan is above ground
-lysimeters
Often used for soil E
-water balance
ET = P-Q
PET (potential evapotranspiration)
The amount of water consumed in a unit of time by
-a short green crop
-completely shading the ground
-of uniform height
-AND NEVER SHORT OF WATER
Estimates of ET are trying to estimate for PET
Two models of ET
-penma Monteith method(this model accounts for energy supply and mechanism of water movement and vegetation control over rate of T)
-Thorn waid method
Estimates of PET
Calculated from meteorological data
-represents max ET possible
-actually ET is < or RARELY = PET
