Evapotranspiration and Infilration
What are controls that cause transpiration to vary? List 4.
1. Different species transpire at different rates (species-specific controls) 2. Amount of water in the soil 3. Plant's ability to transfer water from the soil to its leaves, root depth etc. 4. Ability of the atmosphere to absorb transpired water
What are 4 challenges associated with measuring inputs and outputs in lakes?
1. Ppt across a lake surface can be estimated using techniques that we've discussed before (i.e., gages) 2. Relatively easy to measure streamflow into and out of lakes (use a gage and measure directly) 3. Surface runoff cannot be directly measured, can be estimated, usually by land cover types, topography, and ppt data 4. Groundwater flux (i.e., inputs and outputs) cannot be directly measured, can be estimated
Describe 4 physical characteristics of soil that affect the infiltration capacity.
1. Size of pores in soil (generally decrease with depth) 2. Air trapped in soil (decreases infiltration) 3. Depth of soil (greater infiltration capacity, shallower depth) 4. Texture and structure of soil -Amount of mineral particles in soil (sand, silt, loam, and/or clay) -Other materials contained in soil matrix (i.e., rock) -E.g., soil colloids swell with moisture, (decrease infiltration over time) -Colloids are soil particles that can be "chemically active" (clay or humus)
What are the three ingredients for ET?
1.Flow of energy to the evaporating or transpiring surfaces 2.Flow of liquid water to surfaces (soils and plants) 3.Flow of vapor away from these surfaces
What is albedo? Which has higher albedo: light or dark colored objects?
= amount of shortwave radiation that can be reflected by an object Light-colored surfaces have higher albedo than dark
What is infiltration capacity? What two things determine it?
= maximum rateat which water can enter the soil •Rate determined by: 1. surface conditions (Surface conditions include "covering" of soil surface, plant material or litter -Upper horizon = leaves, stems, undecomposed materials -Lower horizon = decomposed materials acting like "mineral soil" 2. physical characteristics of the soil itself
Define infiltration. What two factors dictate infiltration? Where is infiltration fastest and why?
= water enteringthe soil 1. capillarity(component of matric potential) 2. gravity -Capillarity = process by which water moves through rock, sediment or soil caused by the cohesion between water molecules and an adhesion between water and other materials that pulls water -Dry, unfrozen soil, fast infiltration rate due to physical attraction (matric potential at work) -As water fills pores, rate will drop to a constant
What is pan evaporation? How does it work (equation)?
A technique to estimate ET of water bodies Standard pan gauge = 122 cm in diameter, 25 cm deep, water depth kept between 18 and 20 cm, measured daily E = CeEp •Ep= pan evaporation (mm/day) •Ce= pan coefficient, usually ranges from 0.5 to 0.8, can vary seasonally
When can the infiltration capacity be exceeded?
Actual infiltration rate can exceed "infiltration capacity" when rainfall or snowmelt results in flooding When water reaches a sufficient depth, positive pressure of water (i.e., hydraulic head) can encourage greater infiltration
What is radiant energy? Is it temperature dependent?
All substances with a temp above 0 K emit electromagnetic radiation Amount of radiation is temperature dependent; hotter substances, shorter wavelengths
What is the flow of liquid water in soils controlled by? What is the driving force? define water potential. How does water flow? What is the energy flow equation?
Controlled by water flow through unsaturated soils Driving force = difference in water potential between two points Water potential = relative tendency of water to move from one area to another Water flows from a region of higher free energy to a region of lower free energy (like Ohm's law) Flow (energy flow) = voltage (potential)/ resistance
What are the factors involved for evapotranspiration of a watershed? (equation)
ET = P -Q -ΔS -Δl P is precipitation (mm) Q is streamflow (mm) ΔS is the change in storage in a watershed (mm) Δl is the change in deepseepage (into and out of a watershed)
What equation can be used for estimating lake evaporation that factors in vapor pressure and wind speed? Define the variables.
Eo= N (es-ea) f(u) •N = mass transfer coefficient -Mass transport describes "turbulent eddy movement, this coefficient is meant to capture the propensity of air parcels to move vertically and horizontally through the atmosphere •es-ea = vapor pressure of water surface (mb) -vapor pressure of air (mb) •f(u) = function of wind speed (km/day)
What is the difference between evaporation and evapotranspiration?
Evaporation = changing from liquid to vapor Evaportranspiration(ET) = loss of water from surfaces through combined processes of evaporation and plant transpiration Transpiration is net loss of water from plant leaves by evaporation through plant stomata For a watershed, ET includes evaporation from plant surfaces, waterbodies, soils, and other surfaces
Describe evaporation from soil. What 2 things is needed for evaporation to occur in soils? What happens as soils dry during evaporation?
For wet soils, evaporation depends on energy supply and vapor pressure gradient BUT, as soils dry, gradient of total water potential (Ψs) is established, water moves from region of higher potential to region of lower potential Hydraulic conductivity is also less in dry soils, evaporation rate will be limited in dry soils by rate of water movement
What are the inputs of water to a lake? What are the outputs? What does this equation look like?
Inputs: 1. Precipitation 2. Groundwater 3. Flow 4. Runoff Outputs: 1. Evaporation 2. Groundwater 3. Flow Equation: P + GWi+ Qi+ R-E -GWo-Qo= ΔS
What is net radiation? What is the equation? Define the variables. Can it be negative or positive? What does that mean? How much can be used for evaporation?
Net radiation = resulting radiant energy available at a surface; Primary form of energy for evaporation, transpiration, and snowmelt Rn= (Ws+ ws)(1 -α) + Ia-Ig •Ws+ ws= direct and diffuse shortwave radiation •α= albedo for the surface •Ia-Ig= incoming -emitted longwave radiation Net radiation can be positive or negative Positive = excess energy; must be converted into some other form 80-90% of evaporation
Explain how plants can worsen flood risk?
Plants response to increased CO2 levels can increase surface runoff. Plant stomata will open less widely when exposed to higher levels of CO2, and therefore transpire less. Thus, water remains on the land surface.
What is the energy budget equation for a watershed? Define the variables.
Q* = Qs+ Ql+ Qg •Net radiation (Q*) •Latent heat (Ql), heat absorbed or released during a phase change •Sensible heat (Qs), sensed by instruments, heat we feel as warmth •Soil heat flux (Qg), stored and released from soil, typically ignored because it's nearly 0 over 24 hours •Two other sources of heat... anthropogenic and advective(latent heat arriving from someplace else)
Draw a graph that shows the infiltration capacity soil. Why does the curve look that way?
Rate slows over time in response to downward movement of water being limited by smaller pore space at deeper depths
What is a lysimeter and what does it measure? What is the equation?
Same approach as evaporating pans Lysimeteris filled with soil and vegetation, measures evapotranspiration Allows water to percolate through the bottom of the lysimeter, this is measured (R) E = ΔS -P -R Note that lysimeters are measuring E indirectly by accounting for other processes Weighing lysimeters accounts for change in storage If storage is assumed constant, can estimate without weighing (OK for long time periods)
What type of energy is key to the hydrologic cycle? What does energy determine? How does it vary?
Solar energy is key to the hydrologic cycle Determines amount of energy for latent heat of vaporization Energy varies seasonally and daily
What is shortwave radiation?
Solar radiation Composed of: Direct solar radiation Diffuse radiation = solar radiation scattered (by air molecules) and reflected (by clouds and atmospheric dust)
What is longwave radiation?
Terrestrial radiation Atmosphere and all terrestrial objects emit longwave radiation (temperature dependent) In atmosphere, CO2, O3, and water (liquid and vapor) emit longwave radiation
T/F Transpiration is a more efficient process than evaporation. Explain your answer.
True Large evaporating surface of plant foliage Vegetation is higher than soils, exposed to turbulent airflow above soil boundary layer Plants act as a conduit for water to flow from the soil water reservoir to the active evaporating surface at leaf-atmosphere interface (i.e., stomata)
For ET to occur, what is the BASIC requirement?
VP of water molecules at evaporating surface must be greater than VP in the atmosphere for evaporation to occur.
How does vapor flow work? Describe the "receiving" atmosphere. What does the VP deficit determine? What can tell us about how much the atmosphere can take?
Vapor must flow through a boundary layer to reach turbulent areas of air. The atmosphere (turbulent area) must be dry enough to receive water vapor, it cannot be saturated. The VP deficit determines how much water the atmosphere can take with how fast the air moves away from the evapotranspiring surface. Relative humidity can imply how much water vapor the atmopshere can take
What is the equation for the flow of water in plants? Define the variables.
q = -kw A(Δ Ψpl) =-A(Δ Ψpl/rpl) q = flow (cm3/sec) kw= water permeability (cm/sec*kPa) A = area (cm2) Δ Ψpl= water potential difference (kPa) rpl= resistance of plant components (1/kw)
Where does storage occur in a watershed? (list 5 places)
soil reservoirs lakes "permanent" snow and ice cover deep groundwater
What is the equation for the velocity of soil water? Define the variables.
v = Δ Ψs/rs= -kv δΨ/ δx v = velocity (cm/sec) Δ Ψs= water potential difference (kPa) rs= resistance of any component kv = hydraulic conductivity (cm/kPA* sec) x = distance over which gradient is present
What is the simplified equation of water potential in plants? Define the variables. How does a plant leaf maintain its water potential gradient?
Ψpl= Pt-Po •Pt= turgor pressure •Po= osmotic pressure Leaf of a plant maintains water potential gradient because solute concentrations are increased by photosynthesis
What are the different potentials that affects water potential in plants? Which two are the most important?
Ψpl= Ψo+ Ψp+ Ψt+ Ψg+ Ψm Ψpl: plant water potential Ψo: osmotic or solute potential (solutes ionically attracting water molecules); (important) Ψp:pressure potential (important) Ψt: thermal potentially (usually assumed constant) Ψg: gravitational potential (ignored for small plants, important for large trees) Ψm: matric potential (usually ignored in plants)
What are the different potentials that affects water potential in soils?
Ψs= Ψg+ Ψp+ Ψo+ Ψt+ Ψm Ψs: soil water potential Ψg: gravitational potential (gravity) Ψp:pressure potential (how deep below the surface) Ψo: osmotic or solute potential (solutes ionicallyattracting water molecules) Ψt: thermal potential (usually neglected) Ψm: matric potential (attraction of water to soil particles by both capillary and adsorptive forces); adhesion of water molecule to soil molecule
