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DEF - a collective term for all the processes by which water in the liquid or solid phase at or near the earth's land surfaces becomes atmospheric water vapor.
- the term includes evaporation of liquid water from rivers and lakes, bare soil, and vegative sorces
- the term includes transpiration from within leaves and plants
- the term includes sublimation from ice and snow surfaces
- DEF - an index of the water content that can be held against the force of gravity.
- DEF - water held in the pores after gravity water is taken out
- DEF - Water held after a few days of drainage
- DEF - Quantity of water held against gravity
- ratio of volume of capillary water per unit volume of soilA measurable amount of water per unit of depth
- EX) 23% could be considered a capacity to hold 23 cm of water per 1 m of depth
DEF - PET is the rate at which ET would occur from a large area completely and uniformly covered with growing vegetation which has access to an unlimited supply of water, and without advection or heat storage effects.
Many ways to find PET (aka reference-crop ET)
DEF - when the plant sucks up all of the capillary water, and there is no water left
- As long as suction force of plants exceeds capillary force holding the water in the pores, plants will suck up water
- As some point, the capillary force exceeds the roots ability to take up water
- The percentage of water that cannot be extracted by roots is called the permanent wilting point for that soil
- At this point, plants will wilt and if no new water is added, die
- In the root zone, soil water in storage can vary from field capacity to wilting point
- Vegetation can only use the water that is held between field capacity and the wilting point.
- Soils with large differences between the two are generally good for plants
- DEF - The ability of a material to allow the passage of a liquid, such as water through rocks.
- DEF - The capability of a given substance to allow the passage of a fluid.
- Permeable materials, such as gravel and sand, allow water to move quickly through them, whereas impermeable material, such as clay, allow water to flow slowly through them
- Permeability depends on the size of and the degree of connection among a substance's pores.
- Too much, water runs right out
- Too little, don't get enough
- Water expands greatly.
- As a liquid, 1 gram of water takes up 1 milliliter. At 25° C, 1 gram of water occupies about 42,000 milliliters. Molecules are far apart and not able to bond.
- Water vapor is like other gases. It can compress and expand and exerts pressure. Water vapor exerts a “partial pressure” which is what we care about now.
- The vapor pressure of water changes with temperature and pressure. The volume of a given weight of water vapor varies with temperature and pressure.
- Low vapor pressure = low amounts of water in the air, and high evaporation rates
- High vapor pressure = high amounts of water in the air, and low evaporation rates
DEF- water that falls to the ground either directly through gaps in the canopy or indirectly having drips off leaves, stems or branches.
- Amount of direct throughfall controlled by canopy coverage can be measured using the leaf area index (LAI).
- LAI is a ratio of leaf area to ground surface area. When greater than 1, throughfall is smaller. When less than 1, throughfall is greater.
- Amount of indirect throughfall also controlled by LAI, in addition to canopy storage capacity, and rainfall characteristics.
- high intensity rain means quick throughfall
- long duration rain means more throughfall
- If area is already wet, there will be quicker throughfall
Why is it important?
DEF- The process by which precipitation is caught and held by foliage, twigs, and branches of trees, shrubs, and other vegetation, And/or caught and held by human-made structures.
1. Some studies show no net loss of water but others show evaporation of intercepted water is greater than evapotranspiration from vegetation
2. For light rainfalls, interception can mean water loss due to E and limited wetting of soil
3. Interception reduces erosion by protecting soil from raindrop impact
4. Reduces runoff by reducing the filling of soil pores
5. Subtraction of intercepted water from precipitation during heavy rainfalls is insignificant so no effect on major floods
- DEF- The part that is lost by evaporation, never reaching the surface of the ground, is called interception loss.
- DEF- Interception loss equals the precipitation on the vegetation minus stemflow and throughfall.
Factors which affect interception loss:
1) Plant physiology
DEF- a natural body comprised of soils, liquid, and gasses that occurs on the land surface, occupies space, and is characterized by one or both of the following: horizons that are distinguishable from the initial material as a result of additions, losses, transfers, and transformations of energy and matter or the ability to support rooted plants in a natural environment!
- goes from just being those components by themselves, to a 'living organism' that can support plant life
- The unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants.
DEF- The unconsolidated mineral or organic matter on the surface of the earth that has been subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time
(Soil Science Society of America)
DEF- Water in the unsaturated zone (temporary saturated zone; doesn't stay wet) of porous earth materials
Soil water – 0.001% - not that much, but very important
- 1. 76% of the Earth’s land area surface infiltrates
- 2. Plants use the water (we eat plants or animals that eat plants)
- 3. The water percolates into the groundwater
- DEF- The percentage of pore space (empty space or air) in a soil.
- DEF- The ratio of pore volume to total volume of the soil or rock.
- Sandy soils have large pores but usually clays and other fine-grained soils have higher total porosity due to shape and surface area.
- same amount of pore space in clay/sand, just bigger in sand…doesn't tell you if water can move as a result...
DEF- the dry density of the soil
commonly increases with depth due to compaction by the weight of overlying soil
Transition Zone Between Definitions and Questions
What are the two zones of subsurface water? Compare them.
Soil Water and Groundwater
- Soil water (unsaturated zone)
- Above the water table (vadose zone)
- Some pores contain water and some contain air
- Groundwater (saturated zone – phreatic zone)
- Water completely fills all the soil or rock pores
- Top of this zone is called the water table
What is infiltration?
Why is it important?
DEF- the movement of water from the soil surface into the soil
What do infiltration rates depend on?
Infiltration rates depend on Viscosity and Permeability
- 1) Viscosity (thickness or resistance to flow in a liquid)
- Cold water is more viscous so it flows more slowly
- Warm water is less viscous so if flows more quickly
- 2) Permeability
- Sand is more permeable than clay
- Permeability at the surface is an important factor in determining rates of infiltration
What happens to soil water below the root zone?
- Evaporation to the surface is very slow
- Once full complement of capillary water is stored, it stays there almost indefinitely
- Even in deserts, soil well below the surface will contain some moisture
- Where rainfall is only 2 in a year, the soil a foot or so below the surface has a moisture content of 5-6%
How does soil water move in the unsaturated zone?
water can move up, down, and sideways
Water here is subject to:
- Capillary suction produced by the surface tension in tiny soil pores
- Osmotic suction in roots
- Evaporation bringing water back to surface
all move water upward against gravity
Characteristics of soils may also aid or hinder water movement in the unsaturated zone
What is soil drainage and why is it important?
- DEF- The ability of soil to allow water to move away freely
- DEF- The ability of soils to drain is important in situations of heavy rain or snowmelt.
- Poor drainage affects agriculture, buildings, septic tanks, landfills
Poor drainage can be caused by:
- Low permeability soils – dense fine textured soils
- Topographic position that concentrates water
- Impermeable layer under the soil
Can be recognized by:
- standing water
- water-loving vegetation
- certain types of soils that reflect the prevalence of
- saturated conditions
Why is it important to know soil water content?
- plant growth
How does soil water move? In what directions and why?
- Capillary forces move water in the unsaturated zone
- Gravity can pull it into the groundwater
- In the root zone, water can move:
- Toward roots
- Toward the surface
- Down out of the root zone
Water will move according to the potential energy gradient from high potential to lower
Two types of potential (energy)
- 1. Gravitational
- Gravity as we know it
- 2. Capillary
- Result from cohesion and adhesion forces
- Cohesion causes surface tension
- Water molecules like each other and stick together when in contact with air or other gases
- Adhesion results from the attraction of water molecules to solid surfaces
- Water molecules stick to each other on surfaces
- Water can move up against gravity in narrow pores
- Capillary PE depends on pore size
- e.g. sand to silt as silt pore sizes are smaller so it sucks up the water
- Water moves from wetter to drier soil
Transition Zone Between Chapter 6 and Chapter 7
What is evaporation?
Explain the process of evaporation in some detail.
- DEF- Evaporation is the change of liquid water into a gaseous state and its diffusion into the atmosphere
- DEF- The process whereby atoms or molecules in a liquid state gain sufficient energy to enter the gaseous state
- Transpiration from plants is part of photosynthesis and
- respiration (breathing). Plant transpire water through the opening and closing of stomata (pores) on their leaves.
Different plants transpire at different rates and depending on the amount of water available.
- Evaporation and transpiration are often considered
- together as ET or Et because they are difficult to measure separately and we often don’t care about them separately over land.
Usually measured together, since its next to impossible, and no one cares, about measuring them separately
Why is evaporation important?
1. Water losses from irrigation
2. Crops and water losses
3. Water losses from reservoirs
4. Industrial and domestic cooling
- 5. Heat transfer and moderating of seasons on the Earth; resulting in hurricanes
- temp of water 81 degrees, evaporating, rising, condensing, releasing, convergence, coriolios effect/friction cause spinning, BOOM, huricane
6. Sweating (cooling) heat given off by evaporation…
7. Cleaning water
8. Making salt
9. Water balance calculations
- 10. Modeling climate change
- Water vapor is a green house gas…
What are the three factors needed for evaporation to take place?
- heat (solar energy)
- dry atmosphere
What is humidity?
What is relative humidity?
What is dew point?
- The amount of water in a volume of air is the humidity.
- A volume of air can hold x amount of water for a given temperature and pressure – relative humidity (It’s relative to temperature and pressure – mainly temperature for us.)
- Amount of water that can be held is temperature and pressure dependent.
- how much water the atmosphere can hold at a given temperature.
- Saturated air (can’t hold anymore) has a relative humidity of 100%. Less than 100% means the air can hold more water. The higher the temperature, the more moisture the air can hold.
- RH describes how close the air is to saturation.
- Ratio of actual amount of water vapor in the air to how much the air can hold at a given temperature
- Cold air – low capacity
- Warm air – high capacity
- Relationship of capacity to temperature is not linear
The dewpoint temperature is the temperature at which the air can no longer hold all of its water vapor, and some of the water vapor must condense into liquid water. The dew point is always lower than (or equal to) the air temperature.
If the air temperature cools to the dew point, or if the dew point rises to equal the air temperature, then dew, fog orclouds begin to form. At this point where the dew point temperature equals the air temperature, the relative humidity is 100%.
What factors affect the rate of evaporation?
Be able to briefly explain them.
- vapor pressure
- energy available
- mixing of water
What factors affect evaporation from the NWS Class A pan?
- Most E in real basin or field is from surface of soil or vegetation so water available is limited. (<PE)
- Edge effect – E is higher near edge of pan
- Water is close to the metal pan
- Causes it to be hotter
- Caused evaporation to be higher
- Pan itself will warm and enhance E
- Water in pan is shallow so not much mixing
- In a lake, there is circulation going on that you will not get as much in a pan
- The heat will not be mixing and spreading around as much
- Usually pan will overestimate E so a pan coefficient is used to adjust - 0.70 or 0.75
- (about 3/4 of evaporation in the pan is what is going on in a normal environment)
- give good estimate of values from a nearby lake or reservoir. Error is about ±15%.
- Different Pan coefficients are used for going from pan to crops or dry land.
Pan coefficients are empirical – derived from data and equations based on actual measurements
What do ET processes depend on?
- What's on the land surface?
- Type of vegetation or surface materials.
Transition Zone into Interception
What are direct and indirect throughfall?
2. Canopy storage capacity – amount of water that can be held by the canopy before water starts dripping as indirect throughfall.
Size of trees
Area and water holding capacity of leaves
- 3. Rainfall characteristics - dictate how fast the canopy storage capacity is filled.
- Intensity of rain – high means quick throughfall
- Duration of rain – long means more throughfall
Antecedent (previous) P – already wet means quicker throughfall
What is canopy storage capacity and what controls it?
DEF- amount of water that can be held by the canopy before water starts dripping as indirect throughfall.
- Size of trees
- Area and water holding capacity of leaves
What is stemflow?
What factors affect stemflow?
DEF- Stemflow is the rainfall that is intercepted by stems and branches and flows down the tree (trunk) into the soil.
- Tends to be a small amount – 2-10 percent.
- Tends to funnel P to a smaller area of soil near the base of the tree. Leads to rapid entry of water into the soil.
- Can help to funnel pollutants collected on leaves into the soil.
Compare rain interception with snow interception.
Difference (from rain) in mass of water held and duration.
- Larger amount – longer period
- Available for E or sublimation
- Controlled by tree structure, leaf area, and species.
- In some areas dominated by winter snowfall, up 20 to 50% of gross P lost.
- Unbelievably difficult to measure snow on trees
- How would you do it? Shake the tree and catch it in a big net?
- This is why the % goes from a 10-20% estimate in rain to a 20-50% estimate in snow
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