Quiz 4- Infiltration and Evapotranspiration

infiltration

the entry of water through the surface of the soil, and it is governed by antecedent moisture conditions and soil properties

Greater infiltration means more water is available to plants for growth, increasing the amount of evaporation and transpiration. Also that less water becomes overland flow.

What influences the infiltration rate?

1. soil surface conditions (sealing and compaction)

2. Vegetative cover 

3. soil properties like porosity, hydraulic conductivity, and moisture content 

4. soil heterogeneity 

But MOSTLY soil moisture and unsaturated subsurface flow. 

Tension forces that impact infiltration

adhesion (water to pore), cohesion (water to water), capillary, suction

Smaller pore radius= longer column of water that can be supported 

What determines if water is retained in a soil pore?

The height of the water table ad the size of the pores

Small pores will with water first (largest surface tension forces)

Large pore empty first during drying

hysteresis

the dependence of a system not only in its current environment but also on its past environment

the path there is different from the path back

Hysteresis occurs in wetting and drying of soils 

Field capacity

the point at which gravity flow ceases. Typically occurs at soil suctions of 0.1-0.33 bars. Gravity can remove water in excess of field capacity

Wilting point

the point at which plants wilt from INSUFFICIENT water content. Typically occurs at soil suctions of 15 bars. Evaporation can remove water in excess of wilting point

Plant available water

the amount of water in the soil that can be used by plants; generally field capacity-wilting point 

seasonal soil water balance

change in water storage = to precipitation + irrigation - surface runoff - infiltration - evapotranspiration

Horton equation

used in stormwater models

empirical

if rainfall exceeds infiltration capacity, ,infiltration decreases exponentially

assumes saturation at surface

Green and Ampt assumptions

used to predict cumulative infiltration as a function of time and readily available soil parameters (major advantage) 

1. the soil is homogenous and stable

2. supply of ponded water at surface of soil is not limited 

3. wetting front exists and advances at same rate as water infiltrates 

4. capillary suction is uniform and constant 

5. soil is uniformly saturates above the wetting front. the volumetric water content remains constant below the wetting front

moisture content

ration of the volume of water to the total volume of a unit of porous media

porosity

ratio of interconnected void column to total sample volume

Hydraulic conductivity (K)

volume of water that will flow through a unit soil column in a given time 

capillary suction

measure of the combined adhesive forces that bind the water molecules to solid walls and the cohesive forces that attract water molecules to each other

evapotranspiration

the process that returns water to the atmosphere

evaporation

vaporization from open water, soils, and vegetation surfaces 

transpiration

loss of water through plant respiration

Potential Evapotranspiration

the amount of ET when there is an unlimited water supply

Actual Evapotranspiration (AET)

the amount of ET that occurs when water is limited

When does PET=AET?

over open water

Budyko curve

shows AET vs. PET normalized to precipitation

Latent heat of vaporization

the energy a molecule of water needs to penetrate the water surface and move from liquid to gaseous form. It is usually provided by solar radiation

Factors impacting transpiration (2)

1. type of plant (shaded, trees)

2. Wind (removes water vapor)

Transpiration and Soil texture

deeper roots provides more access to water but it takes more energy to move the plant shoot

Irrigation and ET

1. Too much irrigating increases soil water content, which may raise the water table

2. This can bring salts to the surface by capillary action when drying occurs

3. ET removes excess soil moisture and concentrates the salts, killing plants