LECTURE 3

Lecture 3

Evaporation and Transpiration

Fetter 2.1 & 2.2

Evaporation

When the number of water molecules passing to the vapor state exceeds the number joining the liquid state. Can continue until air is saturated.

Directly proportional to temperature

Dew Point

Temperature at which condensation begins

Rate of Evaporation

is a function of (water temperature, air temperature, absolute humidity)

What drives Evaporation? Sun, wind

How to measure Evaporation, on land with an open pan of water

Map of pan evaporation values for the United States.

Transpiration

Process by which water vapor passes into the atmosphere through the tissue of living plants.

Lumped with Evaporation for convenience in analyzing moisture transfer.

T = is a function of (density and size of vegetation, soil moisture, depth to water, soil structure)

Xerophytes

Shallow, spread roots in the soil, examples are cactus

Phreatophytes

roots tap directly into ground water below the water table - examples are cottonwoods and willows

Hydrophtes

Aquatic plants such as reeds or cattails

Figure 2.2 Diagram of potential and actual evapotranspiration in an area that has coarse soils

with limited soil-moisture storage, warm, dry summers, and cool, moist winters.

Figure 2.3 Diagram of potential and actual evapotranspiration in an area that has fine soils

with ample soil-moisture storage, warm summers, cool winters, and little change in

precipitation throughout the year.

The limited soil moisture causes actual < potential on Figures 2.2 and 2.3.

Evaporation/Transpiration (ET)
	Not possible to separate E from T in field -- total water loss.

		ET = free water evaporation and plant transpiration and soil moisture evaporation

Potential ET
	Thornthwaite, 1944
	"The water loss of which will occur if at no time there is a deficiency of water in the soil for use of vegetation." --assumes soil storage is not depleted

Actual ET
	Limited to evaporation that actually occurs

Methods for predicting ET
	Mass Transfer
			Thornthwaite, Mather, 1955, ET depends only on meteorological conditions.
			Measures potential ET

	Energy balance
		Penman, 1956
		equation based on heat
		usually a lot of estimates

	Empirical methods
		Blaney and Criddle, 1950
		Factors in crops/plants water use

Mean Annual Evapotranspiration

Long-term average annual precipitation for the contiguous United States.

ENV 302 - Lectures