evapotranspiration III

Question 1

what is evapotranspiration?

Answer 1

combined loss of water vapor from within the leaves of plants and evaporation of liquid water from water surfaces, bare soil and vegetative surfaces

Question 2

what are the 5 ways we determine evapotranspiration?

Answer 2

• lysimeter measurements
• inflow-outflow measurements
• micrometereological measurements (eddy covariance method)
• study of groundwater fluctuations
• ET equations

Question 3

what is lysimetry?

Answer 3

• mainly used in agricultutal environments
• crop of interest grown under natural conditions in an isolated tank in large field of same crop

Question 4

what are the disadvantages of lysimetry?

Answer 4

difficult and expensive to construct, requires careful operation and maintenance and primarily for research

Question 5

what is an eddy flux tower?

Answer 5

directly measures how much CO2 or H20 vapor blows in and out of a site in wind gust (vertically)

Question 6

what are the disadvantages of meteorological techniques to determining evapotranspiration?

Answer 6

• expensive
• requires air to be turbulent
• requires flat terrain and homogenous underlying vegetation
• when data acquisition fails, gap-filling the data is difficult
• difficult to separate ecosystem components directly

Question 7

what is potential evapotranspiration?

Answer 7

ET that would occur if there was an adequate soil-moisture supply at all times, takes into account local weather conditions and energy status

Question 8

what is actual ET?

Answer 8

actual evaporation rate from any surface under prevailing conditions of moisture availability and radiative input

Question 9

what is pan evaporation?

Answer 9

evaporation from a US Class-A evaporation pan

Question 10

what is reference crop ET?

Answer 10

ET that would take place under strictly prescribed biologic and surface moisture conditions

Question 11

what is the focus of the blaney-criddle method to measure ET?

Answer 11

air temperature

Question 12

what does the FAO recommend for the Blaney-Criddle equation?

Answer 12

• using site latitude and the literature to identify p for a given month
• use air temperature data from an entire month

Question 13

what is the thornthwaite model for evaluating ET?

Answer 13

uses annual heat index, mostly applicable to arid areas like central and eastern US

Question 14

what is the Penman-Monteith model for evaluating ET?

Answer 14

considers the vegetation canopy as one isothermal leaf

Question 15

what is the hargreaves method for evaluating ET?

Answer 15

estimates grass reference ET, uses air temperature and humidity

Question 16

what is the Hamon equation for evaluating ET?

Answer 16

uses temperature and saturated vapor pressure

Question 17

what are the four types of equations to compute ET and an example for each?

Answer 17

• equations using mean daily air temperature (thornthwaite)
• equations using daily air temperature and solar radiation (jensseen Hafse)
• equations using mean daily air temperature and humidity (Hamon)
• complex equations (penman)

Question 18

when is P-ET negative?

Answer 18

in dry conditions