1. 1
Advanced Soil and Water
Engineering #8
Masahiro Tasumi
Dept. Forest & Env. Sci.
Class schedule
No. Date Contents
1 4/11 Course outline for Advanced Soil and Water Engineering
2 4/18 Physical characteristics of soil I
3 4/25 Physical characteristics of soil II (NO CLASS)
4 5/9 Physical characteristics of soil III
5 5/16 Physical characteristics of soil IV
6 5/23 Micrometeorology I
7 5/30 Micrometeorology II
8 6/6 Micrometeorology III
9 6/13 Micrometeorology IV
10 6/20 Soil water and Evapotranspiration I
11 6/27 Soil water and Evapotranspiration II
12 7/4 Soil water and Evapotranspiration III
13 7/11 Soil and water engineering in operation I
14 7/18 Soil and water engineering in operation II
15 7/25 Discussions
Today’s Topic
Water balance and
crop production
-Micrometeorology III
1. What is Evapotranspiration (ET)?
2. Why ET is important for agriculture?
(and for natural vegetation, large water
circulation of earth, global warming, etc.)
3. How to measure, compute, or estimate ET?
2. 2
What is ET? (Definition)
Let’s read Wikipedia.
What is ET?
- Actual ET - Reference ET
- Relative ET - Pan Evaporation
- Potential ET - Basal ET etc.
1. What is Evapotranspiration (ET)?
2. Why ET is important for agriculture?
(and for natural vegetation, large water
circulation of earth, global warming, etc.)
3. How to measure, compute, or estimate ET?
4. 4
CropWat8 software of FAO
Input data = Weather data, crop data, soil water holding characteristics.
Output data = Crop Water Requirement = Irrigation Requirement
= “ET required by crop” minus Effective precipitation
Points:
(1) In terms of “water quantity”, providing
sufficient water (appears as sufficient ET)
derives maximum crop production and yield.
(2) Crop water requirement is determined based
on ET only. Field and project-level water
requirements are determined based on crop
water requirement, plus water application
efficiencies and losses.
(3) If precipitation does not satisfy the crop
water requirement, better to irrigate.
1. What is Evapotranspiration (ET)?
2. Why ET is important for agriculture?
(and for natural vegetation, large water
circulation of earth, global warming, etc.)
3. How to measure, compute, or estimate ET?
1. Measurements (water balance)
Lysimeter
Evapolation Pan (Class-A Pan)
Japanese government has stopped measurement in 2002.
Disadvantage = Lysimeter ET may not represent ET from
outside of the Lysimeter.
Advantage = Easy to measure and maintain.
Historical data might be found.
Disadvantage = Pan-E is Pan-E. Not ET.
Need adjustments to estimate actual ET.
Other weather-based estimation method is easier
and better.
Advantage = Measured
Actual ET.
5. 5
1. Measurements (meteorology)
Eddy Covariance (Correlation)
Disadvantage = Failure of correlation
between vapor and wind (systematic
under-estimation) ; Fetch problem
Advantage = Measured actual
movement of vapor (~ actual ET).
Bowen Ratio
Disadvantage = Fetch problem
Difficult to use in morning and
evening (and night)
Combination of energy balance
and aerodynamic process
2. Water balance approach
Basin scale water balance
Disadvantage = Impossible to solve water balance only by
reliable measurement data.
Several assumptions, measurement errors, estimation errors
are involved. Requires high technique to improve accuracy.
Advantage = Semi-measured actual ET. Easy to understand
the concept. Easy to conduct if data quality is not required.
Point: Computer
modelling with satellite-
based measurements
has in progress.
Very small and
ignorable
in terms of Energy
Key for
irrigation &
crop water requirement
3. Energy balance approach
Solar Radiation,
Rs
Solar Radiation
reflected, Rs
Incoming Longwave
Radiation, Lin
Outgoing Longwave
Radiation, Lout
Land Surface
Lin reflected,
(1- )Lin
outinBBsn LLR1R
Surface Radiation Balance
Rn = Net Radiation
6. 6
Surface Heat Balance
Rn = Net Radiation
Ground Heat
Flux, G
Sensible Heat
Flux, H
Land Surface
Latent Heat Flux,
lE
Net Radiation, Rn
GEHR n
Net radiation energy is
“immediately” converted
to heat at surface, and be
used for three purposes.
4. Aerodynamic approach
Latent heat, lE
( )
av
21
air
r
q-q
ρ=E
5. Combination method of
aerodynamic and energy balance approach.
We will study from next lecture.
