1. Understanding the concept of water
balance calculation
BY
ALMAZ DEMESSIE
SENIOR AGROMETEOROLOGICAL EXPERT
M.Sc. In Tropical Agricultural Development (UK)
Government of Ethiopia and World Food Programme (WFP)
LEAP Training Material October 16 - 18, 2010
2. Understanding the concept of
water balance calculation
Basically, the water balance is the difference
between the effective amounts of rainfall received
by the crop and theamounts of water lost by the
crop and the soil due to the evaporation,
transpiration and to deep infiltration. The
amounts ofwater, held by the soil and available to
the crop, are also taken into account.The objective
of the water balance model is to convert raw
observations of the atmospheric environment into
a set ofparameters which are of direct importance
for crop production.
3. Evaporation and water-balance
measurements
• Measurement of evaporation from free
water surfaces and from the soil and of
transpiration from vegetation is of great
importance in agricultural meateorology.
Potentialevapotranspiration is defined as
the amount of water which evaporates
from the soil-air interface and from
plants, when the soil is at field capacity.
Actual evapotranspiration is defined as
the evaporation at the soil-air interface,
plus the transpiration of plants, in the
existing conditions of soil moisture.
4. Evaporation and water-balance
measurements contd….
• PET/AET can be measured with evaporimeters
or lysimeters directly.
• It can be also calculated by using different
empirical models like CropWat; statistical
models like Instat
5. • The calculation of reference
evapotranspiration (ETo), i.e., the rate
of evapotranspiration from a
hypothetic reference crop with an
assumed crop height of 12 cm, a fixed
canopy resistance of 70ms-1 and an
albedo of 0.23 (closely resembling the
evapotranspiration from an extensive
surface of green grass), is done
according to the Penman-Monteith
equation (Monteith, 1965, 1981; FAO,
1992b).
6. The calculation procedure uses a
standardized set of input parameters, as
follows:
• Tmax... maximum daily temperature (ºC)
• Tmin... minimum daily temperature (ºC)
• RH ... mean daily relative humidity (%)
• U2... wind speed measurement (ms-1)
• SD... bright sunshine hours per day (hours)
• A... elevation (m)
• L... latitude (deg)
7. Crop coefficients
• The relation between actual and reference
Evapotranspiration and actual evaporation in the
field is based on crop coefficients. ETa is
determined by the crop coefficient approach
whereby the effect of the various weather
conditions areincorporated into ET0 and the
crop characteristics into the Kc coefficient:
• ETa = Kc * ETo
• Exercise1. Examine
8. • The effect of both crop transpiration and soil
evaporation are integrated into a single crop
coefficient. In other words, the Kc coefficient
incorporates crop characteristics and averaged
effects of evaporation from the soil.
Crop coefficients (Kc) are dependent on:
• · Crop
• · Phenological stage of the crop (planting,
vegetative phase, yield formation, ripening etc...)
• · Soils, climate etc. Crop coefficients are known to
be slightly different for different Parts of the
world.
13. • The computation is done dekad-by-dekad
(DEK) and it starts before the planting to
take into account previous rainfall amounts
stored into the soil. From the planting
dekad, the crop water requirements
(WR) are calculated as the potential
Evapotranspiration (PET) times the crop
coefficient (KCR) values.
WR = ET * KCR
14. • IF actual rainfall data (ACT) are available
these are used, otherwise the calculation
uses normal rainfall data(NOR). The
rainfall used in the calculations is called
the working rainfall (WRK). So for a
dekad the following isvalid:
• If ACT is missing
....................................................................
...............then WRK = NOR
• If ACT is not missing then WRK = ACT
15.
16. Agroclimatic classification
The most widely adopted climatic
classification:
The Koppen system and Thorthwait
system
• Koppen attempt to fit climatic data to
observed vegetation limits
• Thornwaite constructed climatic
classification from regular intervals of
his derived moisture indices.
17. Agroclimatic classification contd….
• He introduced a bookkeeping scheme of
moisture gain and moisture losses, also
called the water balance. As long as the
moisture storage is above field capacity, the
water surplus (S) is the difference between
precipitation and reference potentional
evapotranspiration, but when the soil
storage falls below field capacity, the deficit
(D) is the difference between potential and
actual evapotranspiration. His moisture
index is as follows.
Im = 100(S/PET- D/PET)
18.
19.
20. AgroMetShell
Station number:
Crop type Maize:
Cycle length Total water requirements: 12 dekads
Total Water requirement: 561mm
Planting deked: 34
Maximum soil water storage 100mm
Effective/Total rain: 100%
Irrigation applied: No
Pre-season Kcr: .19
21. WRSI
The spatially explicit water requirement
satisfaction index (WRSI*) is an indicator of
crop performance based on the availability of
water to the crop during a growing season.
FAO studies have shown that WRSI can be
related to crop production using a linear yield-
reduction function specific to a crop (FAO,
1977; FAO, 1979; FAO, 1986). More recently,
Verdin and Klaver (2002) and Senay and Verdin
(2001) demonstrated a regional implementation
of WRSI in a grid cell based modeling
environment.
24. No WRSI (%) Drought Severity
class
1 80-100 No drought
2 70-79 Slight drought
3 60-69 Moderate drought
4 50-59 Severe drought
5 <50 Complete crop failure
25.
26. • LGP is describing the period during which crop
growth is not affected by climatic constraints, i.e. the
period of the year when water availability allows crop
growth and when the temperature is not limiting crop
growth. As many studies have indicated, the duration
of the period in which rainfall exceeds selected levels
of evapotranspiration is the most useful index of
agricultural potential. This period refers to the length
of time during which water and temperature permit
crop growth. As FAO (1991) stated, three specific
values are identified for climatic classification,
namely: arid, with LGP of less than 75 days;
Seasonally dry, with LGP of between 75 and 270
days; and humid with LGP of more than 270 days.
Annual or seasonal rainfall is traditionally used to
describe the supply of water to crops, because it is the
primary measurement particularly for rain fed
agriculture.
27. Agroclimatic classification contd….
• LGP is expressing the period during which crop
growth is not affected by climatic constraints or
it characterizes the period of the year when
water availability allows crop growth and when
the temperature is not limiting crop growth. The
method to calculate LGP is FAO methodology
by Frere and Popov (1979). “The growing
period (GP) is defined as the time (days) during
a year when precipitation exceeds half the
potential evapotranspiration (PET) plus the time
(days) necessary to evapotranspire 100mm of
water (or less if 100 mm is not available) from
excess precipitation stored in the soil profile.
28. Additional obsservations for the
better understanding of soil
moisture
Field capacity
• Field capacity is the maximum amount of water
which can be held in the soil after all gravitational
water has seeped out, evaporation from the soil
surface has been prevented, and there is no
direct contact between the soil moisture and the
ground water table.
29. • Soil moisture contd…
There are several methods of determining or of
estimation field capacity. The direct method
consists of selecting a small representative site in
the field, watering it to full capacity, waiting for the
gravitational water to seep down, and then
determining the moisture of the soil. The value
obtained will be the values of the field capacity for
that soil.
31. • Soil moisture contd…
Wilting Point
• Wilting point is the amount of soil
moisture at which permanent wilting
of a plant occurs. Vegetation
consumes soil moisture, and if it is
not replenished by water from
precipitation or irrigation, a time will
come when the plants will start to
wilt, despite the fact that there is still
some moisture in the soil.
32. • Soil moisture contd…
• The moment of permanent wilting occurs
when the soil water is attracted to the solid
soil particles by forces which are greater
than the forces by which the plant’s roots
can extract it.
• Permanent wilting should not be confused
with temporary wilting which often occurs
in the early afternoon hours of hot, dry
days. Permanent wilting means that the
plants cannot regain their turgidity even if
kept in a place with saturated air
34. Crop yield forecasting with water balance calculations
principles
Relation between crop water use and yield
• Doorenbos and Kassam outlined in their FAO publication
“Yield response to water” that there is a clear relation
between crop yield and water use. This relation is the basis
of the use of a water balance calculation in crop
forecasting. It is possible to establish a maximum yield
(Ym) based on a season without water stress and water
deficit. The total Evapotranspiration is then at his
maximum (ETm). In semi-arid circumstances yield is
usually reduced due to water stress leading to a lower
actual yield (Ya) and lower actual Evapotranspiration
(ETa) Yield Reduction is the percentage reduction compared
to a yield obtained without water stress. It is therefore not
measured against maximum yield!
35. Yield response to water
• The authors introduced the so-called yield
response factor (ky) to explain the yield
reduction due to water stress.
• They established the yield response factor for a
large number of crops in a limited number of
climates. Doorenbos and Kassam found this
relation to be near-linear for most crops (see
graph). The general formula is:
• (1- Ya/Ym) = ky * (1 - ETa/ETm)
36. The graph below establishes the relationship for
a number of crops
Horizontal axis: Yield from 0 (no water stress; high yield) to 1 (100% water deficit; no
yield)
Vertical axis: Evapotranspiration deficit from 0 (no deficit) to 100 (100% deficit)
37. In LEAP the Yield Reduction can be
calculated from Total Actual
Evapotranspiration (ETa) and Total Water
Requirement (TWR same value as ETm),
with the formula:-
100 – ((1- (1 – ETa/TWR) *Ky)* 100)
