1. IMPORTANCE OF SATELLITE DERIVED INFORMATION
FOR PROACTIVE EARLY WARNING SYSTEM
ByBy
ALMAZ DEMESSIEALMAZ DEMESSIE
M.Sc. IN TROPICAL AGRICULTURAL DEVELOPMENT(Uk)M.Sc. IN TROPICAL AGRICULTURAL DEVELOPMENT(Uk)
Senior Agro meteorologistSenior Agro meteorologist and Early Warning expert, UNDP/GEF LDCF project Focal Person
Disaster Risk Management and Food Security Sector
Office tel: +251-114430578, Mobile tel: +251 -911-197120
E-mail: demessiead@gmail.com
Strengthening the existing Early Warning Activities in terms of data collection, interpretation and information dissemination in order to
establish timely response mechanism”, organized by DRMFSS and supported by UNDP/GEF LDCF project December 4 – 5, 2015, Debre
Zeit
2. Introduction
Among the satellite derived agro-meteorological
components 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.
5. 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.
6. 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
7. 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 is valid:
If ACT is
missing .......................................................
............................then WRK = NOR
If ACT is not missing then WRK = ACT
8.
9.
10.
11.
12.
13.
14. 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
15.
16. 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.
17. 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.
19. 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.
20. 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
21. 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
notmeasured against maximum yield!
22. 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)
23. 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)
