The importanc of satelite derived information

IMPORTANCE OF SATELLITE DERIVED INFORMATION FOR PROACTIVE EARLY WARNING SYSTEM

ByByALMAZ DEMESSIEALMAZ DEMESSIE

M.Sc. IN TROPICAL AGRICULTURAL DEVELOPMENT(Uk) M.Sc. IN TROPICAL AGRICULTURAL DEVELOPMENT(Uk) Senior Agro meteorologist Senior Agro meteorologist and Early Warning expert, UNDP/GEF LDCF project Focal Person

Disaster Risk Management and Food Security SectorOffice tel: +251-114430578, Mobile tel: +251 -911-197120

E-mail: [email protected]

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

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.

Introduction …Contd

Calculating the Actual Water Balance

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.

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

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

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

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.

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.

Soil moisture contd…

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.

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

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!

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)

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)

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