Drought Monitoring Techniques for Famine Early Warning Systems in Africa - Monitoring and Predicting Agricultural Drought: A Global Study - page 256

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46

to crop production using a linear yield-reduction function specific to a crop.
Regional implementation of WRSI has been demonstrated in a geographic
information system environment (Verdin and Klaver, 2002; Senay and
Verdin, 2003).
WRSI for a season is calculated as the ratio of seasonal actual evapo-
transpiration (AET) to the seasonal crop water requirement (WR):

WRSI = ( AET / WR ) 100 . [19.1]

WR is calculated from potential evapotranspiration (PET) using the crop
coefficient ( K c ) to adjust for the growth stage of the crop:
WR = K c ( PET ) [19.2]

AET represents the actual (as opposed to potential) amount of water
w ithdrawn from the soil water reservoir. When soil water content is above
th e maximum allowable depletion (MAD) (based on crop type), the AET
w ill remain the same as WR (i.e., no water stress). But when the soil water
le vel is below the MAD, the AET will be lower than WR in proportion to
th e remaining soil water content. Soil water content is obtained through a
si mple mass-balance equation where soil water level is monitored by the
w ater-holding capacity of the soil and the crop root depth; i.e.,

[256

Line
——
6.0
——
Norm
PgEn

SW i = SW i − 1 + PPT i − AET i [19.3]

w here SW is soil water content, PPT is precipitation, and i is the time step
index.
The most important inputs to the model are precipitation and PET. PET
va lues are calculated daily for Africa at 1° resolution from a 6-h numerical
m eteorological model output using the Penman-Monteith equation (Shut-
tle worth, 1992; Verdin and Klaver, 2002). Blended satellite-gauge RFE
da ta are obtained from NOAA at 0.1° (

[256

10 km) spatial resolution (Xie
an d Arkin, 1997). In addition, the model uses soil attributes from digital
so ils map of the world (FAO, 1988).
WRSI calculation requires start-of-season (SOS) and end-of-season
(E OS) data for each modeling grid cell. The model determines SOS (or on-
se t of rains) based on simple precipitation accounting. Figure 19.1 shows
th e SOS map for southern Africa during the 2001-02 growing season.
SOS is determined using a threshold amount and distribution of rainfall
re ceived in three consecutive dekads. SOS is established when there is at
le ast 25 mm of rainfall in one dekad, followed by a total of at least 20 mm
of rainfall in the next two dekads. The length-of-growing period for each
pixel is determined by the persistence, on average, above a threshold value
of the ratio between rainfall and PET. EOS is obtained by adding length-
of-growing period to the SOS dekad. The WRSI model can be applied to
different crop types (maize, sorghum, millet, etc.) for which seasonal water
use patterns have been published in the form of a crop coefficient (FAO,
1998).

∼