Monitoring Agricultural Drought in China - Monitoring and Predicting Agricultural Drought: A Global Study - page 360

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sensed aggregated surface temperature may be sufficient to estimate evapo-
transpiration (ET).
Not all energy balance models contain explicit relationships between
soil moisture and surface temperature. Many of those that do are complex
and may contain many parameters that are difficult to measure. Most,
however, can be solved for the case when soil moisture is at field capacity
and is not a limiting factor. The effective ET ( λ E p , where E p is potential
ET) corresponds to a situation where soil water and vegetation condition
do not limit ET while the atmospheric demand and land cover types and
structures are the same. The moisture availability at other times (when soil
water is below field capacity) is then defined as

m a ( EB ) = λ E /λ E p [28.9]

[360

C rop Water Stress Index Using an integrated version of the moisture
av ailability defined in equation 28.9, Jackson et al. (1983) defined the crop
w ater stress index (CWSI) as

Line
——
0.9
——
Shor
PgEn

CWSI = 1 − m ad

E d
E pd

= 1 − [28.10]

w here the subscript d denotes that the quantity is integrated over a day.
R emote sensing data can be used for computing water balancing by pro-
vi ding estimates for ET (Jupp et al., 1997; Tian et al., 1997). Based on the
ra nge of CWSI and W / W f , the surface conditions can be classified as heavy
dr ought, dry, light dry, normal, and moist conditions (table 28.2).

[360

N ormalized Difference Temperature Index Jackson et al. (1983) found
th at an approximation of m a equals a normalized difference temperature
in dex (NDTI):

m a ( EB ) ≈ NDTI

T ∞ −

T s

=

[28.11]

T ∞ −

T 0

where T is theoretical surface temperature if no water were available,

and T 0 is theoretical temperature if maximum amount of water were avail-
able (i.e., the temperature corresponding to E p ).
The NDTI was successfully used to map surface temperatures for
drought monitoring in the North China Plain (Tian et al., 1997). To es-
timate evapotranspiration, a one-layer model was used for a vegetation-
covered surface (Jupp et al., 1997), and a two-layer model was used for
partial vegetation-covered surface (Jupp et al., 1997). A soil-thermal inertia
model was used for low vegetation-covered or bare surface for estimating
soil moisture (Yu and Tian, 1997).

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