Techniques to Predict Agricultural Droughts - Monitoring and Predicting Agricultural Drought: A Global Study - page 43

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Figure 4.1 Variation in soil moisture content during a crop-growing season and its compar-
ison with a threshold value, X 0 . L and D refer to length and dry period, respectively.

1. A wet spell occurs when any time series value at i th instant is
greater than the threshold level. The difference ( X i − X 0 ) is named

as the soil moisture surplus (SMS) when it exceeds zero and is called
a dry spell or soil moisture deficit (SMD) when it is less than zero.
2. A sequence of wet spells preceded and suceeded by a dry spell is
referred to as the duration of wet period, during which there is no
moisture stress and plant growth is normal. If the two successive
dry spells that separate a wet period are X i and X j , then the dura-
tion of this wet period is equal to ( j

[43],

Line
——
-1.1
——
Long
PgEn

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i ).
3. Similarly, if a sequence of dry spells is preceded and suceeded by
a wet spell, it is then referred to as the duration of dry period,
which might restrict the crop development during vegetative or
reproductive phenological phases. If the two successive wet spells
that separate a dry period are X k and X l , then the duration of this
dry period is equal to ( l

[43],

k ).
4. If a dry spell (continuation of SMD) is followed by a wet spell,
then there is a transition from the drought period to wet period
(continuation of SMS) (i.e., X i < X j ).
5. Similarly, if a wet spell is followed by a dry spell, then there is a
transition from the wet period to drought period (i.e., X i > X j ).
6. The maximum dry duration in the record of past soil moisture
observations corresponds to the most critical agricultural drought
period that has occurred in the history of the record site. Such a
critical period is directly related to the critical phenological phases
and is important to crop yield estimation.
7. The summation of water deficits during the whole drought period
gives the total drought severity. This is equivalent to accumulation
of soil moisture needed to offset the agricultural drought, which in
turn is directly related to rainfall surplus.
8. Finally, the division of the accumulated soil moisture by drought
duration can determine the average severity of agricultural drought.
It is possible to calculate almost all of the above objective drought fea-
tures, provided measured soil moisture records are available. Accordingly,
their statistical average, standard deviation, correlation coefficient, skew-
ness, and probability distribution function (PDF) can also be determined.

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