Monitoring Agricultural Drought in China - Monitoring and Predicting Agricultural Drought: A Global Study - page 358

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some places two-crops-in-one-year system is transformed into three-crops-
in-one-year system. This is one of the reasons that drought-damaged ar-
eas have expanded every year rather than shrunk despite the post-1949
improvement of irrigation facilities by the government. Also, sometimes
farmers use water carelessly, and soil erosion occurs due to farming on
steep hills. Some farmers use chemical fertilizers instead of natural fertiliz-
ers, which deteriorate soil quality and lower the water-holding capacity of
soils. Such cultural activities contribute to drought occurrence, for instance,
in the North China Plain.

D rought Monitoring Methods
Th e following models have been developed for monitoring droughts in
C hina, and an operational drought monitoring system is also in place. [358

Water Balance Model

Line
——
-2.8
——
Norm
PgEn

Th e water budget for the root zone of a soil column can be written as (Yang
an d Tian, 1991):

∂ W
∂ t

= ( P + I ) − E − R + D − G [28.1]

w here W is the water content of the root zone (cm), P is precipitation (cm),
I is irrigation (cm), E is evapotranspiration (cm), R is runoff (cm), D is the
di scharge from groundwater (cm), and G is recharge to the groundwater
(c m). Ignoring, for the moment, recharge and discharge and incorporating
irr igation into rainfall, the above equation can be integrated over time t
(d ays or weeks) to derive a simple equation for the incremental change in
th e soil water storage:

[358

W t − W t − 1 = P t − E t − R t [28.2]

The above parameters are difficult to estimate. Typically, soil moisture is
es timated from the amount of rainfall and crop water demand, depending
on air temperature, solar radiation, humidity, wind speed, potential evapo-
tr anspiration ( E p ), and land cover conditions (Monteith and Unsworth,
19 90). Simple water balancing can be performed if initial storage ( W 0 )is
kn own and if operating characteristics for the area that define relationships
be tween moisture availability [ m a ( WB )] and runoff fraction ( r r ) are also
kn own. For example:
m a ( WB ) = E / E p = f [ ( W t − W w )/ W max ] [28.3]

and

r r = R / P = g ( W t / W f ) [28.4]

where W f is the amount of water available in a soil profile at field capacity;
W w is the amount of water in the profile at which evapotranspiration stops