Drought-Related Characteristics of Important Cereal Crops - Monitoring and Predicting Agricultural Drought: A Global Study - page 12

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from a crop system. Water may exit the crop system by evaporation from
the soil, transpiration of weeds, deep drainage beyond the root zone, lateral
flow beneath the soil surface, or runoff. We can sum the daily additions and
losses of water to form a water balance equation:

S = G + P + I − E − T − T w − D − L − R [2.1]

where all the terms are measured in millimeters, S is the amount of the soil
water currently available to the crop, G is the amount of soil
water available on previous day, P is precipitation, I is irrigation, E
is water evaporated from soil surface, T is water transpired by crop, T w is
water transpired by weeds, D is water that drains below the root zone, L is
lateral flow beneath the soil surface, and R is runoff or lateral flow above
the soil surface.
It should be noted that D , L , and R are normally negative values, but
sometimes they may also be positive. Soil water may move upward by cap-
illary action to enter the root zone. In some fields, such as valley bottoms,
subsurface lateral flow or surface runoff may constitute a major source of
water entering the crop system.
The ratio of crop production to the water it uses is called the water use
efficiency (WUE). The WUE is more commonly defined as the ratio of total
dry matter produced to total water used by the crop during the growing
season. Two other common measures of water use include the net WUE,
which is the ratio of marketable yield to total water consumed (Jensen et
al., 1990), and the irrigation WUE, which is the ratio of marketable yield
to total water applied (Hillel, 1990).

[12],

Line
——
-1.5
——
Norm
PgEn

[12],

D rought and Water Deficit Effects on Plants
Although all droughts lead to plant water deficit, not all plant water deficits
result from drought. Disease or pests that damage roots can also cause
plant water deficit that cannot be overcome by adding water to the crop.
Visual symptoms of root pests and diseases may be indistinguishable from
symptoms of drought. Similarly, plants with shallow root systems may
suffer water deficit if upper soil layers dry, even though water is available
in soil layers below the root zone.
As the basic medium of life, water plays many roles in plant growth
and development. Similarly, water deficit affects numerous plant processes.
For crop plants, it is convenient to consider four broad groups of re-
sponses to water deficit: germination processes, turgor-mediated phenom-
ena, substrate-mediated phenomena, and desiccation effects.

G ermination

Crops first require water for germination. Seeds must imbibe enough water
to initiate biochemical processes needed to break down the storage com-
pounds to provide the energy and substrate for growth. Seeds are very resis-