Field Capacity (FC)
This is refers to the maximum quantity of water that the soil
can hold against the forces of gravity. It corresponds to a suction of 0.1bar
(Fonteh & Assoumou, 1996).
Permanent Wilting Point (PWP)
This is the moisture content at which a plant wilts
permanently under conditions of water stress even if it is later placed in a
saturated atmosphere. It is assumed to correspond to a suction of 15bars
(Fonteh & Assoumou, 1996).
Available Water Content (AWC) and Total Available Water
Content (TAWC)
This is the quantity of water that is readily available in a
soil for plant growth. It is expressed as the difference between field capacity
and permanent wilting point.
AWC= èfc - èwp (2.1)
The total Available Soil Water content (TAW) is defined as
the difference in soil moisture content between soil field capacity
(èfc) and wilting point
(èwp). It represents the ultimate amount of
water available to the crop and depends on the texture, structure and organic
matter content of the soil. The total available water in the root zone can be
calculated as follows (Hanks and Ashcroft, 1980):
TAWC = (èfc - èwp) Zr
(2.2)
Where,
TAWC= Total available soil water in the root zone (m)
èfc = Water content at Field capacity
(m3/m3)
èwp = Water content at wilting point
(m3/m3)
Zr = Root Depth (m)
Root depth growth with time can be calculated using the
procedure described by Borg and Grimes (1986) and given by the equation:
Zr = Zrm [0.511 +0.511Sin (3.03 - 1.47)]
(2.3)
Where,
The angle is in radians,
Zr is the root depth in cm,
Zrm is the maximum root depth of the crop in cm, DAP is the
number of days after planting, and
DTM is the number of days to maximum root depth.
The root depth growth rate is 1.2 mm/day for grass and 1.5
mm/day for banana until maximum effective root depth has been reached (Plauborg
et al., 1996). The maximum effective root depth is determined by both
crop and soil type.
Soil Moisture Deficit (SMD)
This is the difference between field capacity and the actual
soil moisture content. It is normally the depth of water that should be
replaced by irrigation (Merriam & Keller,
1975).
P
eff o
Effective Rainfall
? 125
This is the fraction of precipitation that is effectively
used by plants after the deduction of surface run off and deep percolation (Van
Laere, 2003). The effective precipitation depends on a number of variables:
amount, intensity and frequency of rainfall; evaporative demand; terrain
characteristics; soil and crop; groundwater location; management practices;
etc., (Kopec et al., 1984). Due to the difficulty of measuring all
these variables, some authors recommend the use of empirical equations or to
estimate the effective precipitation (Peff) as a percentage of total
precipitation (Ptot). In the last case, a value of 80 % is
recommended when rainfall depth is below 100 mm/month (Rojas & Rolda'n,
1996). Moon and Van der Gulik (1996) stated that the effective precipitation is
ignored if it is under 5 mm/day as this amount is not likely to penetrate the
soil surface and will be evaporated. The effective rainfall could equally be
calculated as proposed by the United States Department of Agriculture Soil
Conservation Service (Smith et al, 1996).
for Ptot < 250mm (2.4)
for Ptot > 250mm (2.5)
Readily Available Water and Depletion Factor
The fraction of total available water that a crop can extract
from the root zone without suffering water stress is the readily available
water (RAW). The depletion factor is the fraction of the total available soil
water that can be depleted from the root zone before moisture stress occurs (P
ranges from 0 to 1). The P values are expressed as a fraction of TAWC with
lower values taken for sensitive crops with limited root systems under high
evaporative conditions, and higher values for deep and densely rooting crops
and low
evaporation rate (Doorenbos et al., 1986). At low
rates of ETc, the p values are higher than at higher rates of Etc.
RAW = P * TAWC (2.6)
Where,
RAW = Readily Available Water,
P= depletion factor (0.35 for banana plants).
