Correct Answer - Option 4 : 1.25 cm
Concept:
Depth of water Held in Root zone:
For ease in calculation water present in voids of soil needs to be expressed as the depth of water.
Let root zone depth = ‘D’ in m
Specific wt. of soil (dry) = γd
The cross-sectional area of soil considered = A
Equivalent depth of water stored in voids of soil = ‘d’.
We know,
\({F_c} = \frac{{Weight.\;of\;water\;retained\;in\;certain\;vol.\;of\;soil}}{{Weight\;of\;same\;volume\;of\;dry\;soil}}\)
\(∴ {F_c} = \frac{{Ad\;{\gamma _w}}}{{A\;D\;{\gamma _d}}}\)
\(∴ {F_c} = \frac{d}{D}\frac{{{\gamma _w}}}{{{\gamma _d}}}\)
\( \Rightarrow d = \frac{{{\gamma _d}}}{{{\gamma _w}}} \times D \times {F_c}\)
It is the depth of water stored in the root zone for full field capacity.
But this entire depth of water cannot be extracted by plants. Hence available moisture will be
\({d_{available}} = \frac{{{\gamma _d}}}{{{\gamma _w}}} \times D \times \left[ {{F_c} - \phi } \right]\)
Where ϕ = permanent wilting point.
Depth of Readily available moisture (d’):
\(d' = \frac{{{\gamma _d}}}{{{\gamma _w}}} \times D \times \left( {{F_c} - {M_0}} \right)\)
Frequency of Irrigation:
The frequency of irrigation is decided based on the maximum allowable deficit and rate of consumptive use.
\({f_w} = \frac{{d'}}{{{C_U}/day}}\)
Where, Cu = Consumptive use/day
Calculation:
Fc = 29 % = 0..29, ϕ = 10% = 0.10, D = 0.8 m,
Relative density of soil (ρd / ρw) = 1.302
Frequency of irrigation = 10 days, Optimum moisture(Mo) = 17%
Depth of Readily available moisture \(\left( {d'} \right) = \frac{{{ρ _d}}}{{{ρ _w}}} \times D \times \left[ {{F_c} - {M_0}} \right]\)
\(∴ d' = 1.302 \times 0.8 \times \left[ {0.29 - 0.17} \right]\)
∴ d' = 0.12499 m = 124.99 mm
∴ Frequency of Irrigation = \(\frac{{d'}}{{{C_u}}} = \frac{{124.99}}{C_u} = 10\;days\)
∴ Cu = 12.49 mm = 1.25 cm.
(1) Saturation Capacity
Saturation capacity is defined as the total water content of a soil when all the pores of the soil are filled with water. This is also known as the maximum water holding capacity of the soil.
(2) Field Capacity
Field capacity is defined as the maximum amount of moisture that can be held by soil against gravity. At field capacity, the large or non-capillary pores of the soil are filled with air and the small or capillary pores are filled with water.
Field capacity is the upper limit of the capillary water or the moisture content available to the plant roots.
\({F_c} = \frac{{Weight.\;of\;water\;retained\;in\;certain\;volume\;of\;soil}}{{Weight\;of\;same\;volume\;of\;dry\;soil}}\)
(3) Permanent wilting point
The permanent wilting point is the moisture content at which the films of water around the soil particles are held so tightly that the plant roots cannot extract enough moisture at a sufficiently rapid rate to satisfy transpiration requirements thus resulting in the wilting of the plants.
(4) Available moisture
The difference in the moisture content of the soil between the field capacity and the permanent wilting point is termed as the available moisture.
This represents the moisture that is stored in the soil in the form of capillary water for being used subsequently by the plants.
(5) Readily available moisture
It is that portion of the available moisture which is most easily extracted by plant roots. It is also known as "Optimum moisture content". Only about 75% of the available moisture is usually readily available.
