This document defines key soil moisture terminology and provides methods for calculating available soil moisture content. It discusses concepts like field capacity, permanent wilting point, saturation, and available water content. Field capacity refers to the water content after gravity drainage, while permanent wilting point is the moisture level at which plants wilt and die. The difference between these levels determines the available water content. Practical problems are provided as examples. One problem involves calculating the total water in the top 30 cm of soil, the depth of wetting from irrigation, and available water at field capacity. Another calculates soil moisture content based on weights of a moist soil sample before and after drying.

1. AG 2105
Soil and Water Management
Practical No. 09
Calculation of available soil moisture in different soils by using FC and PWP
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2. Terminology
• Available soil moisture
• The difference between the amount of water in the soil at field capacity and
the amount at the permanent wilting point.
• Saturation
• Occurs when all the voids in the soil are completely filled with water.
Although there is plenty of water available to the crop at saturation, water
uptake is seriously curtailed by the lack of oxygen in the soil at soil water
contents greater than field capacity.
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4. • Deep percolation
• Water that drains beyond the plant root zone.
• Field capacity
• The water content of the soil where all free water has been drained form the
soil through gravity. Sandy soils may drain within a few hours but fine
textured soils such as clay may take a few days to drain. Proper irrigation
brings soil moisture up to filed capacity.
Terminology cont.
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5. • Permanent wilting point (PWP)
• The soil moisture content at which the plant will wilt and die. While there still
may be water in the soil, the plant is not able to extract sufficient water from
the soil to meet its needs.
• Maximum soil water deficit (MSWD)
• Only a portion of the available water is easily used by the crop. The maximum
soil water deficit is the amount of water stored in the plant’s root zone that is
readily available to the plant. To prevent plant water, stress an allowable
depletion factor is used to calculate the manageable allowable depletion. This
factor varies but is usually around 50%.
Terminology cont.
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8. Field Capacity
• The maximum water which can be held by the soil against gravity.
• It depends on porosity and capillarity.
• Moisture content at field capacity includes hygroscopic water and
capillary water.
𝐹𝐶 =
𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑟𝑒𝑡𝑎𝑖𝑛𝑒𝑑 𝑖𝑛 𝑐𝑒𝑟𝑡𝑎𝑖𝑛 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑜𝑖𝑙
𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑎𝑚𝑒 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑑𝑟𝑦 𝑠𝑜𝑖𝑙
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9. Readily
Available
water
FC
15
1
3
1
10
5
3
1
PWP
Total
Available
water
(AWC)
Soil
moisture
tension
Vol/Vol
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10. Available water content (AWC)
𝐴𝑊𝐶 = 𝐹𝐶 − 𝑃𝑊𝑃
Where;
FC = field capacity
PWP = Permanent wilting point
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11. Practical No. 10
Conversion of soil moisture percentage in to water
depth value
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15. Percent soil water by weight
• This value is calculated by weight loss during oven drying.
𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝑠𝑜𝑖𝑙 𝑤𝑎𝑡𝑒𝑟 𝑏𝑦 𝑤𝑒𝑖𝑔h𝑡(%𝑤) =
𝑤𝑒𝑡 𝑤𝑒𝑖𝑔h𝑡 − 𝑜𝑣𝑒𝑛 𝑑𝑟𝑦 𝑤𝑒𝑖𝑔h𝑡
𝑜𝑣𝑒𝑛 𝑑𝑟𝑦 𝑤𝑒𝑖𝑔h𝑡
× 100
• Percent water by weight can be converted to water content expressed as
percent water by volume if the soil's bulk density is known.
𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝑠𝑜𝑖𝑙 𝑤𝑎𝑡𝑒𝑟 𝑏𝑦 𝑣𝑜𝑙𝑢𝑚𝑒(%𝑣) = (%𝑤)(𝐵𝑢𝑙𝑘 𝑑𝑒𝑛𝑠𝑖𝑡𝑦)
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16. • Equivalent surface depth describes soil water content by how deeply it would
cover the sol if removed and set on top of the sample.
𝐸𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑑𝑒𝑝𝑡ℎ = (%𝑣) (𝑠𝑎𝑚𝑝𝑙𝑒 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑖𝑛 𝑐𝑚)
• Irrigation should begin when 50% of the available water has been depleted. This
value can be calculated as follows:
𝐴𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒 𝑤𝑎𝑡𝑒𝑟 𝑑𝑒𝑝𝑙𝑒𝑡𝑒𝑑, % =
𝑓𝑖𝑒𝑙𝑑 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 (%𝑤) − 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝑤𝑎𝑡𝑒𝑟 (%𝑤)
𝑓𝑖𝑒𝑙𝑑 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 (%𝑤) − 𝑤𝑖𝑙𝑡𝑖𝑛𝑔 𝑝𝑜𝑖𝑛𝑡 (%𝑤)
× 100
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17. Q 01:
Calculate
a) The total water presently contained in the top 30 cm,
b) The depth to which 27.5 mm (1.1 inch) of irrigation would wet this
uniform soil and
c) The available water the soil contains in the top 30 cm when the soil
is at field capacity.
The measurement of the soil is as follows:
• Present water content 18%
• Water content at field capacity 23%
• Permanent wilting percentage 9%
• Bulk density of 0-30 cm depth surface soil 1.3 g/c.c.
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18. • Solution:
• (a) Depth of water (dw)
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19. b) To calculate the depth of wetting by a 27.5 mm (1.1 inch) irrigation, the
following equation is substituted.
c) To calculate the total possible plant available water in the top 30
cm, when the soil is wetted equals field capacity minus
permanent wilting percentage.
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20. Problem 2:
A soil sample taken from a field is placed in the aluminium box, weighed,
dried in an oven at 105°C (221°F) and reweighed.
The measurements are as follows:
• Weight of moist soil plus aluminium box = 159 g
• Weight of oven dried soil plus aluminium box = 134 g
• Weight of empty aluminium box = 41 g
Calculate the moisture content of the soil.
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