AG_2105_Practical_9_10_student_copy.pdf
- 1. AG 2105 Soil and Water Management Practical No. 09 Calculation of available soil moisture in different soils by using FC and PWP 1
- 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. 2
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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. 4
- 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. 5
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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 8
- 10. Available water content (AWC) 𝐴𝑊𝐶 = 𝐹𝐶 − 𝑃𝑊𝑃 Where; FC = field capacity PWP = Permanent wilting point 10
- 11. Practical No. 10 Conversion of soil moisture percentage in to water depth value 11
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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) 15
- 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 16
- 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. 17
- 18. • Solution: • (a) Depth of water (dw) 18
- 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. 19
- 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. 20
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