1. Watershed Management--7
Chapter 2  The watershed management inventory
2.3 Soil
Soil is a porous medium that blankets bedrock.
Main characteristics
Texture: Sand, silt, clay (%)
Soil depth (cm)
Bulk density (g cm2)
Albedo (%)
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2. Watershed Management--7
Chapter 2  The watershed management inventory
Main characteristics
Field capacity (%)
Wilting point (%)
Hydraulic conductivity (cm/sec)
Organic matter (OM) %, organic C (0.58om)
N P K (%)
Cation exchange capacity, CEC (cmol + kg-1)
Salt concentration (%)
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3. Watershed Management--7
Chapter 2  The watershed management inventory
Permeability
Permeability refers to the rate of water movement through a saturated soil (when gravitational forces dominate) and is defined by Darcy’s law.
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4. Watershed Management--7
Chapter 2  The watershed management inventory
Infiltration
Infiltration is the process by which water enters the soil surface.
Infiltration rate (that is, water transport through an unsaturated soil, combination of gravitational forces and capillary action ) can be predicted by Horton’s (essentially empirical) formula (Horton, 1939)
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5. Surface runoff potential
Chapter 2  The watershed management inventory
Hydraulic Soil Group
Soil group
Infiltration rate
Surface runoff potential
A: well-drained beds of sand and gravel
High (>25cm/h)
Low
B: moderately fine to moderately coarse soils such as sandy loams
Low to moderate (6.3 to 25 cm/h)
Low to
moderate

6. Chapter 2  The watershed management inventory
C: fine-textured soils and soils with underlying layer impeding drainage
Low (0.5 to 6.3 cm/h)
High
D: fine clay soils and soils with underlying impermeable layer; soils in areas of permanently high water table
Very low
(<0.5 cm/h)

7. Watershed Management--7
Chapter 2  The watershed management inventory
Evapotranspiration
Evaporation is the net loss of water from a surface resulting from a change in the state of water from liquid to vapor and the net transfer of this vapor to the atmosphere.
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8. Watershed Management--7
Chapter 2  The watershed management inventory
Evapotranspiration
Transpiration is a process in which water is extracted by the plant’s roots, transported upwards through in stem, and diffused into the atmosphere through stomata.
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9. Watershed Management--7
Chapter 2  The watershed management inventory
Evapotranspiration
Evapotranspiration: evaporation from soils, plant surfaces, and water bodies, together with water losses through plant leaves.
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10. Conditions for evapotranspiration
Chapter 2  The watershed management inventory
Conditions for evapotranspiration
A flow of energy to the evaporing or transpiring surface;
A flow of liquid water to these surface;
A flow of vapor away from these surface.
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11. Limitation for evapotranspiration
Chapter 2  The watershed management inventory
Limitation for evapotranspiration
Energy supply limits evaporation from the wet, exposed soil.
Water flow through the soil limits the rate of evaporation at the soil surface.
Transpiration is more efficient than evaporation because of the large surface and the surface is exposed to turbulent airflows.
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12. Limitation for evapotranspiration
Chapter 2  The watershed management inventory
Limitation for evapotranspiration
Turbulent airflows
Lamina airflows
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13. Measurement of evapotranspiration
Chapter 2  The watershed management inventory
Measurement of evapotranspiration
Potted plants
Lysimeters
Tent method
SAP movement method
The paired watershed method
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14. Soil water potential(s)
Chapter 2  The watershed management inventory
Soil water potential(s)
s = g + p + o + t + m
Where g = gravitational potential; p = pressure potential; o = osmotic or solute potential; t = thermal potential; and m = matric potential
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15. Watershed Management--7
Chapter 2  The watershed management inventory
Osmotic potential
Plants obtain atmospheric CO2 required for photosynthesis by diffusion through open leaf stomates. While this is taking place, water in the leaf parenchyma tissues evaporates into the sub-stomatal cavities and diffuses through the open stomates into the atmosphere. This process can create large water potential differences between the leaves and the soil surrounding the roots.
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16. Watershed Management--7
Chapter 2  The watershed management inventory
Soil Texture
Sand
Loamy sand
Silt
Clay
Sandy clay
Silty clay
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17. Watershed Management--7
Chapter 2  The watershed management inventory
Soil Texture
Loam
Sandy loam
Silt loam
Clay loam
Sandy clay loam
Silty clay loam
? Sandy silt loam
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18. Soil water availability
Chapter 2  The watershed management inventory
Soil water availability
Field capacity--the maximum soil water content soil can retain against the force of gravity (-1/3 bar; -33 Kpa)
Wilting point--a limit of soil water content below which soil water is insufficient to enable plant to grow and become wilted (-15 bar; -1500Kpa)
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19. Soil water availability
Chapter 2  The watershed management inventory
Soil water availability
Field capacity
Wilting point
Plant available water
Gravitational water
Unavailable water
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