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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 cm 2 ) Albedo (%). Chapter 2 The watershed management inventory. Main characteristics.
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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 (%) 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 (%) 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. 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) Watershed Management--7
Chapter 2 The watershed management inventory Hydraulic Soil Group
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. 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. 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. Watershed Management--7
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. Watershed Management--7
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. Watershed Management--7
Chapter 2 The watershed management inventory Limitation for evapotranspiration • Turbulent airflows • Lamina airflows Watershed Management--7
Chapter 2 The watershed management inventory Measurement of evapotranspiration • Potted plants • Lysimeters • Tent method • SAP movement method • The paired watershed method Watershed Management--7
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 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. Watershed Management--7
Chapter 2 The watershed management inventory Soil Texture Sand Loamy sand Silt Clay Sandy clay Silty clay 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 Watershed Management--7
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) Watershed Management--7
Gravitational water Field capacity Plant available water Wilting point Unavailable water Chapter 2 The watershed management inventory Soil water availability Watershed Management--7
