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Field Capacity of Water.ppt
- 1. Field Capacity of Water
- 2. FIELD CAPACITY OF WATER • Field capacity (FC) is the amount of soil moisture or water content held in soil after excess water has drained away and the rate of downward movement has decreased, which takes place within 2-3 days after a rain or irrigation in previous soil. • The physical definition of (FC) is the bulk water content present in soil at −33 J/kg of the suction pressure • FC=(θFC/100)×RD Levels of moisture in soil
- 3. TYPES OF WATER IN SOIL • The Soil water is dynamic; removal of water occurs due to drainage, evaporation, and transpiration and addition of water occurs with dewdrops, rainfall, and irrigation . • There are three types of water in soil. 1. GRAVITATIONAL WATER 2. CAPILLARY WATER 3. HYGROSCOPIC WATER
- 4. 1-GRAVITATIONAL WATER • Amount of water that is drained off due to pull of gravity to join ground water table .Larger soil particles that don’t retain water in it and perculates down due to gravity. • This water is not available to the plants for growth
- 5. 2-CAPILLARY WATER • Amount of water that is retained against the pull of gravity due to surface tension. • Capillary water is that water that is held in microspores of the soil, and composes the soil solution because surface tension properties (cohesion adhesion forces) of soil is much greater than that of force of gravity. • As the soil dries out, pore size increases and the capillary water perculates down to gravitational water ,in this way capillary water changes to gravitational water. • Capillary water is the main water that is available to the plants as it is trapped in soil solution right next to the roots of the plants.
- 6. 3-HYGROSCOPIC WATER • Amount of water that is adsorbed on the surface of the soil particles as a very thin film and strongly stick to the soil grain. It is generally not available to the plants because water is so tightly bound to the soil particles by adhesion properties and very little amount of water is taken up by the plants. • Hygroscopic water is found on the soil particles not within the pores. • Example: Clay • Hygroscopic water is helpful for the growth of the plants.
- 7. HOW WATER IS RETAINED IN SOIL • Water retention capacity of soil is due to their colloidal properties and aggregation qualities. • The water is held on the surface of the colloids and other particles and in the pores. The forces responsible for retention of water in the soil after the drainage has stopped are due to surface tension and surface attraction and are called surface moisture tension. • This refers to the energy concept in moisture retention relationships. • The force with which water is held is also termed as suction. • That’s why we say that smaller soil particles retain more water as compared to that of larger soil particles. Water retention capacity of soil.
- 8. SOIL MOISTURE RELATIONSHIP (SATURATION CAPACITY) • Amount of water present in soil when all the soil particles are filled with water. • Larger soil particles don't retain water and water perculates down due to pull of gravity and join gravitational water .Water retain after this is field capacity • Saturation Capacity=Ww/Ws • Where a) Ww is the weight of water b) Ws is the weight of soil Soil water relation.
- 9. FIELD CAPACITY • Weight of water retend in the soil after gravitational water has drained off that is the water exactly required by the plants for their growth.
- 10. PERMANENT WILTING POINT • Critical value below which root cannot extract enough moisture due to increase in soil moisture surface tension. At this point water can’t perculates out of soil because it is strictly adhere to the soil particles. • It is the point when there is no water available to the plants. Permanent wilting point depends on the plant variety, but it is usually around 1500 kPa (15 bars). At this stage, the soil still contains some water, but it is difficult for the roots to extract from the soil particles . • If water is less than permanent wilting point plants wilt and growth is stopped and plants die.
- 11. AVAILABLE WATER • The amount of water actually available to the plants for their growth. • It is determined as field capacity minus the water that will remain in the soil at permanent wilting point. • The available water content depends greatly on the soil texture and structure. • θAWC = θFC - θPWP • Where a) θAWC is maximum available moisture content (%v/v) b) θFCis the moisture content at field capacity point (%v/v) c) θPWP is the moisture content at permanent wilting point(%v/v) Different Level of water in Soil
- 12. READILY AVAILABLE WATER • Amount of water that is easily available to the plants. • Readily available =75% of the available water • (Total water) Vv = Vwy/V + Vwr/V • Where: a) Vwy is the volume of the water yield that is lost in form of gravitational water b) Vwr is the volume of the water retend in the soil particles and available to the plants c) V is the total volume of the soil d) Vv = Sy + Sr e) Where Sy is specific yield ,Sr is specific retention SPECIFIC YIELD (Sy) • Ratio of the volume of the water yield to the total volume of the soil. SPECIFIC RETENTION (Sr) • Ratio of the volume of water retained against gravity to the total volume the soil.
- 13. FACTORS THAT INFLUENCE THE FIELD CAPACITY OF WATER • The various factors that influence the water holding capacity 1) PREVIOUS SOIL WATER HISTORY 2) SOIL STRUCTURE 3) SOIL TEXTURE 4) TYPE OF CLAY 5) ORGANIC MATTER 6) TEMPERATURE 7) DEPTH OF WETTING 8) EVAPOTRANSPIRATION 9) SOIL MOISTURE CONTENT
- 14. PREVIOUS SOIL WATER HISTORY, SOIL STRUCTURE , And SOIL TEXTURE • A wetting soil and a drying soil hold different amounts of water. A soil that is saturated and then dries has a higher field capacity than a soil that is being wetted. • Soil structure has direct relation to the amount of water it contain. • The smaller the soil particles , greater will be water retention capacity of soil. Clay has more water holding capacity than sand. Smaller soil particles (clay) have more pores or capillary spaces so they have high water holding capacity. Larger soil particles (sand) have few capillary spaces so they have less water holding capacity
- 15. TYPE OF CLAY and ORGANIC MATTER • The higher the content of montmorillonite, greater is the amount of water retained in it. Montmorillonite is a type of clay, a very soft group of minerals that form when they precipitate from water solution as microscopic crystals. • Soil organic matter helps in retaining water. Organic matter aids in cementing particles of clay, silt, sand that aggregates into substance which increases the water holding capacity. Decomposition of organic matter add essential nutrients in the soil which are helpful for the growth of pants.
- 16. TEMPERATURE and EVAPOTRANSPIRATION • Temperature influences the amount of water held, particularly if the soil has been previously wetted. The amount of water retained at field capacity decreases as the soil temperature increases. This results in increased runoff from a watershed as soil warms and water retention capacity of the soil decreases. • Combination of water that is lost from the soil through evaporation and transpiration through plants as a part of their metabolic process. The rate and pattern of extraction of water by plant roots from soil can affect the gradients and flow directions in the profile
- 17. DEPTH OF WETTING • Wetter the profile is at the outset greater is the depth of wetting and greater is the extraction of water by plants roots from the soil can affect the flow of gradients. And at field capacity , the soil is wet and contain all the water it can hold.
- 18. SOIL MOISTURE CONTENT • Soil moisture content indicates the amount of soil present in water present in soil. It is commonly expressed as amount of soil present in depth of 1m of a soil. • Example: when amount of water (in mm of water depth) of 150mm in a depth of of 1m of a soil, soil moisture content is 150mm/m.
- 19. IMPORTANCE OF FIELD CAPACITY • Soil water serves as a solvent and water is up to field capacity it act as carrier of food nutrients for plant growth. • The yield of a crop is more often determined by the amount of water available rather than the deficiency of other food nutrients. • Soil water regulates soil temperature. • Microorganisms require water for their metabolic activities. • Field capacity of water helps in chemical and biological activities of soil. • Water is essential for photosynthesis and plants can synthesize the organic matter if water is supplied to the plants upto field capacity.
- 20. ESSENTIAL CONDITIONS TO DETERMINE THE FIELD CAPACITY • Saturate the soil profile to the depth under study by adding excess of irrigation water. • Minimize the evaporation losses from the surface. • Eliminate the transpiration losses by working on non- cropped fields. • Select the plots containing uniform and free draining soil. • Observe the time rate of decrease in moisture content.
- 21. METHODS OF MEASURING FIELD CAPACITY There are various methods of measuring field capacity of water but two methods are mainly described. 1)Pressure based method. 2)Flux based method.
- 22. 1.PRESSURE-BASED METHOD • Pressure based method was introduced by Richard and Weaver to estimate the field capacity to determine with a soil water content at a matrics potential of -33kPa. • For Pressure based method , Field capacity is determined by retention curve method. Water retention curves for sand clay and loam
- 23. 2. FLUX –BASED METHOD • From the Flux based method, the field capacity can be estimated from the unsaturated soil hydraulic conductivity function with a predefined negligible free drainage flux. • With the unsaturated soil hydraulic conductivity under the field capacity conditions.
- 24. Thank You