NRES 201 Lecture 16 Outline of Last Lecture I Properties of water II Capillarity and soil water III Energetics of soil water IV Classification of soil water Outline of Current Lecture V Measurement of soil water VI Movement of water in soils VII Factors affecting soil water availability Current Lecture Measurement of soil water Water content Gravimetric techniques o Mass based estimation The most common method of measuring soil water content Heat sample at 110 C for 48 hours cool and weight Always calculated on a dry weight basis o Expressed as A proportion kg kg Volume based estimation o Bulk density Db must be determined o Useful for calculating profile water storage or depth of wetting Time Domain Reflectometry TDR o Non Destructive technique for sensing electrical polarization in response to an applied voltage Much higher for water than for soil solids or air o Two parallel probes are inserted into the soil o Measures volumetric water content o Convenient and rapid but expensive Measurement of soil water Water potential Tensiometers o Measure matric potential o Water filled plastic tube with These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute An airtight seal at the top A vacuum gauge A porous ceramic cup at the bottom o Limited measurement range 0 to 85 kpa Resistance blocks o Consists of Two electrodes embedded in a porous block of gypsum A resistance meter o Useful range 90 to 1500 kpa o Used to automate irrigation Pressure plate o Standard lab method for relating soil water content and matric potential o Known pressure applied to soil sample on a porous plate in a sealed chamber o Water forced downward into an unpressurized cell o Soil water content determined gravimetrically after equilibration Movement of water in soils Saturated flow Some fundamentals o Water movement by gravity o Occurs when macro and micropores are water filled o Caused by precipitation or irrigation o Begins with infiltration Water movement into the surface layer of soil Infiltration o Caused in part by gravity and also by the affinity of water for soil solids o Most rapid for course textured soils Due to extensive macro porosity o Slowest for fine textured soils o Decreases with time due to Breakdown of soil structure upon wetting Clogging of soil pores by displaced solids Swelling of clay particles The filling of surface soil pores when drainage is limited by low subsoil permeability Movement of water in soils Unsaturated flow Characteristics o Water movement due to differences in matric potential o Requires continuous water films o Occurs when macropores are filled with air Responsible for o Lateral water movement away from saturated zone due to furrow or drip irrigation o Capillary rise Factors affecting soil water availability Texture Organic matter content Compaction Salinity Soil and rooting depths
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