NRES 201 Lecture 16 Outline of Last Lecture I. Properties of waterII. Capillarity and soil waterIII. Energetics of soil waterIV. Classification of soil waterOutline of Current Lecture V. Measurement of soil waterVI. Movement of water in soilsVII. Factors affecting soil water availability Current LectureMeasurement of soil water: Water content- Gravimetric techniqueso Mass-based estimation The most common method of measuring soil water content Heat sample at 110ºC for 48 hours, cool, and weight Always calculated on a dry-weight basiso Expressed as: A proportion (kg/kg)- Volume-based estimationo Bulk density (Db) must be determinedo 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 airo Two parallel probes are inserted into the soilo Measures volumetric water contento Convenient and rapid but expensiveMeasurement of soil water: Water potential- Tensiometerso Measure matric potentialo 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 bottomo Limited measurement range - 0 to -85 kpa- Resistance blockso Consists of: Two electrodes embedded in a porous block of gypsum A resistance metero Useful range: -90 to <-1500 kpa o Used to automate irrigation - Pressure plateo Standard lab method for relating soil water content and matric potentialo Known pressure applied to soil sample on a porous plate in a sealed chambero Water forced downward into an unpressurized cello Soil water content determined gravimetrically after equilibrationMovement of water in soils: Saturated flow- Some fundamentals:o Water movement by gravityo Occurs when macro and micropores are water filledo Caused by precipitation or irrigationo Begins with infiltration Water movement into the surface layer of soil- Infiltrationo Caused in part by gravity and also by the affinity of water for soil solidso Most rapid for course - textured soils: Due to extensive macro porosityo 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 potentialo Requires continuous water filmso Occurs when macropores are filled with air- Responsible for:o Lateral water movement away from saturated zone due to furrow or drip irrigationo Capillary riseFactors affecting soil water availability:- Texture- Organic matter content- Compaction- Salinity - Soil and rooting
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