Vadose/unsaturated. (Basic concepts) Lucila Candela ERASMUS

Transcription

1 The Vadose/unsaturated Zone (Basic concepts) 1 Lucila Candela ERASMUS

2 Contents Nature of vadose zone Energy status Parameters and determination Water Balance Hydraulic conductivity 2

3 (nobody s land, Meinzer, 1942) Soil Drinking water Vadose zone Capillary fringe Groundwater

4 ±0,0 m 5,0 m 7,0 m IMPERMEABLE LAYER 27,0 m 4

5 General characteristics ti Geologic media between land surface and regional water table Their pores are only partially filled with water; water content is lower than porosity n Water pressure p is less than atmospheric pressure; pressure head lower than zero Soil water potential h is measured with a tensiometer Hydraulic conductivity K and water content depend on pressure head For unsaturated flow: < 0; =() y K= K() Solids Air (O 2 ) Water Bacteria CO 2

6 Hydraulic conductivity K and water content depend on pressure headhysteretic behaviour

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8 1. Nature of vadose zone Is a thermodynamic open system air water solid particles With exchange of materials and energy Soil consists of : - solid, liquid and gas - organic and mineral Particles of different sizes organic and mineral - living and dead Air (O 2 ) Solids Water 8 Bacteria CO 2

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14 Specific surface The solid-particle surface area of a soil divided by the solid-particle mass or volume, expressed in m 2 kg -1 specific surfaces of some mineral compounds: Kaolinite m 2 /g Illite m 2 /g Montmorillonite it m 2 /g Organic Matter...> 800 m 2 /g 14

15 2. Energy status-soil water potential The amount of work that must be done per unit of a specified quantity of pure water in order to transport reversibly and isothermally an infinitesimal quantity of water from a specified source to a specified destination. ψt ψg ψp ψo Ψ t Ψ g Ψ p Ψ o Hydraulic potential (total soil water potential) Matric potential (capillary/adsorptive forces) Pressure potential (soil-air pressure) Osmotic potential (solute potential) 15

16 3. Parameters and water content V t V p V a V l air water m a 0 m l m t Porosity Water content (volumetric/gravimetric) Density Flow/transport parameters V s solid m s volume: mass V a air m a V 1 water m 1 16 V s solid m s Soil-water retention curve

17 SCALE!!! catchmentmark : 100 x 100 m Lysimeter : 1x 1 x 1 m Experimental plot Laboratorieforsøg: LABORATORY A O ca 100 g

18 3.1. Laboratory determinations ti Characteristics (VZ) Physico chemical Mineralogy O.M content Water content Density Hydraulic Grain distribution Clay content Porosity... ****ASTM, SSA, OECD. Suction measurements (direct) Tensiometers Psicrometers Gypsum cell. (applied suction) Suction plate lt Pressure cell Osmotic pressure.

19 Sievingi Oven Presure plate Tensiometer 19

20 Disturbed-undisturbed soil samples (destructive). Sampling devices Bd Bodenbohrer bh Spaten ASTM task group on vadose zone Monitoring 20

21 3.2. Field determinations (subsurface) Water/moisture content Radioactive Method (neutron probe) FDR-Method (dielectric constant) TDR-Method (dielectric constant) 21

22 TDR TRIME RS44 Tube Access Probe (portable) 22

23 Loam Sand Clay 23 Cepuder, 2004

24 4. Components of soil-water budget Evapotranspiration Infiltration:Inf Recharge Water retention: field capacity, plants 24

25 Climatic Water Balance Basis is the water balance equation ET = P - RO + S + D ET evapotranspiration ti P precipitation RO surface Runoff S change of soil water Storage D drainage 25

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27 Evapotranspiration Loss of water from a given area and during a specified period of time, by Evaporation from the soil surface and by Transpiration from plants in [mm d -1 ] Potential Evaporation / Transpiration / Evapotranspiration: Actual Evaporation / Transpiration / Evapotranspiration: Infiltration The entry of water into soil. Infiltration ti flux (or rate): The volume of water entering a specified cross-sectional area of soil per unit time [L t -1 ] Cummulative volume 27

28 (De Marsilly) 28 infiltration-rates for different soils: sand, loam, clay (Cepuder, 2004)

29 Field calculation: Double Ring Infiltrometer 29

30 Lysimeter 30

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32 Evapotranspiration estimation direct methods: Water balance indirect methods: Evaporation from different pans Calculation from climatological data 32

33 5. Hydraulic conductivity. Darcy s Law Darcy law describes the rate of flow of water through saturated porous media. v = k. i K saturated K obtained in the lab by direct application of the Darcy equation to a saturated soil column of a uniform cross-sectional area dh k dt h L clay K [cm/s] 3, (0,0003 m/d) medium silt 4, coarse silt 2, fine sand 4, coarse sand 7, gravel > (1296 m/d) 33

34 5 1 Unsaturated Hydraulic Conductivity 5.1.Unsaturated Hydraulic Conductivity in the vadose zone K is a highly nonlinear function of water content k(θ) (or matric potential ). Is a function of the fluid properties, media properties and water content (or gas): t h K q (D a rc y ) z h K q t 1 z h h K z t z q t R ic h a r d s 34

35 k (h) k (θ) 35

36 Field determinations (field-saturated hydraulic conductivity. Guelph Permeameter Disc Infiltrometer (unsat) 36

37 5.2.Hydraulic potential.tensiometers Device for determination of pressure difference between soil water phase and atmosphere, commonly equal to the matrix potential per Vol. Measurement range limited to bars Old tensiometers, were equipped with a vacuum manometer, modern devices have a pressure transducer, whose electronic signal is recorded with a data logger 37