Soil Moisture and Groundwater Recharge. Hsin-yu Shan Department of Civil Engineering National Chiao Tung University

Transcription

1 Soil Moisture and Groundwater Recharge Hsin-yu Shan Department of Civil Engineering National Chiao Tung University

2 Soil Physics A scientific field that focus on the water in the vadose zone Major concern of agricultural engineering Relationship between water content and pressure Hydraulic conductivity of unsaturated soil

3 Soil in the Vadose Zone Solids: Soil particles/aggregates Organic materials Fluids: Water (Aqueous solution) Air Other liquid

4 Porosity and Water Content of Soil Volume Weight V a Air V w a v V w Water w w Solids Air Water V s ws Solids

5 Porosity: ratio of volume of the void to the total volume e e V V V V V n s v v v + = + = = 1 Void ratio: volume of the voids to the volume of the solids s v V V e = Bulk density: s s b V w ρ =

6 Gravimetric water content: ratio of weight of water to the dry weight of solids w = ww x 100% w s Volumetric water content: ratio of pore water to the total volume V θ = w V Degree of saturation: ratio of the volume of pore water to the total volume of voids Vw Sr = x 100% V v

7 Capillary and the Capillary Fringe Capillary: water molecules at the water table subject to an upward attraction due to surface tension of the air-water interface and the molecular attraction of the liquid and the solid phases.

8 Tension If fluid pressures are measured above the water table, they will be found to be negative with respect to local atmospheric pressure

9 Air Pressure If air in the pores is connected, the air pressure is equal to the local atmospheric pressure If air in the pores is not connected and is in the forms of air bubbles such as in the capillary fringe, the air pressure is equal to the pressure of water, which is negative.

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11 Capillary rise in a tube σcosλ i λ s f r R λ σ λ=0 R=r h c = 2σ cosλ ρ gr w h c water

12 Fig. 6.2 IDEALIZED pore diameter in a sediment with cubic packing. The equivalent capillary tube has a radius of 0.2 the diameter of the grains

13 P atm z(m) P atm ψ p 0.4 ψ h (=0) ψ g water 0 y (J/kg) y r l (kj /cu.m = kp a) y /g (J/N = m)

14 z(m) water ψp ψh ψg =gz ψ (J/Kg)

15 1 2 z (m) ψ g = gz h heads (m)

16 Height of Capillary Rise Sediments Grain Diameter (cm) Pore Radius (cm) Capillary Rise (cm) Fine silt Coarse silt Very fine sand Fine sand Medium sand Coarse sand Very coarse sand Fine gravel

17 Capillary Fringe Capillary pores in the vadose zone can draw up water from beneath the water table below which the pores are saturated with water. The zone above the water table, in which the pores are saturated is termed capillary fringe

18 The liquid pressure in the capillary zone is negative Capillary fringe is a part of vadose zone The zone of aeration is best defined as the zone where the soil moisture is under tension

19 The capillary fringe is higher in finegrained soils than in a coarse-grained soils Smaller pore opening creates greater tension

20 Pore-Water Tension in the Vadose Zone Fluid pressures in the vadose zone are negative The negative pressure head is measured in the field with a tensiometer

21 Tensiometer A tube that is closed at the top and filled with water The tube is connected to a pressure gauge A ceramic cup at the bottom of the tube to provide a porous membrane The ceramic cup should be saturated with water before use

22 d z 1 b Soil Surface c z 2 a Tensiometer (1)

23 C B z 1 E D A l Soil Surface z 2 Tensiometer (2)

24 If the suction is lower than the entry pressure of the membrane, only water can go through it When the suction in the soil pulls water out from the tensiometer, the water in the tube is under tension and will cause the pressure gauge to indicate the magnitude of such a tension

25 z (m) Air Entry Pressure

26 Soil Water Water in the vadose zone that is available to growing plants. This is not a very exact definition. Avoid using it.

27 Field Capacity The soil moisture content of a layer at which the the force of gravity acting on the water equals the surface tension. Related to the specific retention Depends on specific retention, evaporation depth, and the unsaturated permeability characteristic curve of the soil.

28 Field capacity is related to specific retention but has different units It depends upon specific retention, evaporation depth, and the unsaturated permeability characteristic curve of the soil

29 The concept is vague Gravity drainage may take a long period to occur Some definitions: Water content of soil after 48 hours of gravity drainage Water content of soil under a suction of 0.3 bar

30 Moisture content of a silt loam as a function of time since saturation Time (days) θ (%)

31 Wilting Point The soil moisture content below which the plant roots cannot withdraw water from the soil. Some defined it to be the soil moisture content under a suction of 15 bars. The available water capacity of a soil is the difference between the field capacity and the wilting point.

32 Fig. 6.5 Hypothetical fluctuation of soil moisture for a sandy loam soil through an annual cycle in a region with a moderate amount of rainfall (500 to 750 mm per year) and heavy rains in the spring

33 Fig. 6.6 Water-holding properties of soils based on texture. The available water supply for a soil is the difference between field capacity and wilting point

34 Water Potential The potential energy, or force potential of ground water consists of two parts: elevation and pressure (velocity related kinetic energy is neglected)

35 Suction of Water in Soils Fluid pressures in the vadose zone are negative, owing to tension of the soil-surface-water contact The negative pressure head is measured in the field with a tensiometer

36 d b z 1 Soil Surface z 2 c a Suction Calibrate Before Use

37 Suction Matric suction Elevation head Pressure head Osmotic suction

38 Head (Water Potential) Gravity potential, Z Elevation head Moisture potential, ψ Suction head Can be several orders of magnitude greater than the gravity potential 1 bar 10 m of water column

39 Soil Water Characteristics Defines the relationship between water content and water potential (suction)

40 θ h m (m) SWCC of a Coarse Sand

41 SWCC of various soils

42 Soil Water Characteristic Curve

43 Absorption and desorption characteristics with primary scanning curves

44 Poorly sorted P ressure head (cm) h b W ell sorted Entry pressure h b Water content,θ

45 Drying scanning curve ψ Main drying curve Main wetting curve Wetting scanning curve Volumetric water content, θ Absorption and desorption characteristics with primary scanning curves

46 Hysteresis Ink bottle effect Trapping of air Advancing and receding contact angle

47 Determination of SWCC Laboratory Pressure plate apparatus Filter paper Thermocouple Psychrometer Centrifuge Field Tensiometer Water content measurement

48 soil Ceramic plate water z (m) Effluent port that can vary elevation ψ g = gz h heads (m)

49 Theory of Unsaturated Flow Gravity potential, Z Moisture potential, ψ(θ v ) Negative value suction resulted from soil-water attraction At moisture contents close to specific retention, the gravity potential is greater When the soil is very dry, the moisture potential may be several orders of magnitude greater than gravity potential

50 Darcy s law is valid for flow in the unsaturated zone Flow in water saturated pores Larger cross-sectional area to conduct flow Flow in pores with air in them Smaller cross-sectional area to conduct flow Total potential φ φ = ψ (θ v ) + Z

51 Fig. 6.7 The relationship between hydraulic conductivity and volumetric water content

52 ψ -150 cm water -100 Drying Wetting Hydraulic conductivity (10-4 Relationship between hydraulic conductivity and soilmoisture head

53 Fig. 6.9 Idealized curves showing relationships of volumetric water content, hydraulic conductivity, and soilmoisture head

54 Fig Moisture profiles showing the downward passage of a wave of infiltrated water. The soil is saturated at a water content of 0.29 and has a field capacity water content of 0.06

55 Coarse vs. Fine-Grained Materials At lower volumetric water content: Coarse material may have very few saturated pores Fine-grained soils may have most of the pores still saturated Thus, the unsaturated hydraulic conductivity of a clay may be greater than that of a sand or gravel

56 Fig Typical soil-moisturepotential-hydraulic conductivity curves for a sandy soil showing the crossover effect for increasing moisture potential

57 Water-Table Recharge When the front of infiltrating water reaches the capillary fringe, it displaces air in the pore spaces and cause the water table to rise.

58 Fig Hydrograph of a shallow well in a watertable aquifer in Long Island.

59 Rate of Recharge The rate of water-table recharge depends on: Thickness of the unsaturated zone The thinner the zone, the faster the rise of water table May generate a localized ground-water mound

60 Fluctuation of Ground-Water Table Water table shows a seasonal fluctuation Rising during periods of recharge Falling when there is no precipitation or when evapotranspiration exceeds precipitation

61 Fig Monthly hydrograph of water levels in a watertable monitoring well on eastern Long Island, New York

62 Depth to water (feet) Fig Hydrograph of a water-table monitoring well showing effect of discharge by evaporation on the water table elevation

63 Depth to water (feet)