Soils & Hydrology ( Part II)

Transcription

1 Soils & Hydrology ( Part II) 9. Soil Water 10. Precipita>on and Evapora>on 11. Infiltra>on, Streamflow, and Groundwater 12. Hydrologic Sta>s>cs and Hydraulics 13. Erosion and Sedimenta>on 14. Wastes in Soil and Water

2 Global Water Budget Volumes in PL, Flows in PL/yr 1 PL = 1 petaliter = L = 1000 GL

3 Residence Time: The >me water spends in a reservoir: τ = V / Q τ (tau) is the residence >me (years) V is the volume of the reservoir (liters) Q is the flow through reservoir (liters per year) Think of a bathtub... If you start with an empty bathtub τ = how many minutes it takes to fill the bathtub V = size of tub, (say 100 gallons) Q = flow into tub, (say 10 gallons per minute) τ = V / Q = 100 / 10 = 10 minutes If you start with a full bath There is no inflow And you let it drain at a constant rate Will it take the same >me to drain? You add and drain water at the same rate Will a tracer take the same >me?

4 Water on Planet Earth

5 Soil surfaces have nega%ve charge! Water is a polar molecule The H- side has a posi>ve (+) charge The O- side has a nega>ve (- ) charge Water is bound to: other water molecules by cohesive (water- water) forces the soil surface by adhesive (soil- water) forces Think of water as a bar magnet soil is a nega>ve box it s>cks to other water magnets the posi>ve end s>cks the box

6 Soils = Air + Water + Solids

7 Capillary Rise: The wicking effect caused by small pores Rise is greater in finer materials: clays > silts > sands > gravels h = 0.15 / r h is the height of rise in tube, cm r is the radius of tube, cm

8 Energy: The driving force for water flow The total energy is the sum of three types of energy E = GE + KE + PE GE = gravita>onal or poten>al energy like a water balloon at the top of a building KE = kine>c or iner>al energy like when the balloon is dropped and it s picking up speed PE = pressure energy like when the balloon hits the ground before it explodes Total Energy is expressed as a head, or height of water: h = z + p z is the eleva>on head p is the pressure head

9 Soil Tension A nega%ve pressure that accounts for moisture held in the soil by capillary forces A small tension means water is not bound >ghtly A large tension means that water is bound >ghtly We use the symbol ψ (psi) = - p to represent the tension: It s a nega>ve pressure, or suc>on Remember, dry soil sucks!

10 Kartchner Caverns, AZ

11 Buggytop Cave Chimney Rock Cave

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13 Some Defini>ons Bulk Density: BD = Mass Soil / Volume Soil Porosity: PS = Volume Voids / Volume Soil PS = 1 - BD / PD Water Content (theta): θ v = Volume Water / Volume Soil θ g = Mass Water / Mass Soil θ v = θ g BD Water Depth: D w = θ v D s D s is depth of soil Rela>ve Satura>on (capital theta): Θ = Volume Water / Volume of pores Θ = Θ v / PS

14 Classifica>on of Soil Water Satura>on = SAT The water content when the pores are completely filled with water. Satura>on corresponds to pressure poten>als of zero, and above (posi>ve pressure). This is the same as saying p = 0, ψ = 0 At satura>on, the volumetric water content equals the porosity.

15 Field Capacity = FC The water content held after rapid gravitational drainage has occurred. Field capacity is sometimes described as the amount of water a soil can hold against gravity. This is not completely true, however, as water continues to drain slowly by gravity at pressures below field capacity. The tensions associated with field capacity are between ψ = 0.1 and 0.3 bars, equal to 100 to 300 cm.

16 Wil>ng Point = WP The amount of water held when plant roots can no longer extract water from the soil. This tension is usually assumed to be ψ =15 bars Different plants have different wil>ng points Xeriphytes (dry- loving plant) can go down to ψ = 75 bars Phreatophytes (water- loving plants) can only go down to ψ = 5 bars

17 Air Dry = AD The amount of water held by soil when it is exposed to the atmosphere. Related to the rela>ve humidity. Soils let in moist air are weuer than soils let in dry air. Soils in caves and greenhouses are moist Soils in the desert are dry ψ varies from 75 to over 1000 bars depending on the RH Oven Dry = OD The amount of water held once the soil has been dried in a 105 C oven for 48 hours. ψ is about 10,000 bars in the oven.

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19 Lab 9 - This week in lab Moisture Characteris4c Curves Plot of tension (pressure) vs. water content (satura>on) Measure water content at various tensions Saturated, Field Capacity, Wil>ng Point, Air Dry, Oven Dry Determine plant available water content Demonstrate some concepts Capillary rise a func>on of par>cle and void size How to measure field water tension and satura>on Tensiometer, Time Domain Reflectometry (TDR), Gypsum Blocks

20 Moisture Characteris>c Curve A plot of water content, θ, vs soil tension, ψ.

21 Plant Available Water = AW = FC - WP The water in the soil between field capacity, ψ = 0.1 bar, and the wil%ng point, ψ = 15 bars: The water bound less >ghtly than the field capacity is termed gravita%onal water because gravity easily drains this water before the plants can get it. Water bound beyond the wil>ng point is unavailable, because plant roots can not pull hard enough to overcome absorp%on of the water to the soil

22 Soil Tensiometer A small reservoir to hold water This dial gives soil tension (negative pressure) This part buried in soil

23 Time Domain Reflectometer Used to measure soil moisture An electrical pulse is sent down the rod The pulse bounces off the end and returns to the source The wetter the soil, the longer the delay in returning

24 Capillary Rise: The wicking effect caused by small pores Rise is greater in finer materials: clays > silts > sands > gravels h = 0.15 / r h is the height of rise in tube, cm r is the radius of tube, cm

25 Hysteresis: Caused by air blocking water in pores Ink Bottle effect

26 Soil Water Movement Hydraulic head: h = z + p h is total head, the driving force z is eleva>on head, water moves from high to low eleva>on p is pressure head, water moves from high to low pressure Ψ is matric tension, which is a nega>ve pressure (- p), so that water moves from low to high tension Hydraulic gradient: G = Δh / Δx Δh is the change in head Δx is the distance This is the slope, or gradient, of the head. The steeper the slope, the higher the gradient

27 Soil water can move from dry to wet

28 Darcy s Law Water Flux: q = - K G = - K (Δh / Δx ) K is the hydraulic conduc>vity, or how permeable the soil is A big K (sands, gravels) means that water flows fast A small K (clay, rock) means that water flows more slowly q is the flux, or flow per unit area The nega>ve shows that the flux is moving from high to low It has a nega>ve slope Total Flow: Q = q A A is the cross- sec>onal area For example, the cross- sec>onal area of a pipe is π r 2 The bigger the pipe, the more the flow Q is the total flow

29 Soil Water Movement Total Head: h = z + p = z Ψ Hydraulic Gradient, G change in energy w/ distance G = Δh / Δx Hydraulic conductivity, K a function of pore size (d) and connectivity K = f(d 2 )

30 Hydraulic Conductivity Typical Values Gravel, lava, caves K = 10 cm/s Sands K = 20 cm/hr Soils K = 5 cm/day Clays K = 0.9 cm/yr

31 Darcy s Law, Q = A K G A = The area of flow The greater the area, the greater the flow. K = The hydraulic conduc>vity or permeability The higher the conduc>vity, the greater the flow. G = ΔH / L = The magnitude of the driving force (L = Δx) The steeper the water slope, the greater the flow

32 Unsaturated Hydraulic Conduc>vity Less than the Saturated Conduc>vity!! q = K u G = K K r G K u = K K r = unsaturated hydraulic conduc>vity K r = rela>ve hydraulic conduc>vity

33 Plant Stomata Control Water Loss Plant Water Uptake Water is pumped through plants partly by the pull or tension of the atmosphere The tension in the atmosphere is generally hundreds of bars The moisture in the soil is only weakly bound. We can measure the tension in the plant, and find the tension is lowest in the roots, and highest in the leaf

34 Plant Water Uptake Plant Available Water Soil water that is poten>ally used by plants Limited by soil tension, from wil>ng point to field capacity PAW = FC WP

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36 Chapter 9 Quiz Soil Water 1. Adhesion Cohesion (circle one) is the aurac>on of the water to the soil surface. 2. What is the volumetric water content if the bulk density is 1.33 kg/l (same as g/cm 3 ) and the gravimetric water content is 0.25? 3. What is the depth of water in a foot of soil if the volumetric water content is 0.10 (10%)? 4. Is available water primarily held in: Macropores (greater than 100 μm) Mesopores (between 0.1 and 100 μm) Micropores (smaller than 0.1 μm) 5. Which soil type has the greatest plant available water? Least? Clay Sand Silt Loam Clay Loam