Chapter 9

Soils & Hydrology (3060 - Part II)Soil WaterPrecipitation and EvaporationInfiltration, Streamflow, and GroundwaterHydrologic Statistics and HydraulicsErosion and SedimentationWastes in Soil and Water 1

Global Water BudgetVolumes in PL, Flows in PL/yr1 PL = 1 petaliter = 1015 L = 1000 GL2Residence Time: The time water spends in a reservoir: = V / Q (tau) is the residence time (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 = how many minutes it takes to fill the bathtubV = size of tub, (say 100 gallons)Q = flow into tub, (say 10 gallons per minute) = V / Q = 100 / 10 = 10 minutesIf you start with a full bathThere is no inflowAnd you let it drain at a constant rateWill it take the same time to drain?You add and drain water at the same rateWill a tracer take the same time?

3Water on Planet Earth

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Soil surfaces have negative charge! Water is a polar molecule The H-side has a positive (+) chargeThe O-side has a negative (-) chargeWater is bound to:other water molecules by cohesive (water-water) forcesthe soil surface by adhesive (soil-water) forcesThink of water as a bar magnetsoil is a negative boxit sticks to other water magnetsthe positive end sticks the box

5Soils = Air + Water + Solids

6Capillary Rise: The wicking effect caused by small poresRise is greater in finer materials:clays > silts > sands > gravels h = 0.15 / rh is the height of rise in tube, cmr is the radius of tube, cm

7Energy: The driving force for water flow The total energy is the sum of three types of energyE = GE + KE + PEGE = gravitational or potential energy like a water balloon at the top of a buildingKE = kinetic or inertial energylike when the balloon is dropped and its picking up speedPE = pressure energylike when the balloon hits the ground before it explodesTotal Energy is expressed as a head, or height of water:h = z + pz is the elevation headp is the pressure head

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Soil TensionA negative pressure that accounts for moisture held in the soil by capillary forcesA small tension means water is not bound tightlyA large tension means that water is bound tightly

We use the symbol (psi) = - p to represent the tension:Its a negative pressure, or suctionRemember, dry soil sucks!9

Kartchner Caverns, AZ

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12Some DefinitionsBulk Density:BD = Mass Soil / Volume SoilPorosity: PS = Volume Voids / Volume SoilPS = 1 - BD / PDWater Content (theta):v = Volume Water / Volume Soil g = Mass Water / Mass Soilv = g BDWater Depth:Dw = v DsDs is depth of soilRelative Saturation (capital theta): = Volume Water / Volume of pores = v / PS13Classification of Soil WaterSaturation = SATThe water content when the pores are completely filled with water. Saturation corresponds to pressure potentials of zero, and above (positive pressure). This is the same as saying p = 0, = 0At saturation, the volumetric water content equals the porosity.14Field 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 = 0.1 and 0.3 bars, equal to 100 to 300 cm.

15Wilting 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 =15 bars Different plants have different wilting pointsXeriphytes (dry-loving plant) can go down to = 75 bars Phreatophytes (water-loving plants) can only go down to = 5 bars

16Air Dry = AD The amount of water held by soil when it is exposed to the atmosphere. Related to the relative humidity. Soils left in moist air are wetter than soils left in dry air. Soils in caves and greenhouses are moistSoils in the desert are dry varies from 75 to over 1000 bars depending on the RHOven Dry = ODThe amount of water held once the soil has been dried in a 105C oven for 48 hours. is about 10,000 bars in the oven.

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18Lab 9 - This week in labMoisture Characteristic CurvesPlot of tension (pressure) vs. water content (saturation)Measure water content at various tensionsSaturated, Field Capacity, Wilting Point, Air Dry, Oven DryDetermine plant available water contentDemonstrate some conceptsCapillary rise a function of particle and void sizeHow to measure field water tension and saturationTensiometer, Time Domain Reflectometry (TDR), Gypsum Blocks

Moisture Characteristic Curve A plot of water content, , vs soil tension, .

20Plant Available Water = AW = FC - WPThe water in the soil between field capacity, = 0.1 bar, and the wilting point, = 15 bars:The water bound less tightly than the field capacity is termed gravitational water because gravity easily drains this water before the plants can get it.Water bound beyond the wilting point is unavailable, because plant roots can not pull hard enough to overcome absorption of the water to the soil

21Soil Tensiometer

This part buried in soilThis dial gives soil tension (negative pressure)A small reservoir to hold water22Time Domain ReflectometerUsed to measure soil moistureAn electrical pulse is sent down the rodThe pulse bounces off the end and returns to the sourceThe wetter the soil, the longer the delay in returning

23Capillary Rise: The wicking effect caused by small poresRise is greater in finer materials:clays > silts > sands > gravels h = 0.15 / rh is the height of rise in tube, cmr is the radius of tube, cm

24Hysteresis: Caused by air blocking water in pores

Ink Bottle effect25Soil Water MovementHydraulic head: h = z + ph is total head, the driving forcez is elevation head, water moves from high to low elevationp is pressure head, water moves from high to low pressure is matric tension, which is a negative pressure (-p), so that water moves from low to high tensionHydraulic gradient: G = h / x h is the change in headx is the distanceThis is the slope, or gradient, of the head.The steeper the slope, the higher the gradientSoil water can move from dry to wet

27Darcys LawWater Flux: q = - K G = - K (h / x )K is the hydraulic conductivity, or how permeable the soil isA big K (sands, gravels) means that water flows fastA small K (clay, rock) means that water flows more slowlyq is the flux, or flow per unit areaThe negative shows that the flux is moving from high to lowIt has a negative slopeTotal Flow: Q = q AA is the cross-sectional area For example, the cross-sectional area of a pipe is r2The bigger the pipe, the more the flowQ is the total flow

Soil Water MovementTotal Head: h = z + p = z

Hydraulic Gradient, Gchange in energy w/ distanceG = h / x

Hydraulic conductivity, Ka function of pore size (d) and connectivityK = f(d2)29Typical ValuesGravel, lava, cavesK = 10 cm/sSandsK = 20 cm/hrSoilsK = 5 cm/dayClaysK = 0.9 cm/yr

Hydraulic Conductivity30Darcys Law, Q = A K GA = The area of flowThe greater the area, the greater the flow.K = The hydraulic conductivity or permeabilityThe higher the conductivity, the greater the flow.G = H / L = The magnitude of the driving force (L = x)The steeper the water slope, the greater the flow

31Unsaturated Hydraulic ConductivityLess than the Saturated Conductivity!!q = Ku G = K Kr GKu = K Kr = unsaturated hydraulic conductivityKr = relative hydraulic conductivity

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Plant Stomata Control Water LossPlant Water UptakeWater is pumped through plants partly by the pull or tension of the atmosphereThe tension in the atmosphere is generally hundreds of barsThe 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 leaf33Plant Water UptakePlant Available WaterSoil water that is potentially used by plantsLimited by soil tension, from wilting point to field capacityPAW = FC WP

35Chapter 9 Quiz Soil WaterAdhesion Cohesion (circle one) is the attraction of the water to the soil surface.What is the volumetric water content if the bulk density is 1.33 kg/L (same as g/cm3) and the gravimetric water content is 0.25?

What is the depth of water in a foot of soil if the volumetric water content is 0.10 (10%)?

Is available water primarily held in:___ Macropores (greater than 100 m)___ Mesopores (between 0.1 and 100 m)___ Micropores (smaller than 0.1 m)Which soil type has the greatest plant available water? Least?___Clay ___ Sand ___ Silt ___ Loam ___ Clay Loam