ssc 200 - the cycling of water through soil

distribution of water on earth

97.25 oceans, 2 ice, .7 groundwater

condensation

water vapor to liquid. rises by convection, temp drops as warm air rises and it changes into liquid or ice

precipitation

condensed water vapor becomes too heavy and falls

infiltration

portion of precipitation that seeps into the ground. varies with slope, vegetation, soil type, saturation level

runoff

precipitation that flows over the ground to watershed

evaporation

liquid to gas with heat

transpiration

plants take in water through roots and release through leaves

evapotranspiration

cumulative losses from evaporation, transpiration

N magnifying P problem

nitrogen lost by volatilization and denitrification, so the ratio of P to N in biosolids is way too high, leading to phosphorus buildup in the soil

important properties of soil water

soil water content, direction and rate of flow

importance of water to plants

transports minerals through soil, provides physical support in cells, transports sugars and minerals throughout plant

polarity of water molecules

h2o molecules have a slightly negative and a slightly positive end

cohesion

water molecules attracted to each other due to H bonds

adhesion

water molecules attracted to solid surfaces, adsorption to soil surface via H bonds

surface tension

air-water interface created due to force imbalance within water

water repellency

adhesive forces at water-solid interface are less than the cohesive force of water

capillarity

water moves up a hydrophobic wick

hydrophilic

adhesion and cohesion cause water to cling to surfaces

macropores

biggest size, water drains due to gravity

mesopores

medium sized, water is plant available

micropores

smallest size, water held too tightly due to capillary forces

capillary rise in macropores

gravitational pull stronger than adhesion, water flows freely

capillary rise in micropores

adhesion forces greater than gravity, water pulled into pore

positive soil water potential

energy is stored and can be used for work

negative soil water potential

energy must be expended to move water

soil water potential

shows how hard plants work to extract water

water flows from _____ to _____ soil water potential

high to low

matric force/potential

results from adsorption (cohesion and adhesion), negative potential, has the greatest effect on release of water from soil to plants

as soil dries, matric potential…

decreases from zero to a more negative number

osmotic potential

salts - measure of soil solute concentration including salts and organic compounds, greater solute concentration means more negative potential

water moves _______ the higher concentration of solutes to reach equilibrium concentration

toward

gravitational potential

positive potential, work can be done by water

soil water potential equation

matric + osmotic + gravitational = total potential

ideal soil composition for plant growth

50% solid - 45% mineral, 5% OM. 50% pore space - half air, half water.

saturation

macro, meso, and micropores are full of water, soil water potential = 0 kPa. soil becomes anaerobic

gravitational water

water held in macropores lost to gravitational drainage

field capacity

amount of water a soil can hold against gravity, where gravitational water stops draining. macropores have been drained, water held in meso and micropores. amount held 24 hrs after saturation.

wilting point

water held tight as a thin film around soil particles, adhesive forces much greater than gravity. soil water potential is so low plants can’t extract water, held in micropores only.

hydroscopic coefficient

soil water content of air-dried soil

plant-available water

between field capacity and wilting point, dominant potential is matric, water held in mesoporesg

gravitational water

between saturation and field capacity, water held in macropores and not available to plants

unavailable water

between wilting point and oven dry, dominant potential is matric, water held in micropores

energy status is ______ in wet soils, _______ in dry soils

higher, lower

electrical resistance

resistance of electrodes embedded in a porous block is proportional to its water content. wetter block, lower resistance. works better at lower water content

tensiometers

estimate soil moisture by measuring tension due to adhesive forces with soil solids

time domain reflectrometry (TDR)

estimate soil moisture by using radar to transmit signals into soil and analyzing reflected signals, presence of water slows down the electromagnetic pulse

soil water content

amount of water in soil by weight or volume

gravimetric measuring approach

compare initial weight to oven-dry weight. W = (wet weight - dry weight)/oven dry weight

saturated water content

(saturated - oven dry)/oven dry

field capacity water content

(field capacity - oven dry)/oven dry

wilting point water content

(wilting point - oven dry)/oven dry

hygroscopic water content

(air dry - oven dry)/oven dry

gravitational water content

(saturated - field capacity)/oven dry

plant available water content

(field capacity - wilting point)/oven dry

unavailable water content

(wilting point - oven dry)/oven dry

volumetric measuring approach

volume of water per volume of soil, water volume/soil volume = water mass/soil volume

depth of water

depth of soil * volumetric water content

saturated flow

all pores filled with water, free movement of gravitational water through macropores, downward only

unsaturated flow

capillary flow through micropores in any direction

hydraulic conductivity (K)

describes how fast water moves through soil, measure of soil’s ability to transmit water. sandy soils > clayey soils.

infiltration

water enters soil from the surface

measuring infiltration

double or single ring infiltrometer

factors that affect infiltration

texture, bulk density, slope, temp, weather, root channels and creatures create paths, tillage

leaf area index

average density of vegetation above the soil surface, affects evapotranspiration

irrigation

need related to rooting depth, minimize water contact with dry air

drainage

improve soil gas exchange with atmosphere, natural and constructed flows

channeled flow

micro topography results in runoff into channels, reduces infiltration surface area

sheet flow

micro topography is smoothed, water flows evenly at uniform depth

crop vigor

surface drains divert rainfall away from saturated soils, allowing internal drainage so crops don’t drown

spike aeration

push hole into soil with spike, reduces aeration by compressing soil

core aeration

pulls up plugs of soil, letting air into root zone