Soil moisture#Geophysical methods

File:ECMWF soil moisture forecast, East Asia, 2022-04-12.png

; Field capacity

: A flooded field will drain the gravitational water under the influence of gravity until water's adhesive and cohesive forces resist further drainage at which point it is said to have reached field capacity.{{sfn|Wadleigh|1957|p=48}} At that point, plants must apply suction to draw water from a soil. By convention it is defined at 0.33 bar suction.{{sfn|Wadleigh|1957|p=48}}{{sfn|Richards|Richards|1957|p=50}}

; Available water and unavailable water

: The water that plants may draw from the soil is called the available water.{{sfn|Wadleigh|1957|p=48}}{{sfn|Richards|Richards|1957|p=56}} Once the available water is used up the remaining moisture is called unavailable water as the plant cannot produce sufficient suction to draw that water in.

; Wilting point

: The wilting point is the minimum amount of water plants need to not wilt and approximates the boundary between available and unavailable water. By convention it is defined as 15 bar suction. At this point, seeds will not germinate,{{sfn|Wadleigh|1957|p=39}}{{sfn|Wadleigh|1957|p=48}}{{sfn|Richards|Richards|1957|p=52}} plants begin to wilt and then die unless they are able to recover after water replenishment thanks to species-specific adaptations.{{cite journal |last1=Snyman |first1=Henny A. |last2=Venter |first2=W.D. |last3=Van Rensburg |first3=W.L.J. |last4=Opperman |first4=D.P.J. |journal=Journal of the Grassland Society of Southern Africa |volume=4 |issue=2 |title=Ranking of grass species according to visible wilting order and rate of recovery in the Central Orange Free State |url=https://www.researchgate.net/publication/232842548 |year=1987 |pages=78–81 |doi=10.1080/02566702.1987.9648075 |access-date=14 August 2022 }}

{{Further|Soil water (retention)|Water retention curve}}

Water is retained in a soil when the adhesive force of attraction that water's hydrogen atoms have for the oxygen of soil particles is stronger than the cohesive forces that water's hydrogen feels for water oxygen atoms.{{sfn|Donahue|Miller|Shickluna|1977|pp=72–74}} When a field is flooded, the soil pore space is completely filled by water. The field will drain under the force of gravity until it reaches what is called field capacity, at which point the smallest pores are filled with water and the largest with water and gases.{{cite web |url=https://www.fao.org/3/r4082e/r4082e03.htm |title=Soil and water |publisher=Food and Agriculture Organization of the United Nations |access-date=21 August 2022 }} The total amount of water held when field capacity is reached is a function of the specific surface area of the soil particles.{{cite journal |last1=Petersen |first1=Lis Wollesen |last2=Møldrup |first2=Per |last3=Jacobsen |first3=Ole H. |last4=Rolston |first4=Dennis E. |journal=Soil Science |volume=161 |issue=1 |title=Relations between specific surface area and soil physical and chemical properties |url=https://www.researchgate.net/publication/232162864 |year=1996 |pages=9–21 |doi=10.1097/00010694-199601000-00003 |access-date=21 August 2022 |bibcode=1996SoilS.161....9P }} As a result, high clay and high organic soils have higher field capacities.{{cite journal |last1=Gupta |first1=Satish C. |last2=Larson |first2=William E. |journal=Water Resources Research |volume=15 |issue=6 |title=Estimating soil water retention characteristics from particle size distribution, organic matter percent, and bulk density |year=1979 |pages=1633–35 |doi=10.1029/WR015i006p01633 |bibcode=1979WRR....15.1633G |citeseerx=10.1.1.475.497 |url=https://www.researchgate.net/publication/252286022 |access-date=21 August 2022 }} The potential energy of water per unit volume relative to pure water in reference conditions is called water potential. Total water potential is a sum of matric potential which results from capillary action, osmotic potential for saline soil, and gravitational potential when dealing with downward water movement. Water potential in soil usually has negative values, and therefore it is also expressed in suction, which is defined as the minus of water potential. Suction has a positive value and can be regarded as the total force required to pull or push water out of soil. Water potential or suction is expressed in units of kPa (10³ pascal), bar (100 kPa), or cm H₂O (approximately 0.098 kPa). Common logarithm of suction in cm H₂O is called pF.{{Cite web |url=https://agriinfo.in/?page=topic&superid=4&topicid=277 |title=Soil water potential |publisher=AgriInfo.in |access-date=15 March 2019 |archive-url=https://web.archive.org/web/20170817091337/http://www.agriinfo.in/?page=topic&superid=4&topicid=277 |archive-date=17 August 2017 |url-status=dead }} Therefore, pF 3 = 1000 cm = 98 kPa = 0.98 bar.

The forces with which water is held in soils determine its availability to plants. Forces of adhesion hold water strongly to mineral and humus surfaces and less strongly to itself by cohesive forces. A plant's root may penetrate a very small volume of water that is adhering to soil and be initially able to draw in water that is only lightly held by the cohesive forces. But as the droplet is drawn down, the forces of adhesion of the water for the soil particles produce increasingly higher suction, finally up to 1500 kPa (pF = 4.2).{{cite journal |last1=Savage |first1=Michael J. |last2=Ritchie |first2=Joe T. |last3=Bland |first3=William L. |last4=Dugas |first4=William A. |journal=Agronomy Journal |volume=88 |issue=4 |title=Lower limit of soil water availability |url=https://www.researchgate.net/publication/309079210 |year=1996 |pages=644–51 |doi=10.2134/agronj1996.00021962008800040024x |bibcode=1996AgrJ...88..644S |access-date=21 August 2022 }} At 1500 kPa suction, the soil water amount is called wilting point. At that suction the plant cannot sustain its water needs as water is still being lost from the plant by transpiration, the plant's turgidity is lost, and it wilts, although stomatal closure may decrease transpiration and thus may retard wilting below the wilting point, in particular under adaptation or acclimatization to drought.{{cite journal |last1=Al-Ani |first1=Tariq |last2=Bierhuizen |first2=Johan Frederik |journal=Acta Botanica Neerlandica |volume=20 |issue=3 |title=Stomatal resistance, transpiration, and relative water content as influenced by soil moisture stress |url=https://natuurtijdschriften.nl/pub/539770/ABN1971020003008.pdf |year=1971 |pages=318–26 |doi=10.1111/j.1438-8677.1971.tb00715.x |access-date=21 August 2022 }} The next level, called air-dry, occurs at 100,000 kPa suction (pF = 6). Finally the oven dry condition is reached at 1,000,000 kPa suction (pF = 7). All water below wilting point is called unavailable water.{{sfn|Donahue|Miller|Shickluna|1977|pp=75–76}}

When the soil moisture content is optimal for plant growth, the water in the large and intermediate size pores can move about in the soil and be easily used by plants. The amount of water remaining in a soil drained to field capacity and the amount that is available are functions of the soil type. Sandy soil will retain very little water, while clay will hold the maximum amount. The available water for the silt loam might be 20% whereas for the sand it might be only 6% by volume, as shown in this table.

The above are average values for the soil textures.

Water moves through soil due to the force of gravity, osmosis and capillarity. At 0 to 33 kPa suction (field capacity), water is pushed through soil from the point of its application under the force of gravity and the pressure gradient created by differences in the pressure of water; this is called saturated flow. At higher suction, water movement is pulled by capillarity from wetter toward drier soil. This is caused by water's adhesion to soil solids, and is called unsaturated flow.{{sfn|Donahue|Miller|Shickluna|1977|p=85}}{{cite web |url=http://eagri.org/eagri50/AGRO103/lec03.pdf |title=Soil water movement: saturated and unsaturated flow and vapour movement, soil moisture constants and their importance in irrigation |website=Tamil Nadu Agricultural University |access-date=28 August 2022 }}

Water infiltration and movement in soil are controlled by six factors:

1. Soil texture
2. Soil structure. Fine-textured soils with granular structure are most favourable to infiltration of water.
3. The amount of organic matter. Coarse matter is best and if on the surface helps prevent the destruction of soil structure and the creation of soil crusts.
4. Depth of soil to impervious layers such as hardpans or bedrock
5. The amount of water already in the soil
6. Soil temperature. Warm soils take in water faster while frozen soils such as permafrost may not be able to absorb depending on the type of freezing.{{sfn|Donahue|Miller|Shickluna|1977|p=86}}

Water infiltration rates range from 0.25 cm per hour for high clay soils to 2.5 cm per hour for sand and well stabilized and aggregated soil structures.{{sfn|Donahue|Miller|Shickluna|1977|p=88}} Water flows through the ground unevenly, in the form of so-called gravity fingers, because of the surface tension between water particles.{{cite journal |last1=Cueto-Felgueroso |first1=Luis |last2=Juanes |first2=Ruben |journal=Physical Review Letters |volume=101 |issue=24 |title=Nonlocal interface dynamics and pattern formation in gravity-driven unsaturated flow through porous media |year=2008 |pages=244504 |doi=10.1103/PhysRevLett.101.244504 |pmid=19113626 |bibcode=2008PhRvL.101x4504C |s2cid=21874968 |url=https://art1lib.com/book/33134808/c9c15f |access-date=28 August 2022 |archive-date=28 August 2022 |archive-url=https://web.archive.org/web/20220828075737/https://art1lib.com/book/33134808/c9c15f |url-status=dead }}{{cite web |url=http://soilandwater.bee.cornell.edu/research/pfweb/educators/intro/fingerflow.htm |title=Finger flow in coarse soils |website=Cornell University |access-date=28 August 2022 }}

Tree roots, whether living or dead, create preferential channels for rainwater flow through soil,{{cite journal |last1=Ghestem |first1=Murielle |last2=Sidle |first2=Roy C. |last3=Stokes |first3=Alexia |journal=BioScience |volume=61 |issue=11 |title=The influence of plant root systems on subsurface flow: implications for slope stability |year=2011 |pages=869–79 |doi=10.1525/bio.2011.61.11.6 |doi-access= }} magnifying infiltration rates of water up to 27 times.{{cite journal |last1=Bartens |first1=Julia |last2=Day |first2=Susan D. |last3=Harris |first3=J. Roger |last4=Dove |first4=Joseph E. |last5=Wynn |first5= Theresa M. |journal=Journal of Environmental Quality |volume=37 |issue=6 |title=Can urban tree roots improve infiltration through compacted subsoils for stormwater management? |url=https://www.researchgate.net/publication/23411104 |year=2008 |pages=2048–57 |doi=10.2134/jeq2008.0117 |pmid=18948457 |bibcode=2008JEnvQ..37.2048B |access-date=28 August 2022 }}

Flooding temporarily increases soil permeability in river beds, helping to recharge aquifers.{{cite journal |last1=Zhang |first1=Guohua |last2=Feng |first2=Gary |last3=Li |first3=Xinhu |last4=Xie |first4=Congbao |last5=P |first5=Xiaoyu |journal=Water |volume=9 |issue=7 |title=Flood effect on groundwater recharge on a typical silt loam soil |year=2017 |pages=523 |doi=10.3390/w9070523 |doi-access=free |bibcode=2017Water...9..523Z }}

Water applied to a soil is pushed by pressure gradients from the point of its application where it is saturated locally, to less saturated areas, such as the vadose zone.{{cite journal |last1=Nielsen |first1=Donald R. |last2=Biggar |first2=James W. |last3=Erh |first3=Koon T. |journal=Hilgardia |volume=42 |issue=7 |title=Spatial variability of field-measured soil-water properties |year=1973 |pages=215–59 |doi=10.3733/hilg.v42n07p215 |doi-access=free }}{{cite journal |last1=Rimon |first1=Yaara |last2=Dahan |first2=Ofer |last3=Nativ |first3=Ronit |last4=Geyer |first4=Stefan |journal=Water Resources Research |volume=43 |issue=5 |title=Water percolation through the deep vadose zone and groundwater recharge: preliminary results based on a new vadose zone monitoring system |year=2007 |pages=W05402 |doi=10.1029/2006WR004855 |bibcode=2007WRR....43.5402R |doi-access=free }} Once soil is completely wetted, any more water will move downward, or percolate out of the range of plant roots, carrying with it clay, humus, nutrients, primarily cations, and various contaminants, including pesticides, pollutants, viruses and bacteria, potentially causing groundwater contamination.{{cite web |last1=Weiss |first1=Peter T. |last2=LeFevre |first2=Greg |last3=Gulliver |first3=John S. |title=Contamination of soil and groundwater due to stormwater infiltration practices: a literature review |year=2008 |citeseerx=10.1.1.410.5113 |url=https://conservancy.umn.edu/bitstream/handle/11299/115341/pr515.pdf |website=Saint Anthony Falls Laboratory |access-date=28 August 2022 }}{{cite journal |last1=Hagedorn |first1=Charles |last2=Hansen |first2=Debra T. |last3=Simonson |first3=Gerald H. |journal=Journal of Environmental Quality |volume=7 |issue=1 |title=Survival and movement of fecal indicator bacteria in soil under conditions of saturated flow |year=1978 |pages=55–59 |doi=10.2134/jeq1978.00472425000700010011x |bibcode=1978JEnvQ...7...55H |s2cid=774611 |url=https://art1lib.com/book/66877050/85c295 |access-date=28 August 2022 |archive-date=28 August 2022 |archive-url=https://web.archive.org/web/20220828075738/https://art1lib.com/book/66877050/85c295 |url-status=dead }} In order of decreasing solubility, the leached nutrients are:

• Calcium
• Magnesium, Sulfur, Potassium; depending upon soil composition
• Nitrogen; usually little, unless nitrate fertiliser was applied recently
• Phosphorus; very little as its forms in soil are of low solubility.{{sfn|Donahue|Miller|Shickluna|1977|p=90}}

In the United States percolation water due to rainfall ranges from almost zero centimeters just east of the Rocky Mountains to fifty or more centimeters per day in the Appalachian Mountains and the north coast of the Gulf of Mexico.{{sfn|Donahue|Miller|Shickluna|1977|p=80}}

Water is pulled by capillary action due to the adhesion force of water to the soil solids, producing a suction gradient from wet towards drier soil{{cite journal |last1=Ng |first1=Charles W.W. |last2=Pang |first2=Wenyan |journal=Journal of Geotechnical & Geoenvironmental Engineering |volume=126 |issue=2 |title=Influence of stress state on soil-water characteristics and slope stability |url=https://www.researchgate.net/publication/245293642 |year=2000 |pages=157–66 |doi=10.1061/(ASCE)1090-0241(2000)126:2(157) |bibcode=2000JGGE..126..157N |access-date=4 September 2022 }} and from macropores to micropores.{{cite journal |last1=Germann |first1=Peter Fritz |last2=Beven |first2=Keith |journal=European Journal of Soil Science |volume=32 |issue=1 |title=Water flow in soil macropores. I. An experimental approach |url=https://art1lib.com/book/9196284/294c32 |year=2006 |pages=1–13 |doi=10.1111/j.1365-2389.1981.tb01681.x |access-date=4 September 2022 |archive-date=4 September 2022 |archive-url=https://web.archive.org/web/20220904080543/https://art1lib.com/book/9196284/294c32 |url-status=dead }} The so-called Richards equation allows calculation of the time rate of change of moisture content in soils due to the movement of water in unsaturated soils.{{cite journal |last=Richards |first=Lorenzo A. |year=1931 |title=Capillary conduction of liquids through porous mediums |journal=Physics |volume=1 |issue=5 |pages=318–33 |doi=10.1063/1.1745010 |bibcode=1931Physi...1..318R |url=https://art1lib.com/book/15496078/0feb63 |access-date=4 September 2022 |archive-date=4 September 2022 |archive-url=https://web.archive.org/web/20220904080536/https://art1lib.com/book/15496078/0feb63 |url-status=dead }} Interestingly, this equation attributed to Richards was originally published by Richardson in 1922.{{cite book |last1=Richardson |first1=Lewis Fry |title=Weather prediction by numerical process |date=1922 |publisher=Cambridge University Press |location=Cambridge, United Kingdom |pages=262 |url=https://archive.org/details/weatherpredictio00richrich |access-date=4 September 2022 }} The soil moisture velocity equation,{{cite journal |last1=Ogden |first1=Fred L. |last2=Allen |first2=Myron B. |last3=Lai |first3=Wencong |last4=Zhu |first4=Julian |last5=Douglas |first5=Craig C. |last6=Seo |first6=Mookwon |last7=Talbot |first7=Cary A. |year=2017 |title=The soil moisture velocity equation |journal=Journal of Advances in Modeling Earth Systems |volume=9 |issue=2 |pages=1473–87 |doi=10.1002/2017MS000931 |bibcode=2017JAMES...9.1473O |doi-access=free }} which can be solved using the finite water-content vadose zone flow method,{{cite journal |last1=Talbot |first1=Cary A. |last2=Ogden |first2=Fred L. |year=2008 |title=A method for computing infiltration and redistribution in a discretized moisture content domain |journal=Water Resources Research |volume=44 |issue=8 |pages=W08453 |doi=10.1029/2008WR006815 |bibcode=2008WRR....44.8453T |doi-access=free }}{{cite journal |last1=Ogden |first1=Fred L. |last2=Lai |first2=Wencong |last3=Steinke |first3=Robert C. |last4=Zhu |first4=Julian |last5=Talbot |first5=Cary A. |last6=Wilson |first6=John L. |year=2015 |title=A new general 1-D vadose zone solution method |journal=Water Resources Research |volume=51|issue=6 |pages=4282–4300 |doi=10.1002/2015WR017126 |bibcode=2015WRR....51.4282O |s2cid=119834716 |doi-access= }} describes the velocity of flowing water through an unsaturated soil in the vertical direction. The numerical solution of the Richardson/Richards equation allows calculation of unsaturated water flow and solute transport using software such as Hydrus,{{cite web |last1=Šimůnek |first1=Jiri |last2=Saito |first2=Hirotaka |last3=Sakai |first3=Masaru |last4=Van Genuchten |first4=Martinus Th. |year=2013 |url=https://www.researchgate.net/publication/271515313 | title=The HYDRUS-1D software package for simulating the one-dimensional movement of water, heat, and multiple solutes in variably-saturated media |access-date=11 September 2022 }} by giving soil hydraulic parameters of hydraulic functions (water retention function and unsaturated hydraulic conductivity function) and initial and boundary conditions. Preferential flow occurs along interconnected macropores, crevices, root and worm channels, which drain water under gravity.{{cite journal |last=Bouma |first=Johan |journal=Geoderma |volume=3 |issue=4 |title=Soil morphology and preferential flow along macropores |url=https://www.researchgate.net/publication/223095848 |year=1981 |pages=235–50 |doi=10.1016/0378-3774(81)90009-3 |bibcode=1981AgWM....3..235B |access-date=11 September 2022 }}{{cite journal |last1=Luo |first1=Lifang |last2=Lin |first2=Henry |last3=Halleck |first3=Phil |journal=Soil Science Society of America Journal |volume=72 |issue=4 |title=Quantifying soil structure and preferential flow in intact soil Using X-ray computed tomography |year=2008 |pages=1058–69 |doi=10.2136/sssaj2007.0179 |bibcode=2008SSASJ..72.1058L |citeseerx=10.1.1.455.2567 |url=https://art1lib.com/book/64277435/dc8b4b |access-date=11 September 2022 |archive-date=11 September 2022 |archive-url=https://web.archive.org/web/20220911075914/https://art1lib.com/book/64277435/dc8b4b |url-status=dead }}

Many models based on soil physics now allow for some representation of preferential flow as a dual continuum, dual porosity or dual permeability options, but these have generally been "bolted on" to the Richards solution without any rigorous physical underpinning.{{cite journal |last1=Beven |first1=Keith |last2=Germann |first2=Peter |journal=Water Resources Research |volume=49 |issue=6 |title=Macropores and water flow in soils revisited |year=2013 |pages=3071–92 |doi=10.1002/wrcr.20156 |bibcode=2013WRR....49.3071B |s2cid=53132908 |url=https://boris.unibe.ch/117045/7/Beven_et_al-2013-Water_Resources_Research.pdf |access-date=11 September 2022 }}

Of equal importance to the storage and movement of water in soil is the means by which plants acquire it and their nutrients. Most soil water is taken up by plants as passive absorption caused by the pulling force of water evaporating (transpiring) from the long column of water (xylem sap flow) that leads from the plant's roots to its leaves, according to the cohesion-tension theory.{{cite journal |last1=Aston |first1=Mervyn J. |last2=Lawlor |first2=David W. |journal=Journal of Experimental Botany |volume=30 |issue=1 |title=The relationship between transpiration, root water uptake, and leaf water potential |url=https://www.researchgate.net/publication/269624495 |year=1979 |pages=169–81 |doi=10.1093/jxb/30.1.169 |access-date=18 September 2022 }} The upward movement of water and solutes (hydraulic lift) is regulated in the roots by the endodermis{{cite journal |last=Powell |first=D.B.B. |journal=Plant, Cell and Environment |volume=1 |issue=1 |title=Regulation of plant water potential by membranes of the endodermis in young roots |url=https://art1lib.com/book/9318022/34afe2 |year=1978 |pages=69–76 |doi=10.1111/j.1365-3040.1978.tb00749.x |bibcode=1978PCEnv...1...69P |access-date=18 September 2022 |archive-date=20 September 2022 |archive-url=https://web.archive.org/web/20220920171859/https://art1lib.com/book/9318022/34afe2 |url-status=dead }} and in the plant foliage by stomatal conductance,{{cite journal |last1=Irvine |first1=James |last2=Perks |first2=Michael P. |last3=Magnani |first3=Federico |last4=Grace |first4=John |journal=Tree Physiology |volume=18 |issue=6 |title=The response of Pinus sylvestris to drought: stomatal control of transpiration and hydraulic conductance |year=1998 |pages=393–402 |doi=10.1093/treephys/18.6.393 |pmid=12651364 |doi-access=free }} and can be interrupted in root and shoot xylem vessels by cavitation, also called xylem embolism.{{cite journal |last1=Jackson |first1=Robert B. |last2=Sperry |first2=John S. |last3=Dawson |first3=Todd E. |journal=Trends in Plant Science |volume=5 |issue=11 |title=Root water uptake and transport: using physiological processes in global predictions |year=2000 |pages=482–88 |doi=10.1016/S1360-1385(00)01766-0 |pmid=11077257 |bibcode=2000TPS.....5..482J |s2cid=8311441 |url=https://sperry.biology.utah.edu/publications/Jackson_2000_Trends-in-Plant-Science.pdf |access-date=11 November 2022 }} In addition, the high concentration of salts within plant roots creates an osmotic pressure gradient that pushes soil water into the roots.{{cite journal |last=Steudle |first=Ernst |journal=Plant and Soil |volume=226 |issue=1 |title=Water uptake by plant roots: an integration of views |url=https://art1lib.com/book/11264229/4f53a7 |year=2000 |pages=45–56 |doi=10.1023/A:1026439226716 |bibcode=2000PlSoi.226...45S |s2cid=3338727 |access-date=18 September 2022 |archive-date=20 September 2022 |archive-url=https://web.archive.org/web/20220920170553/https://art1lib.com/book/11264229/4f53a7 |url-status=dead }} Osmotic absorption becomes more important during times of low water transpiration caused by lower temperatures (for example at night) or high humidity, and the reverse occurs under high temperature or low humidity. It is these processes that cause guttation and wilting, respectively.{{sfn|Donahue|Miller|Shickluna|1977|p=92}}{{cite journal |last1=Kaufmann |first1=Merrill R. |last2=Eckard |first2=Alan N. |journal=Plant Physiology |volume=47 |issue=4 |title=Evaluation of water stress control with polyethylene glycols by analysis of guttation |year=1971 |pages=453–56 |doi=10.1104/pp.47.4.453 |pmid=16657642 |pmc=396708 |url=https://art1lib.com/book/49458316/cb3e5a |access-date=18 September 2022 |archive-date=20 September 2022 |archive-url=https://web.archive.org/web/20220920171213/https://art1lib.com/book/49458316/cb3e5a |url-status=dead }}

Root extension is vital for plant survival. A study of a single winter rye plant grown for four months in one cubic foot (0.0283 cubic meters) of loam soil showed that the plant developed 13,800,000 roots, a total of 620 km in length with 237 square meters in surface area; and 14 billion root hairs of 10,620 km total length and 400 square meters total area; for a total surface area of 638 square meters. The total surface area of the loam soil was estimated to be 52,000 square meters.{{sfn|Wadleigh|1957|p=46}} In other words, the roots were in contact with only 1.2% of the soil volume. However, root extension should be viewed as a dynamic process, allowing new roots to explore a new volume of soil each day, increasing dramatically the total volume of soil explored over a given growth period, and thus the volume of water taken up by the root system over this period.{{cite journal |last1=Kramer |first1=Paul J. |last2=Coile |first2=Theodore S. |journal=Plant Physiology |volume=15 |issue=4 |title=An estimation of the volume of water made available by root extension |year=1940 |pages=743–47 |doi=10.1104/pp.15.4.743 |pmid=16653671 |pmc=437871 |url=https://art1lib.com/book/46874586/93398e |access-date=18 September 2022 |archive-date=20 September 2022 |archive-url=https://web.archive.org/web/20220920170507/https://art1lib.com/book/46874586/93398e |url-status=dead }} Root architecture, i.e. the spatial configuration of the root system, plays a prominent role in the adaptation of plants to soil water and nutrient availability, and thus in plant productivity.{{cite journal |last=Lynch |first=Jonathan |journal=Plant Physiology |volume=109 |issue=1 |title=Root architecture and plant productivity |year=1995 |pages=7–13 |doi=10.1104/pp.109.1.7 |pmid=12228579 |pmc=157559 |url=https://art1lib.com/book/64045845/0c6b21 |access-date=18 September 2022 |archive-date=20 September 2022 |archive-url=https://web.archive.org/web/20220920171647/https://art1lib.com/book/64045845/0c6b21 |url-status=dead }}

Roots must seek out water as the unsaturated flow of water in soil can move only at a rate of up to 2.5 cm per day; as a result they are constantly dying and growing as they seek out high concentrations of soil moisture.{{cite journal |last1=Comas |first1=Louise H. |last2=Eissenstat |first2=David M. |last3=Lakso |first3=Alan N. |journal=New Phytologist |volume=147 |issue=1 |title=Assessing root death and root system dynamics in a study of grape canopy pruning |year=2000 |pages=171–78 |doi=10.1046/j.1469-8137.2000.00679.x |doi-access=free |bibcode=2000NewPh.147..171C }} Insufficient soil moisture, to the point of causing wilting, will cause permanent damage and crop yields will suffer. When grain sorghum was exposed to soil suction as low as 1300 kPa during the seed head emergence through bloom and seed set stages of growth, its production was reduced by 34%.{{sfn|Donahue|Miller|Shickluna|1977|p=94}}

Only a small fraction (0.1% to 1%) of the water used by a plant is held within the plant. The majority is ultimately lost via transpiration. At the same time evaporation from the soil surface is also substantial, the transpiration:evaporation ratio (T/ET) varying according to vegetation type and climate, peaking in tropical rainforests and dipping in steppes and deserts.{{cite journal |last1=Schlesinger |first1=William H. |last2=Jasechko |first2=Scott |journal=Agricultural and Forest Meteorology |volume=189/190 |title=Transpiration in the global water cycle |url=https://art1lib.com/book/23350490/f2e7f4 |year=2014 |pages=115–17 |doi=10.1016/j.agrformet.2014.01.011 |access-date=25 September 2022 |bibcode=2014AgFM..189..115S |archive-date=25 September 2022 |archive-url=https://web.archive.org/web/20220925093313/https://art1lib.com/book/23350490/f2e7f4 |url-status=dead }} Transpiration plus evaporative soil moisture loss is called evapotranspiration. Evapotranspiration plus water held in the plant totals to consumptive use, which is nearly identical to evapotranspiration.{{sfn|Donahue|Miller|Shickluna|1977|p=94}}{{cite book |last1=Erie |first1=Leonard J. |last2=French |first2=Orrin F. |last3=Harris |first3=Karl |date=1968 |title=Consumptive use of water by crops in Arizona |location=Tucson, Arizona |publisher=The University of Arizona |url=https://repository.arizona.edu/bitstream/handle/10150/607084/TB169.pdf |access-date=25 September 2022 }}

The total water used in an agricultural field includes surface runoff, drainage and consumptive use. The use of loose mulches will reduce evaporative losses for a period after a field is irrigated, but in the end, the total evaporative loss (plant plus soil) will approach that of uncovered soil, while more water is immediately available for plant growth.{{cite journal |last1=Tolk |first1=Judy A. |last2=Howell |first2=Terry A. |last3=Evett |first3=Steve R. |journal=Soil and Tillage Research |volume=50 |issue=2 |title=Effect of mulch, irrigation, and soil type on water use and yield of maize |url=https://art1lib.com/book/8870946/822a70 |year=1999 |pages=137–47 |doi=10.1016/S0167-1987(99)00011-2 |bibcode=1999STilR..50..137T |access-date=25 September 2022 |archive-date=25 September 2022 |archive-url=https://web.archive.org/web/20220925093343/https://art1lib.com/book/8870946/822a70 |url-status=dead }} Water use efficiency is measured by the transpiration ratio, which is the ratio of the total water transpired by a plant to the dry weight of the harvested plant. Transpiration ratios for crops range from 300 to 700. For example, alfalfa may have a transpiration ratio of 500; as a result, 500 kilograms of water will produce one kilogram of dry alfalfa.{{sfn|Donahue|Miller|Shickluna|1977|pp=97–99}}

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• {{cite book |title=Soils: An Introduction to Soils and Plant Growth |last1=Donahue |first1=Roy Luther |last2=Miller |first2=Raymond W. |last3=Shickluna |first3=John C. |year=1977 |publisher=Prentice-Hall |isbn=978-0-13-821918-5 |url=https://archive.org/details/soilsintroductio00dona }}
• {{cite book |title=Arizona Master Gardener |url=https://cc1lib.vip/book/21179861/878c50 |publisher=Cooperative Extension, College of Agriculture, University of Arizona |access-date=25 September 2022 }}{{Dead link|date=October 2024 |bot=InternetArchiveBot |fix-attempted=yes }}
• {{cite book |title=Soil: The Yearbook of Agriculture 1957 |editor-last=Stefferud |editor-first=Alfred |year=1957 |publisher=United States Department of Agriculture |url=http://archive.org/stream/yoa1957#page/n18/mode/1up |oclc=704186906 }}
• {{harvc |name-list-style=harv |last=Richards |first=L.A. |last2=Richards |chapter=Soil Moisture |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n68/mode/1up }}
• {{harvc |name-list-style=harv |last=Wadleigh |first=C.H. |chapter=Growth of Plants |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n57/mode/1up }}

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{{Soil science topics}}

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Category:Soil

Category:Water