Field capacity

{{short description|Concept in hydrology}}

Field capacity is the amount of soil moisture or water content held in the soil after excess water has drained away and the rate of downward movement has decreased. This usually occurs two to three days after rain or irrigation in pervious soils of uniform structure and texture. The nominal definition of field capacity (expressed symbolically as θ_fc) is the bulk water content retained in soil at −33 kPa (or −0.33 bar) of hydraulic head or suction pressure. The term originated from Israelsen and West{{cite journal |author1=Israelsen, O.W. |author2=West, F.L. |year=1922 |title=Water holding capacity of irrigated soils |journal=Utah State Agricultural Experiment Station Bull |volume=183 |pages=1–24 |url=http://digitalcommons.usu.edu/uaes_bulletins/149/}} and Frank Veihmeyer and Arthur Hendrickson.{{cite journal |author1=Veihmeyer, F.J. |author2=Hendrickson, A.H. |year=1931 |title=The moisture equivalent as a measure of the field capacity of soils |journal=Soil Science |volume=32 |issue=3 |pages=181–193 |doi=10.1097/00010694-193109000-00003 |bibcode=1931SoilS..32..181V |s2cid=95457706 }}

Veihmeyer and Hendrickson{{cite journal |author1=Veihmeyer, F.J. |author2=Hendrickson, A.H. |year=1949 |title=The application of some basic concepts of soil moisture to orchard irrigation |journal=Proc. Wash. State Hort. Assoc. |volume=45 |pages=25–41 }} realized the limitation in this measurement and commented that it is affected by so many factors that, precisely, it is not a constant (for a particular soil), yet it does serve as a practical measure of soil water-holding capacity. Field capacity improves on the concept of moisture equivalent by Lyman Briggs. Veihmeyer & Hendrickson proposed this concept as an attempt to improve water-use efficiency for farmers in California in 1949.{{cite journal |last=Novak |first=Viliam |author2=Jan Havrila |title=Method to estimate the critical soil water content of limited availability for plants |journal=Biologia, Bratislava |date=2006 |volume=61/Suppl. 19: S289—S293, 2006 |issue=S19 |pages=6 |doi=10.2478/s11756-006-0175-9 |bibcode=2006Biolg..61S.289N |url=http://147.213.145.2/biohydrology/Biologia2006Papers/Novak.pdf |accessdate=8 October 2013 |url-status=dead |archiveurl=https://web.archive.org/web/20130718000216/http://147.213.145.2/biohydrology/Biologia2006Papers/Novak.pdf |archivedate=18 July 2013 }}

Field capacity is characterized by measuring water content after wetting a soil profile, covering it (to prevent evaporation), and monitoring the change soil moisture in the profile. A relatively low rate of change indicates when macropore drainage ceases, which is called Field Capacity; it is also termed drained upper limit (DUL).

Lorenzo A. Richards and Weaver{{cite journal |author1=Richards, L.A. |author2=Weaver, L.R. |year=1944 |title=Moisture retention by some irrigated soils as related to soil moisture tension |journal=Journal of Agricultural Research |volume=69 |pages=215–235 |url=http://afrsweb.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P0029.pdf |access-date=2013-08-16 |archive-url=https://web.archive.org/web/20131216073434/http://afrsweb.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P0029.pdf |archive-date=2013-12-16 |url-status=dead }} found that water content held by soil at a potential of −33 kPa (or −0.33 bar) correlate closely with field capacity (−10 kPa for sandy soils).