Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Water infiltration, retention and movement / Capillary rise from shallow groundwater and salt accumulation
Capillary rise from shallow groundwater and salt accumulation
Water tables within a metre or two of the surface can resupply the root zone by capillary rise, but evaporation concentrates dissolved salts near the surface unless leaching removes them.
Boundary: where this process starts
Starts with: Capillary fringe above the water table: rise follows Jurin h = 0.15/r cm, about 0.3-0.5 m in fine sand and 1-2 m in silt
Boundary: where it ends
Ends after: Leaching requirement LR = ECiw / (5 ECe - ECiw): irrigating with 1 dS/m water on a crop tolerating ECe 4 dS/m needs about 5% extra water; Lower transpiration and a surface salt crust cut upward flux, partly self-limiting the salinisation
Line thickness = size of impact (1 to 5)% on a line = chance a fork takes that branch📄 = pieces of evidence on a stepNegative effectLoops backHypothesisResource used up (arrow into the step)Resource used, not used upResource made or released (arrow out of the step)
What am I looking at? The concepts on every map
- Participant
- Someone or something that takes part in a process or influences it: a plant, a microbe, a river, a machine, a force. Participants are the rows of a map.
- Resource
- Something a step uses or uses up: water, oxygen, sunlight, a tool. Each step has an arrow down to the resources it uses.
- Step
- One thing that happens in a process. Every step has at least one participant and carries evidence.
- Decision fork
- A step where the process can go several ways. Every outcome is labeled with a percentage, and the percentages add up to 100%. It is not limited to yes or no.
- Loop
- A line that carries the process back to an earlier step. We record whether it reinforces or balances, how strong it is (weak, moderate, strong), and its gain (0 to 1): the share of the effect that returns on each pass.
- Size of impact
- Every line says how much the earlier step affects the next one (1 to 5), in which direction (+, -, 0), and why. Thicker lines mean bigger impact.
- Evidence
- The support for a claim: a source and what it shows. Every step, line, change and vote needs it. Click any step or line to read its evidence.
- Boundary
- Where a process starts and where it ends. Every process, and every subprocess, has a start and an end.
- Subprocess
- A step that is itself a whole process, with its own boundary, steps and lines. Click a step marked ↘ to open it. Subprocesses nest as deep as the evidence goes.
- Description
- A plain-language summary of what a process is and what it covers.
- Hypothesis
- A proposed new connection between two parts of the map that nobody has linked. It carries a prediction, a way to test it, and a count of papers on each topic and on both. It is an unverified idea until someone tests it.
Every step, with its evidence
The same information as the map, as a list you can search and share. Click a participant or resource to see everywhere it appears.
Capillary fringe above the water table: rise follows Jurin h = 0.15/r cm, about 0.3-0.5 m in fine sand and 1-2 m in silt
Evidence (1) · added by Fern-C
- Lohman 1972, USGS Professional Paper 708, Ground-water hydraulics: Table of capillary rise by grain size: tens of centimetres in sand to metres in silt. (confidence 0.55)
Evaporation and root uptake pull water up when K at the table depth permits: the maximum steady upward flux falls roughly as depth to the table raised to a power of 2-4
Evidence (1) · added by Fern-C
- Gardner 1958, Soil Science 85:228-232: Steady-state solutions for evaporation from a water table give a maximum flux decreasing as a power of the table depth set by the K(h) exponent. (confidence 0.55)
Shallow groundwater can supply tens of percent of crop water use where the table is within about 1-2 m
Evidence (1) · added by Fern-C
- Ayars, Christen, Soppe & Meyer 2006, Irrigation Science 24:147-160, The resource potential of in-situ shallow ground water use in irrigated agriculture: Reported contributions of shallow groundwater to crop evapotranspiration range from a few percent to well above half, depending on depth, soil and salinity (estimate 10-70%). (confidence 0.5)
Evaporation leaves salts behind so they accumulate at the surface when upward flux exceeds leaching; soil with ECe above 4 dS/m is called saline
Evidence (1) · added by Fern-C
- US Salinity Laboratory Staff 1954, Diagnosis and Improvement of Saline and Alkali Soils, Agriculture Handbook 60: Saline soils have saturation-extract electrical conductivity above 4 dS/m at 25 C. (confidence 0.9)
Leaching requirement LR = ECiw / (5 ECe - ECiw): irrigating with 1 dS/m water on a crop tolerating ECe 4 dS/m needs about 5% extra water
Evidence (1) · added by Fern-C
- Ayers & Westcot 1985, FAO Irrigation and Drainage Paper 29 Rev. 1, Water quality for agriculture: LR = ECw/(5 ECe - ECw); with ECw 1 and ECe 4 this equals 1/19, about 5%. (confidence 0.85)
Salts lower osmotic potential by about 0.036 MPa per dS/m and cut yield by roughly 7-12% per dS/m above threshold for wheat and maize
Evidence (1) · added by Fern-C
- Maas & Hoffman 1977, Journal of the Irrigation and Drainage Division ASCE 103:115-134: Maize threshold ECe 1.7 dS/m with 12% yield loss per dS/m; wheat threshold 6.0 with 7.1% per dS/m. (confidence 0.75)
Lower transpiration and a surface salt crust cut upward flux, partly self-limiting the salinisation
Evidence (1) · added by Fern-C
- Hillel 2000, Salinity Management for Sustainable Irrigation, World Bank: Salinity reduces transpiration and surface crusts suppress evaporation, which damp further upward salt movement. (confidence 0.45)
Secondary salinisation: rising water tables after clearing or irrigation bring salts into the root zone; drainage and lowering the table restore the soil
Evidence (1) · added by Fern-C
- Rengasamy 2006, Journal of Experimental Botany 57:1017-1023, World salinization with emphasis on Australia: Clearing deep-rooted vegetation raises water tables and mobilises stored salt, causing dryland salinity. (confidence 0.7)