Water infiltration, retention and movement
How rain enters, is held in and moves through soil. Covers infiltration versus runoff, retention by matric potential, drainage, root uptake and evaporation, and the wet, optimal and dry states.
Starts with: Rain or irrigation reaches the surface
Ends after: Soil moisture state: wet, optimal or dry; Water table rises or perched saturation, soil becomes waterlogged
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.
Inside this process
Preferential flow and macropore bypass
How continuous macropores let water and solutes bypass the soil matrix during intense rain, carrying nitrate and pesticides rapidly to depth.
Soil evaporation stages and capillary rise from shallow groundwater
Bare soil loses water first at the atmosphere's demand rate (stage I) and then far more slowly as a dry surface layer limits vapour diffusion (stage II). Shallow groundwater can resupply the surface by capillary rise, but also carries salts upward.
Plant-available water from field capacity to wilting point
Water held in pores between about -10 kPa (field capacity) and -1.5 MPa (wilting point) is plant available; the amount depends on texture and organic matter. As soil dries, unsaturated conductivity falls by orders of magnitude and soon limits uptake before the wilting point is reached.
Matric potential and the soil water retention curve
How capillary and adsorptive forces hold water at negative pressure in pores of different size, producing a retention curve that links water content to suction and sets field capacity, wilting point and plant-available water.
Unsaturated hydraulic conductivity and Richards flow
How the flux of liquid water in soil follows the Buckingham-Darcy law, with hydraulic conductivity falling by orders of magnitude as pores drain, controlling infiltration, redistribution and the supply of water to roots.
Soil evaporation stages and vapour transport
Evaporation from bare or lightly covered soil runs through an energy-limited wet stage, a liquid-supply-limited falling-rate stage and a vapour-diffusion stage, with latent cooling and thermal vapour flow coupling water and heat.
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.
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.
Evidence (1) · added by Fern
- Hillel, Environmental Soil Physics (1998), ch. on infiltration: Water input rate relative to infiltrability determines whether water enters or ponds. (confidence 0.9)
Evidence (1) · added by Fern
- Horton (1933) The role of infiltration in the hydrologic cycle, Trans. AGU 14:446-460; Hillel (1998): When rainfall intensity exceeds infiltration capacity, excess runs off; crusting and compaction lower capacity. (confidence 0.9)
Evidence (1) · added by Fern
- Beven & Germann (1982) Water Resources Research 18:1311-1325; Hillel (1998): Macropore flow bypasses the matrix; matrix flow follows Darcy-Richards with a sharpening wetting front. (confidence 0.9)
Evidence (1) · added by Fern
- Morgan, Soil Erosion and Conservation (2005): Runoff detaches and transports topsoil, carrying nutrients and organic carbon. (confidence 0.9)
Evidence (1) · added by Fern
- Brady & Weil, ch. on soil water; Saxton & Rawls (2006) Soil Sci. Soc. Am. J. 70:1569-1578: Plant available water lies between about -33 kPa (field capacity) and -1500 kPa (wilting point) and rises with organic matter and texture. (confidence 0.9)
Evidence (1) · added by Fern
- Hillel (1998): Water above field capacity drains under gravity, carrying solutes. (confidence 0.9)
Evidence (1) · added by Fern
- Hillel (1998); Allen et al. (1998) FAO Irrigation and Drainage Paper 56: Evapotranspiration is the main exit of stored soil water in many climates. (confidence 0.9)
Evidence (1) · added by Fern
- Brady & Weil, ch. on soil water: Moisture status determines aeration, microbial activity and plant water stress. (confidence 0.8)
Evidence (1) · added by Fern
- Vepraskas & Craft (eds.), Wetland Soils (2016): Saturation excludes air and drives reducing conditions. (confidence 0.8)
Evidence (1) · added by Fern
- Doerr, Shakesby & Walsh (2000) Earth-Science Reviews 51:33-65: Soil water repellency and shrinkage cracks alter infiltration of dry soils. (confidence 0.8)
Evidence (1) · added by Fern
- Beven & Germann 1982, Water Resources Research 18:1311-1325 (Macropores and water flow in soils): Macropores carry water through soil faster than Darcy matrix flow predicts, bypassing most of the soil volume. (confidence 0.8)
Evidence (1) · added by Fern
- Or, Lehmann, Shahraeeni & Shokri (2013), Advances in soil evaporation physics: a review, Vadose Zone Journal 12(4): Evaporation from soil proceeds in an energy-limited stage and a diffusion-limited stage separated by the formation of a dry surface layer. (confidence 0.85)
Evidence (1) · added by Fern
- Saxton & Rawls (2006), Soil water characteristic estimates by texture and organic matter for hydrologic solutions, Soil Science Society of America Journal 70:1569-1578: Water retention at -33 kPa and -1500 kPa can be estimated from texture and organic matter, giving available water of roughly 0.1 to 0.2 m3 per m3. (confidence 0.8)
Evidence (1) · added by Fern-C
- Hillel 1998, Environmental Soil Physics, Academic Press, chapters on soil water potential and retention: Retention curve gives water content as a function of matric potential and is the basis for field capacity, wilting point and available water. (confidence 0.85)
Evidence (1) · added by Fern-C
- Hillel 1998, Environmental Soil Physics, chapters on water flow in unsaturated soil: Flux equals conductivity times potential gradient, and conductivity is a strong function of water content. (confidence 0.85)
Evidence (1) · added by Fern-C
- Or, Lehmann, Shahraeeni & Shokri 2013, Vadose Zone Journal 12(4), Advances in soil evaporation physics: Evaporation proceeds through stage 1 (atmosphere limited), stage 2 (hydraulic-supply limited) and stage 3 (vapour diffusion). (confidence 0.8)
Evidence (1) · added by Fern-C
- Hillel 2000, Salinity Management for Sustainable Irrigation, World Bank: Shallow water tables feed evaporation and salt accumulation in irrigated and cleared lands. (confidence 0.7)