soil water

what is the soil plant atmosphere continuum

the continuous movement of water from the soil, through plants, and into the atmosphere, driven by differences in water potential

how much water is on earth and how much is usable

earth has around 1400 million km³ of water, covering around 71% of the surface, but most is saline or inaccessible and not active in the annual hydrologic cycle

what is the hydrologic cycle

the movement of water from earth’s surface to the atmosphere and back again, driven by solar energy through evaporation, condensation and precipitation

what role does solar energy play in the hydrologic cycle

about one third of incoming solar energy is absorbed by water, stimulating evaporation (liquid water to water vapour)

what water pools cycle most actively

water with short residence times, including the atmosphere, soil moisture, lakes, rivers, shallow groundwater, and the surface layer of oceans

what is the residence time of water in key reservoirs

atmosphere around 10 days, rivers more than 20 days, soil moisture 1 month

what are FC, PWP, and RAW

• field capacity: soil water after 2-3 days of drainage

• permanent wilting point: water content where 50% of plants permanently wilt

• readily available water: ½ paw, easily extracted by plants

what is plant available water

the water plants can extract from soil = field capacity - permanent wilting point

what types of water are held in unsaturated soils

• capillary water: held in small pores by capillary forces

• hygroscopic water: tightly bound to soil particles and organic matter

what is soil water retention

the soil’s ability to hold water against gravity under saturated or unsaturated conditions

what controls water flow in saturated soils

darcy’s law, influenced by:

• soil water potential gradient

• hydraulic conductivity (k), which depends on water properties and soil texture/structure

what is infiltration rate and steady infiltration rate

• infiltration rate: volume of water entering soil per unit area per unit time

• steady infiltration rate: constant rate reached after prolonged wetting; key for irrigation and drainage planning

what is infiltration and why is it important

infiltration is the entry of water into soil; it determines how much water enters the soil versus becoming runoff and is critical for water management

how does soil water move

water moves from areas of higher water potential to lower water potential, follwing a universal tendency toward equilibrium

why is soil water potential important

it controls water entry into soil, direction and rate of water flow, plant water uptake, drainage, capillary rise, solute transport, and soil temperature changes

what is soil water potential

the energy status of soil water that determine its ability to do work, including movement and plant uptake

what is soil water content and how is it measured

• gravimetric water content: mass based, direct lab method

• volumetric water content: volume based, supports indirect measurement and sensors

what is soil water

water contained in soil, described by both its amount (soil water content) and energy state (soil water potential)

what happens to precipitation that falls on land

about 65% becomes soil water, which is eventually transpired by plants or drains into groundwater

where do we find high water potential and where do we find low water potential

high water potential near roots and in soil, low water potential in atmosphere