4 Water flow in soils

A collection of 80 vocabulary flashcards covering soil water flow, the water balance equation, virtual water footprints, Darcy's law, and soil hydraulic properties.

Professor Paul Hallett

The instructor for the Water flow in soils course at the University of Aberdeen.

Soil Physics (1996)

A textbook available at the Sir Duncan Rice Library authored by T. J. Marshall, J. W. Holmes, and C. W. Rose.

Critical freshwater withdrawal

A proportion of available freshwater resources exceeding $100\%$, according to UN-Water (2021) data.

High freshwater withdrawal

A proportion of available freshwater resources between $>75\%$ and $100\%$.

Medium freshwater withdrawal

A proportion of available freshwater resources between $>50\%$ and $75\%$.

No stress freshwater withdrawal

A proportion of available freshwater resources between $0\%$ and $25\%$.

Aqueduct Water Risk Atlas

A tool provided by the World Resources Institute to map and analyze water stress and food demand.

India Baseline Water Stress

A condition where $54\%$ of India faces high to extremely high water stress based on withdrawals versus available supply.

Water Balance Equation

$$\Delta\Theta = P + I + C - E - T - D - R$$

$$\Delta\Theta$$

The change in soil water content.

$$P$$

Precipitation in the soil hydrological cycle.

$$I$$

The amount of irrigation applied to the soil.

$$C$$

Capillary rise, representing upward movement of water from groundwater to the root zone.

$$E$$

Soil evaporation in the water balance equation.

$$T$$

Transpiration from plants in the soil water store.

$$D$$

Deep drainage, which is water that flows out of the root zone.

$$R$$

Runoff or runon in the hydrological cycle.

Interception

The process by which vegetation stops water from hitting the ground.

Infiltration Rate

The speed at which water that hits the soil permeates into it.

Seepage

Another term for water that flows out of the root zone, also known as deep drainage.

Water Use Efficiency (Eco-physiologist)

The relationship between $\text{CO}_2$ assimilation and transpiration.

Water Use Efficiency (Agronomist)

Yield per unit area ($Y$) divided by the water used to produce that yield.

Virtual water

The volume of freshwater used to produce a product, measured at the place where the product was made.

One Drop (Virtual Water Symbol)

An illustration equivalent to $50\,dm^3$ of virtual water.

Green water

The rainfall and soil water storage component of the water footprint.

Blue water

The surface or groundwater sources component of the water footprint.

Grey water

The amount of fresh water required to assimilate pollutants to meet specific water quality standards.

Water footprint of one burger ($150\,g$ beef)

$2500\,dm^3$ of water.

Water footprint of one beef steak ($300\,g$)

$4650\,dm^3$ of water.

Water footprint of one apple ($150\,g$)

$125\,dm^3$ on average.

Water footprint of $\text{1}\ dm^3$ of milk

$1000\,dm^3$ of water.

Water footprint of one package of toast ($500\,g$)

$650\,dm^3$ of water.

Water footprint of one pot of tea ($750\,cm^3$)

$90\,dm^3$ of water.

Water footprint of one pot of coffee ($750\,cm^3$)

$840\,dm^3$ of water.

Water footprint of one pound ($500\,g$) of barley

$650\,dm^3$ of water.

Water footprint of one pound ($500\,g$) of sorghum

$1400\,dm^3$ of water.

Water footprint of one pound ($500\,g$) of millet

$2500\,dm^3$ of water.

Water footprint of $\text{1}\ dm^3$ of apple juice

$1140\,dm^3$ of water.

Water footprint of one piece of cheese ($500\,g$)

$2500\,dm^3$ of water.

Mekonnen and Hoekstra (2011)

Researchers who determined the global average water footprint for crops consists of $68\%$ green, $16\%$ blue, and $15\%$ grey water.

Big 'Splendour' apple tree

A tree requiring $60\,dm^3/\text{day}$ at peak use and $6200\,dm^3$ seasonally to produce $750$ apples.

Favourable soil properties

Good structure, good drainage, and high organic matter leading to better water retention in pores and less run-off.

Degraded soil properties

Poor structure, poor drainage, low organic matter, and the presence of a surface crust leading to increased run-off.

Infiltration capacity

A measure of how much water the soil can hold, determined by wetness and permeability.

Matric Potential

The gradient that causes water to flow within the soil.

Early-time flow stage

The first few minutes of wetting dominated by capillarity and described by sorptivity.

Steady-state flow stage

The stage describing the ability of water to move through soil, characterized by hydraulic conductivity.

Sorptivity ($S$)

A parameter for early-time flow measured in $m\,s^{-1/2}$ or $cm\,h^{-1/2}$.

Equation for Sorptivity

$\text{Sorptivity} = \frac{\text{Infiltration Amount}}{\text{Time}^{1/2}}$.

Hydraulic Conductivity ($K$)

A parameter measured in $m\,s^{-1}$ representing the ease with which water moves through soils.

Factors of Hydraulic Conductivity

Soil texture, density, structure, and water content.

Darcy's Law (1856)

The law stating that flow rate is proportional to the hydraulic head and inversely proportional to the length of the soil column.

$Q$ in Darcy’s Law

The volume of water flow per unit time ($\text{Volume}/\text{Time}$).

Hydraulic head ($h$)

The pressure or energy of water that drives flow through the soil.

Double the length of soil ($L$)

In Darcy's Law, this results in the rate of flow being slowed to $50\%$.

Double the head of water ($h$)

In Darcy's Law, this results in the rate of flow being increased to $200\%$.

Hydraulic Conductivity ($K$) Formula

$K = \frac{V \times L}{h \times A \times t}$ where $V$ is volume, $L$ is length, $h$ is head, $A$ is area, and $t$ is time.

$K_{sat}$ for Beach sand

Approximately $36\,cm/h$; also used for golf course greens.

$K_{sat}$ for Very sandy soils

Approximately $18\,cm/h$; characterized as being too fast to filter pollutants.

$K_{sat}$ for typical Agricultural use

Approximately $1.8\,cm/h$; considered suitable for most urban and agricultural uses.

$K_{sat}$ for Clayey soils

Approximately $0.18\,cm/h$; considered too slow for most uses.

$K_{sat}$ for compacted material

Less than $3.6 \times 10^{-5}\,cm/h$; extremely slow.

Guelph Permeameter

A constant head field infiltrometer used to measure hydraulic conductivity in the field.

Double-Ring Infiltrometer

An automated field tool used to measure the infiltration rate and saturated flow.

Saturated Flow Gradient

$\text{Gradient} = \frac{\text{Difference in total potential between points}}{\text{Distance between the points}}$.

Infiltration Darcy’s Equation

$\frac{\text{Volume flow}}{\text{Area} \times \text{time}} = Q = K_{sat} \times \text{gradient}$.

Wetting Front

The boundary where infiltrating water moves into dry soil, pulled by matric suction.

Transmission Zone

The soil region behind the wetting front where water is being transported.

Ponding Time

The time at which the rainfall rate exceeds the soil's infiltration rate, leading to surface accumulation.

Thatch layer

A layer found in 'Bad' turf areas (e.g., St Andrews Old Course) that negatively impacts hydraulic conductivity.

Water Repellent soil

Soil where the contact angle is $\ge 90^{\circ}$, resulting in no infiltration.

Sub-critical Water Repellent soil

Soil where the contact angle is between $0^{\circ}$ and $90^{\circ}$, causing impeded infiltration.

Non-repellent soil

Soil where the contact angle is approximately $0^{\circ}$, meaning infiltration is not impeded.

Causes of soil repellency

Surface waxes from leaves, decomposing organic matter, plant root exudates, and fungal hyphae.

Lattice-Boltzmann Model

A pore-scale model used to visualize water flow in repellent versus non-repellent soils.

Effect of Fire on Repellency

Fire can create or enhance a water repellent layer in the soil, leading to increased flood risk.

Beechgrove Experimental Plot

A site where nitrogen levels were found to affect water sorptivity due to changes in fungal biomass and repellency.

High Nitrogen ($120\,kg/ha$) effect

Associated with greater fungal biomass and differences in water repellency compared to no added Nitrogen.

Infiltration rate ($f$)

The actual rate of water entry into soil, which decreases over time until reaching steady-state.

Capillary Rise source

The movement of water from the water table upward into the unsaturated zone.