GEOS2060 Week 5: Hydrology and Hydrogeology

Three water "zones" the Critical Zone intersects

Zone 1 atmospheric & surface water; Zone 2 vadose (unsaturated) zone; Zone 3 groundwater (saturated) zone

Zone 1 (atmospheric and surface water) is dealt with in which field/unit?

Hydrology (GEOS2050)

Zone 2 (vadose zone) is dealt with in which field/unit?

Soil Science (GEOS2060)

Zone 3 (groundwater) is dealt with in which field/unit?

Hydrogeology (GEOS2050)

Vadose zone (unsaturated zone)

Unsaturated groundwater zone above the water table where pores contain rock, water AND air

Saturated zone

Zone below the water table where pores/cracks are full of water (rock and water)

Capillary fringe

Zone just above the water table where water is drawn up by capillary action; saturated with water at negative pressure

θs (theta s) on a water content profile

Volumetric water content at saturation

θr (theta r) on a water content profile

Residual volumetric water content

Pools (USGS water cycle)

Places where water is stored (e.g. the ocean, groundwater, ice sheets, lakes)

Fluxes (USGS water cycle)

The ways water moves between pools (e.g. evaporation, precipitation, discharge, recharge, human use)

Percentage of all water on Earth that is available for human use

Only ~1%

Of the usable ~1% of water, how much comes from groundwater?

99% (0.86% from lakes, 0.02% from rivers)

Share of all freshwater that is icecaps and glaciers

68.7%

Share of all freshwater that is groundwater

30.0%

Share of all freshwater that is surface water

0.3%

Share of all freshwater classed as "other"

0.9%

If all groundwater were pooled evenly across the land surface like a lake, how deep would it be?

~180 m (Gleeson et al. 2015)

Groundwater age classes (Gleeson et al.)

Modern ≤ 50 years; Young ≤ 100 years; Old > 100 years

Modern groundwater volume vs older groundwater storage (Gleeson et al.)

Modern groundwater ≈ 347,180 km³; older groundwater storage ≈ 21.97 million km³

Other water pools in the Gleeson et al. figure

Atmosphere 12,000 km³; vegetation 1,000 km³; soil water 16,000 km³; surface water 100,000 km³

How does the proportion of modern groundwater (R_modern) change with depth?

It decreases with depth - most modern groundwater is in the shallow subsurface

Hydrological (water) cycle

The continuous movement of water on, above and below the surface of the Earth

Precipitation

Release of condensed water from clouds as rain, snow, sleet or hail that falls back to the Earth's surface

Evaporation

Conversion of water from liquid to vapour (from open water bodies/surfaces) into the air

Evapotranspiration

Combined process of evaporation from the Earth's surface and transpiration from plants

Condensation

Water vapour in the atmosphere cools and condenses into liquid or solid forms, leading to cloud formation

Infiltration (surface water context)

Movement of water from the land surface into the soil and underlying rock layers

Surface runoff / streamflow

Flow of water over the land surface, eventually leading to streams, rivers, lakes and oceans

What influences spatial and temporal precipitation patterns?

Climate and topography - mountains, oceans and prevailing wind patterns

Why do areas near the equator typically receive more precipitation?

Convergence of trade winds and the presence of tropical rainforests

Most of Australia's precipitation falls as

Rainfall

Annual precipitation comparison in the lecture (Australia, Las Vegas, Beijing, London)

Australia 165 mm/yr; Las Vegas 114 mm/yr; Beijing 635 mm/yr; London 752 mm/yr

Why is global evaporation highest around the equator?

Largest rainfall there and high incoming solar radiation (needed for the phase change); other factors include proximity to coast

How is evaporation measured/mapped across Australia?

Pan evaporation records from ~300 stations (Bureau of Meteorology)

Where is average annual pan evaporation highest in Australia?

The arid interior (up to ~4000 mm/yr); lowest in the SE coast and Tasmania

Interception

Precipitation caught by vegetation canopy (interception storage) before reaching the ground; may evaporate

Throughfall

Precipitation that passes through the vegetation canopy and reaches the ground

Depression storage

Water held in small surface depressions that must fill before overland flow occurs

Overland flow

Water flowing over the land surface towards a stream channel

Interflow

Lateral flow of water through the shallow unsaturated soil zone to a stream

Return flow

Subsurface water that re-emerges at the surface downslope and then flows as overland flow

Percolation

Downward movement of excess water through the soil/vadose zone toward the water table

Baseflow

Groundwater discharge into a stream that sustains flow between rainfall events

Groundwater recharge

Water moving down through the unsaturated zone to the water table (adds to groundwater storage)

Runoff generation pathway (Freeze and Cherry 1979)

Precipitation → interception/throughfall → infiltration → unsaturated soil moisture storage → groundwater recharge → groundwater storage; overland flow, interflow and baseflow feed channel storage → runoff

Streamflow (discharge)

The volume of water moving down a stream or river per unit of time

Four main sources of water for streamflow

Channel precipitation; surface runoff/overland flow; interflow; groundwater flow/baseflow

Respiration equation as given in the lecture (photosynthesis)

6 CO2 + 6 H2O + Energy = C6H12O6 + 6 O2 (carbon dioxide + water + sunlight = sugar + oxygen)

Transpiration

Water vapour lost from a plant to the atmosphere

Stomata

Small openings on leaves through which transpiration primarily occurs

Evapotranspiration equation

Evapotranspiration = transpiration + evaporation

Factors controlling water infiltration rate (lecture list)

Surface cover (sealed vs natural), vegetation, soil texture, soil moisture

Effect of surface cover on infiltration

Sealed (developed) surfaces reduce infiltration and increase runoff; natural surfaces allow more infiltration

Sandy vs clayey soils: which infiltrate faster?

Sandy soils infiltrate faster than clayey soils

Effect of soil moisture on infiltration rate

Dry soils have higher initial infiltration rates

Effect of vegetation on infiltration

Vegetation improves soil structure and water infiltration

Infiltration importance

Vital for soil moisture replenishment and groundwater recharge (and reduces surface runoff)

Natural pre-development site vs developed site with no infiltration planning

Natural: more evapotranspiration and shallow/deep infiltration, less runoff. Developed: more runoff, less infiltration

Irrigation

Artificial application of water to support crop or vegetation growth (a human modification of the water cycle)

Types of irrigation systems

Surface, drip, sprinkler and subsurface irrigation

Benefits of irrigation

Enhances crop yields and optimises soil moisture

Risks of irrigation

Waterlogging, salinisation and high costs (must be made sustainable)

Ring infiltrometer

Cylindrical ring placed in the soil with a water reservoir at the top; infiltration rate measured from the decrease in water level

Guelph permeameter

Cylindrical probe with a porous base allowing infiltration at constant water level; infiltration rate from the drop in the reservoir over time

What does a Guelph permeameter tell you?

How easily water can flow through the soil

Water balance model - guiding principle

Based on conservation of mass

Components of the water balance model

Input (precipitation, surface inflow); storage (reservoirs, soil, aquifers, snowpacks); output (evaporation, transpiration, surface outflow); infiltration to the vadose zone; net balance

Aridity index (AI) formula

AI = P / PET (mean annual precipitation ÷ mean annual potential evapotranspiration)

A lower aridity index means

A drier climate (precipitation is small relative to potential evapotranspiration)

Hyper-arid aridity index

AI < 0.03 (extremely dry, desert climates)

Arid aridity index

0.03 ≤ AI < 0.20 (very low precipitation)

Semi-arid aridity index

0.20 ≤ AI < 0.50 (low to moderate rainfall)

Dry sub-humid aridity index

0.50 ≤ AI < 0.65 (moderate rainfall but some dryness)

Humid aridity index

AI ≥ 0.65 (sufficient to abundant rainfall)

Why is Australia called the driest inhabited continent?

Low average annual precipitation and the lowest annual river discharge of the inhabited continents

Köppen-Geiger classification - what is it based on?

Categorises global climates using temperature and precipitation patterns

Five main Köppen-Geiger climate types

Tropical, Dry, Temperate, Continental, Polar

Why is the Köppen-Geiger system useful?

Helps understand regional climate variations and impacts on the environment; widely used in climatology, geography and environmental science; framework for studying climate change

Köppen type BWh

Hot desert (dominates central Australia)

Köppen type BSh

Hot semi-arid

Köppen type Aw

Savanna (northern Australia)

Köppen type Cfa

Humid subtropical

Köppen type Cfb

Oceanic

Köppen type Csa

Hot-summer Mediterranean

Köppen type Csb

Warm-summer Mediterranean

Köppen type BSk

Cold semi-arid

Köppen type BWk

Cold desert

Köppen types Am and Af

Am = Monsoon; Af = Rainforest

Porosity

The pore space in rocks and sediments that can be filled with water and air

Example: pore space in sand

~30% - i.e. 300 L of void space within 1,000 L of volume

Typical soil composition pie chart (volume)

~45% mineral matter, 25% water, 25% air, 5% organic matter

Role of the vadose zone in water movement

Crucial in the movement of water from the surface to the groundwater

Why does moisture content in the vadose zone matter?

It influences water availability for plants and the transport of nutrients and contaminants

Vadose zone as a buffer

Moderates the impact of surface activities on groundwater, affecting water quality and recharge rates

Free water

Held in soil pores and can drain through gravity (gravitational water)

Capillary water

Held within the small pores and capillary spaces of soil by capillary action

Pellicular water

Thin film around soil particles held by surface tension

Hygroscopic water

Tightly bound to the surfaces of soil particles by adsorption forces

Which forces hold each water type? (free, capillary, pellicular, hygroscopic)

Free = gravity (drains); capillary = capillary action; pellicular = surface tension; hygroscopic = adsorption