Water

Question 1

UK Trends?

Answer 1

UK trend:
• Most high ground in West
• Majority of weather from West
• West = wettest

Question 2

Precipitation trends?

Answer 2

• Not exactly but broadly speaking:
• Pressure > vapour pressure leads to precipitation.
• Pressure < vapour pressure leads to evaporation.
• Low temperature leads to more precipitation.
• High altitude corresponds to low pressure but also low temperature.
• Generally, precipitation is greater at higher altitudes.
• NB The term precipitation includes both rainfall and snowfall

Question 3

Water calculations?

Answer 3

Streamflow = Precipitation – Evaporation – Change in Storage
Evapotranspiration = Rainfall – Streamflow
Change in storage over decades should be negligible

Daily rainfall:

Given in mm/per day
To get rainfall into km3:
(1) divide by 1,000,000 to get into km.
(2) multiply by the area (in km2) of the catchment to get km3.
Now you have a volume of rainfall that lands in a day, which you can say has units of km3/day.

Question 4

Types of evaporation?

Answer 4

Evaporation – Evaporation of water from an open water surface.
Evapotranspiration – Includes wet canopy (top of vegetation) evaporation and transpiration.
Potential Evaporation (PE) – The amount of water that would evaporate and transpire given an unlimited source of water. (what could happen)
Actual Evaporation (AE) – The actual amount of water that evaporates and transpires. (What will actually happen)

Question 5

Factors affecting evaporation?

Answer 5

• Sunshine: The energy required to provide the latent heat necessary to change water to vapour mostly derives from solar radiation.
• Temperature: Higher temperatures control how readily water is evaporated and how much water the air can hold.
• Humidity: There is a limit to how much water a parcel of air can hold. When the air is vapour saturated evaporation will cease. (Relative humidity)
• Wind: Air parcels circulate in multi-scale Eddy patterns, replacing vapour saturated air with dryer air from above. The rate of dry air replenishment is heavily controlled by the local wind-speed.
• Soil water: Evapotranspiration is ultimately limited by the availability of water in the soil.
• Plant type: Evapotranspiration is also affected by the plant type. Various plant types transpire at different rates and/or lead to additional loss by evaporation due to various rates of rainfall interception (rainfall sometimes puddles on leaves).

Question 6

Types of aquifer?

Answer 6

Aquifer: A saturated permeable geological unit that stores groundwater and allows it to flow under normal conditions.
Aquiclude: A saturated geological unit that is incapable of transmitting significant quantities of water.
Aquitard: A ‘less permeable’ bed in a stratigraphic sequence (may be significant in a regional groundwater balance, but insignificant for a local production well).
Aquifer types:
Confined:
An aquifer confined between two aquitards or aquicludes, such that pressures in the aquifer are everywhere greater than atmospheric.

Unconfined:
An aquifer with a free water surface as the upper boundary (the water table or phreatic surface) at which pressure is atmospheric.

Question 7

Types of catchments?

Answer 7

Surface-water catchments:
• River flow reacts quickly to rainfall events.
• There are periods of very low flow

Groundwater catchments:
• River flow is less “flashy”.
• Substantial summer flows.
• Most of the flow (the baseflow) is supplied by groundwater (see red line) through springs and upwelling in river channels.

Question 8

How does flow work?

Answer 8

How groundwater flows:

• The water flows through the voids, or pore spaces, in the rock.
• This may be in the minute spaces between the grains of a sandstone or in the small cracks and fractures that are more usual in limestones and chalk.
• Interested in how interconnected pore spaces are

Porosity = Volume of voids/ Total volume of the rock

Question 9

Ways to measure hydraulic head?

Answer 9

Look at Darcy’s Law
Observation well
Essentially an open borehole. Water level in such a well represents the average hydraulic head over the length of the open section. In unconfined aquifers, the water level in an observation well is often assumed to be the level of the prevailing watertable.
Piezometer
A device for measuring hydraulic head at a point (or averaged over a relatively small depth) in the ground. The water level in an open piezometer well is not necessarily the same as the water table level in unconfined conditions.