4 Intro Hydrology: Water balance and precipitation

What are the elements of the hydrological cycle?

• Evapotranspiration

• Condensation

• Precipitation

• Snow

• Runoff

• Soil water

• Ground water

What is the history of the hydrological cycle?

concept dates back to 3000 BP (before present), described based on guess work, mythology and religious convictions

What are issues of the hydrological cycle?

• only show one direction (e.g. no precipitation above the oceans which is actually the biggest flux)

• groundwater is a river below the surface

• no human activities (e.g. groundwater pumping, hydro power)

What does a newer water cycle look like?

What is Einzugsgebiet in English? 2 terms

catchment

watershed

How do you define the boundaries of the catchment?

surface topography → water divides

We take the highest points in the landscape and look at which direction (→ which direction) water flows when it rains

What can you use to see the surface topography?

map with contour lines

What is the issue with water divides?

Where is this especially relevant?

subsurface water divides

The topography only shows surface water divides, majority of the water flows through permeable layers of soil

Geology matters, especially in Karst areas

Why is the delineation of water divides difficult for flat area?

Countries with lots of flat areas don’t have many high points in the landscape

How can catchments interact with each other?

A smaller catchment can be included in a bigger catchment etc.

Water balance:

• Definition

Accounting for the catchment area, includes the influx, outflux and storage

Water balance:

• Formula and units

Water balance:

• What is important for unit conversion?

Discharge → specific discharge

Height → volume

Water balance:

• What is the height of 1L water on 1 m²?

Water balance:

• Example Rietholzbach

• What are the steps?

What is the water balance in Switzerland?

How has the water balance in Switzerland changed from 1900 to 2010?

slight changes in precipitation and discharge because of climate change

Precipitation:

• How does it form?

• Where is which type typical?

convective: typical for summer thunderstorms

orographic: mountaneous regions

frontal: regions that receive tropical air

Precipitation:

• Side view of orographic precipitation

Precipitation:

• How is precipitation spatially distributed across Maui, Hawaii?

Precipitation:

• How is precipitation spatially distributed across Switzerland? (annual precipitation)

• Alpennordseite: konvektiv

• Alpensüdseite: orographisch, frontal

• ganze Schweiz

Precipitation:

• How is precipitation spatially distributed across Switzerland? (precipitation days)

in Ticino and Valais there is a low number or precipitation days, however when it occurs it’s in form of a heavy rainfall

Precipitation:

• How is precipitation spatially and seasonally distributed across Switzerland? (1-day max precipitation)

summer: afternoon/evening, convective, thunderstorms

winter: entire day

Precipitation:

• Where does what percentage of Europe’s rainfall occur?

What is the relation between maximum intensity and duration?

What is the intensity duration curve used for?

intense - short

light - long

→ used for predictions

How is precipitation measured?

• precipitation gauges

• totalisator (with windshield)

What are measurement errors and issues?

• catch errors

• splash (ok if 1.5 - 2m above ground)

• evaporation

• low intensity rain / fog / snowflakes

• instrument errors (continuous gauges)

• observer errors (manual read gauges)

• wind (increased wind speed at the gauge orifice carry away small drops)

How should a rain gauge be located?

2x the height of the obstruction (preferably 4x) away

45° cone above rain gauge should be empty

How is the precipitation distributed spatially in the Thur catchment?

• mountains increase rainfall on the windward side (luv)

What can be observed about the change in average intensity with area in the Thur catchment?

creating the average for high-intensity events creates a gap between the averages, because the sizes of the areas are different

How does average rainfall behave in relation to area?

there can be a high intensity for a point (small area) and a low intensity for a big area

What ways are there to determine the spatial distribution of a catchment?

• arithmetic mean

• Thiessen polygons

• Isohyet method

How does the arithmetic mean work?

What is the disadvantage?

each gauge is weighted equally and the mean is calculated

Disadvantage: usually gauges are located in low-altitude areas, even if the catchment also has high elevation areas

How do the Thiessen polygons work?

What is the advantage?

◆ The area is divided into n subregions, each centered on each of the n rain gauges

◆ All points in each subregion are closer to their central gauge than any other gauge

◆ Assume uniform precipitation for each subregion

◆ Each gauge represents a sub-area A_i

◆ The spatial average is calculated by weighting the individual stations with their representative area

Advantage: suitable for flat countries, because there are little changes in altitude

How does the isohyet method work?

interpolate the measured values at the gauges

→ draw isohyet lines of equal precipitation

→ calculate spatial average by weighing the gauges with their area