Urban Hydrologyand Water Budget Calculation

Geog310 Urban Climatology

Early settlement occurs near a water body for

1. Source of water supply

2. Transportation

3. Sewage disposal

The water budget is a way of keeping track of all inputs and outputs to the total water available to a city. The equation,

• P = E + R + ΔS,

where P is precipitation, E is evaporation, R is runoff and ΔS is change in ground storage, represent the components of the water budget.

Inputs to urban system is greater than rural, since

(1) precipitation is augmented, and

(2) importation of water from outside for municipal use. (rural may have irrigation).

Evaporation and sub-surface storage are less in cities due to removal of vegetation and replacement by impervious surfaces.

Urban runoff is greater than rural. Partly this is due to sewage systems, the rest due to waterproofing surfaces and artificial routing, eg storm sewers.

Understanding Water BalancesUnderstanding Water Balances

Root zone

Water table

EvapotranspirationRainfall

Stormwater runoff

Infiltration

Natural water cycleNatural water cycle Urban Water CycleUrban Water Cycle

Root zone

Water table

EvapotranspirationRainfall

Stormwater runoff

Infiltration

Garden watering

Wastewater discharge

Water Supply

Storm discharge shows • (a) urban basins respond

faster to storm input, so peak occurs earlier;

• (b) amount runoff increases with urbanization, not basin size. (Therefore storm sewers must be designed to accommodate very large volumes in short period of time.)

• Flood frequency increases with urbanization, as does flood intensity

• urban aggravates the trouble of flood (water shed contribute). Impervious pavement shorten time rain or meltwater reaches the water courses. Storm drain system deliberately designed to carry the water away from residential and business district reduce the lag times even further potential lead to high cost of flooding.

Water shed or drainage basin: an area where all precipitation contribute to a particular stream.

Vegetated area, water is temporary intercepted by leaves, soil absorbs water and store water, forest litter is effective storage medium. All these significantly retarded surface runoff.

Return period: recurrence intervals for maximum rain or peak flood.

• Urbanization leads to greater sediment loads in rivers and decreased water quality.

• Since evaporation generally less, cooling by evaporation is less.

Water budget calculation based on Thornthwaite and Mather’s method (handout)

Potential evapotranspiration (PE): water loss from a large homogeneous,vegetation-covered area (albedo of 22 to 25%) that never suffers from a lack of water.

(It is primary a function of climatic conditions (energy from the sun) and is not a function of type of vegetation, type of soil, soil moisture content or land management practices).

• If we know air temperature and latitude of the location, we can find PE from tables designed by Thornthwaite and Mather (they follow each other seasonally)

• We can calculate AE based on PE, P and soil moisture condition.

Actual evapotranspiration (AE): depend on all factors in addition to the climatic factors.

Whenever precipitation exceeds the climatic demand for water, the soil moisture storage will increase, a water surplus may develop, and the groundwater table may rise.

• If P is greater than PE, AE=PE, water storage increases (to p-PE). If water storage becomes larger than capacity (150mm for example), the amount of runoff is p-PE-change in storage (storage of water in soil of next month - current month)

• If P is smaller than PE, AE=p+change in storage. Deficit=PE-AE.

• If P equals PE, AE=PE, no storage change, no runoff