Lecture3 hydrology cycle dung

Precipitation Flux of water from atmosphere to earth surface... Convective Precipitation Hot earth surface Convective precipitation occurs when the air near the ground is heated by the

Earth's Water and the Hydrologic Cycle

Components of Earth’s water

Supply of Water Resources

Small fraction (.014%) is readily available for human use

HYDROLOGY | fresh water

HYDROLOGY | fresh water | rivers & lakes

Hydrologic cycle

The processes and pathways involved in the circulation

of water from land and water bodies to the

atmosphere and back again

Changes in the formation of H2O (Ice, liquid, gas)

Earth's Water and the Hydrologic Cycle

Components of Earth’s water

What is significant of hydrology cycle?

Within 2.6 % of Freshwater on the earth

Soil and Groundwater storage

For Active Circulation

Water budgets-Water balance

Input = Output + Storage

Large Scale

Scales of Water Budgets

Budget: Input, Output, and Storage

Global water balance

Land (148.7 million km 2 )

(29% of earth area)

Ocean (361.3 million km 2 ) (71% of earth area)

Units are in volume per year relative to precipitation on

land (119,000 km 3 /yr) which is 100 units

What conclusions can we draw from these data?

HYDROLOGIC THINKING !

Precipitation, snow, seasonality, fire, soil, nutrients, flora/fauna,

ground water, floods, rivers, climate change?

ARID

SEMI-ARID

NATIVE PRAIRIE

RAINFORESTS

Precipitation

(Flux of water from atmosphere to earth surface)

Local Precipitation (Watershed scale)

Mechanisms causing air masses to lift

and cool, resulting in precipitation

Cold front (produces cumulus cloud) Warm front (produces stratus cloud)

Frontal Systems （前線性降雨）

• Boundary between air masses with different properties is called a

front

• Cold front occurs when cold air advances towards warm air

• Warm front occurs when warm air overrides cold air

Mưa địa hình -Orographic precipitation

Warm, moist air

a “rain shadow”;

Orographic Precipitation when air is forced to rise because of the physical presence of elevated land

Convective Precipitation

Hot earth surface

Convective precipitation occurs when the air near the ground is heated by the earth’s warm surface This warm air rises, cools and creates precipitation

Convective Storm

• Short duration (minutes to a few hours)

• High intensity, may exceed 3 cm/hr

• Small scale

– Flash floods

(e.g Big Thompson, CO)

– Usually summer;

– Often accompanied by lightning;

– High spatial variability (see FC flood)

Precipitation Measurement

• Rain gauges

– Storage, – Weighing, – Tipping-bucket

• Problems (ERRORS)

– Obstacles, – Wind eddies, – Evaporation

Tipping-Bucket Rain Gauge

How do we characterize precipitation?

year;

hyetograph;

Hyetograph = Bar graph of

precipitation over time

Hourly rainfall for a site near

Charlottesville, Virginia

Hourly precipitation usually very

heterogeneous (variable) over time

Hornberger et al., 1998, Fig 2.2

 Depth

we use maximum 30-minute intensity

Differences in amounts per unit time vary with location;

WHY?

What kind of rainfall data do we

have in Vietnam?

Why a water balance isn’t

simple

Precipitation Accuracy: Wind

Source: L Dingman, Physical Hydrology

Hydrologic Reality

in space (e.g., precipitation, infiltration, soil moisture, evapotranspiration);

variable over time (hours, days, months, and years);

estimates or indices, not TRUTH;

Rainstorm on Bobcat fire: 16 August 2000

Rainstorm on Bobcat fire: 15 August 2001

7

2

7.4 7.6

13.2 10.7

15.2 17.4

Upper Jug

Why a water balance is not simple: estimating precipitation over a basin?

How can we estimate precipitation over a basin?

1 Mean

near settlements, so not well

distributed;

small, flat, and relatively uniform

How can we estimate precipitation over a basin?

1 Mean

2 Thiessen polygons

Thiessen method for estimating rainfall

Step 1

Step 2

Thiessen method for estimating rainfall

Step 3

Thiessen method for estimating rainfall

Step 4

Thiessen method for estimating rainfall

Thiessen method

Ai

Pi

Ai Y

P(Y): Precipitation of watershed Y

A i : Area of i polygon

P i : Precipitation of i polygon

How can we estimate precipitation over a basin?

1 Mean

2 Thiessen polygons

How can we estimate precipitation over a basin?

How well the gauges in

a watershed represent the precipitation over the entire watershed

area?

Watershed

Precipitation

How well the gauges in

a watershed represent the precipitation over the entire watershed

area?

Watershed

Precipitation

PA = 1,9 in; PB = 2,5 in; PC= 3,0 in; PD = 3,2 in Please calculate watershed precipitation using Thiessen method

Excercise

Number square of AA=66, AB=86, AC=134, AD=50

Need to predict probability of different events (floods, storms, etc.)

of annual precipitation leading to crop

failure or shortage of water supply;

Frequency Analysis for Precipitation

future event

Weather system vary from one year to the next (also seasonally)

Predicting probability

of a given event

1 Take a series of data:

- Most common is annual maximum

(most extreme event in a year);

- Can use annual totals, daily values,

7-day minimum, or almost any time series;

Annual Precip for LA, CA 1934-1953 Year Depth (in)

Predicting probabilities

storms), or smallest to largest (droughts);

rank, even if the data are the same;

given value is uncertain!

Predicting probabilities

plotting position formula

rank and n is number of years;

because a more extreme event can always

occur, and we can’t have 100% probability of anything!

Annual Precip for LA, CA 1934-1953

Drawing Frequency curve

Amount of Precipitation

Recurrence Interval of Precipitation (yrs) or Probability

Recurrence Interval (yr)

n

Tr   1  1

Tr: Recurrence Interval n: number of years m: rank of the event

A measure of how often-on average-an event

of this magnitude OR a more extreme event will occur

n



Example (Oume AMeDAS station)

Automated Meteorological Data Acquisition System

200 mm/24hrs event is approximately 12 year recurrence interval

Commonly Used Units and Dimensions

for Water in Watershed

•Rainfall (total or accumulated): mm (for snow depth – m)

•Volumetric Flow or Discharge: m3/s

•Volumetric flow or Discharge (groundwater well production): l/s;

•Streamflow or Discharge per unit area (sometimes called ‘specific

or unit discharge’): m 3 /s per km 2

•Volume of Flow: Large volumes - m3 Small volumes- liter (l)

•1 liter = 0.001 m3

•Flow velocity: m/s