Class notes
Hydrology: Chapter 2
- Course
- GLY5827
- Institution
- Florida State University
Florida State University: Hydrology (Ming Ye) Chapter 2: Precipitation
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September 18, 2024Introduction
Precipitation
Primary input of water to a catchment
A key source parameter in the water budget equation
A detailed treatment of the precipitation process is most often a subject of meteorology, an dis beyond
this hydrology course
Our discussion of precipitation will focus on the methods for indirectly quantifying precipitation
inputs to catchment and on describing these inputs using statistical techniques
Evapotranspiration
Primary output of water from a catchment
Evapotranspiration depends both on properties of the land surface and the state of the near-surface
and is, therefore well within the domain of hydrology
Estimate the rates, timing, and spatial distribution of these water fluxes between the land and atmosphere
Historical examples
Ancient india (400 BC)
Measurements of precipitation were used as a basis for taxation, because agricultural production was
presumed to be proportional to rainfall amounts
Colorado river Management
In the colorado river compact of 1922, the average annual discharge (from 1896-1921) is 16.8 million
acre-feet, but the actual average annual discharge from 1922 to 1976 (dry era) is 13.9 million
acre-feet. As a result, there was not enough water to be apportioned to the colorado states
It is critical to consider spatial and temporal variation of precipitation and evapotranspiration
Distribution of precipitation
Precipitation at any place is distributed as follows:
A portion known as the interception is retained on buildings, trees, shrubs, and plants. This is
eventually evaporated
Some of the remaining precipitation is evaporated back into the atmosphere directly
Another portion is infiltrated into the ground. A part of the infiltration in the root zone is
consumed by plants and trees and ultimately transpired into the atmosphere
If the precipitation exceeds the combined evaporation and infiltration, puddles known as depression
storage are formed. Evaporation takes place from these puddles
After the puddles are filled, the water begins flowing over the land surface to join a stream channel.
This is known as surface runoff. Some evaporation takes place from the stream surface
Studying precipitation and evapotranspiration for the catchment annual water budget
There are infinite combinations of et and rs that will satisfy the equation (p = rs + et)
, Precipitation
Precipitation: the deposition of liquid water droplets and ice particles that are formed in the atmosphere and
grow to a sufficient size so that they are returned to the earth’s surface by gravitational settling
Types of precipitation
Ordinarily classified according to the phase it is in when it reaches the surface of a collector or the
ground:
Solid (snow and ice crystals, including sleet and hail) or
Liquid (rain and freezing rain)
Are dew and fog considered as precipitation? NO
When cloud particles become too heavy to remain suspended in the air, they fall to the earth as
precipitation
Formation of precipitable water in the atmosphere
Creation of saturated condition in the atmosphere
Condensation of water vapor into liquid water
Growth of small droplets by collision and coalescence until they become large enough to precipitate
Saturated condition (vapor pressure and saturated e)
Condensation: saturated conditions within the air + presence of condensation nuclei, small particles such as
dust or previously formed water or ice particles
Coalescence: small droplets into larger droplets, through collision of small droplets with each other or with
larger drops, that gives rise to precipitable raindrops or ice crystals
Some facts:
Minimum average precipitation: death valley, california
Maximum average precipitation: kauai, Hawaii
Most of the precipitation falling on the continental US are from the bordering oceans, even in the
interior of the continent
DO NOT FORGET TEMPORAL VARIATION!!!
Major types of precipitating systems
Thunderstorms (including squall lines and tornadoes)
Tropical systems (including hurricanes, typhoons, and tropical cloud clusters)
Frontal storms (also known as extratropical cyclones)
Orographic storms (produced by forced lifting of air over mountain barriers)
Precipitation measurements: point measurements
The rainfall (or snowfall) at any place are measured by gages of self-recording or manual observation type
Point precipitation: amount of precipitation deposited at a particular station representing a point in space
Point precipitation usually is expressed in depth units (volume divided by collector cross-sectional
area) in inches or millimeters at any place within a given time
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