- A hydrologic system is as a structure or volume in space, surrounded by a boundary, that accepts water and other inputs, operates on them internally, and produces them as outputs.
- The structure (for surface or subsurface flow) or volume in space (for atmospheric moisture flow) is the totality of the flow paths through which the water may pass from the point it enters the system to the point it leaves.
- The boundary is a continuous surface defined in three dimensions enclosing the volume or structure.
- A working medium enters the system as input, interacts with the structure and other media, and leaves as output.
- Physical, chemical and biological processes operate on the working media within the system; the most common working media involved in hydrologic analysis are water, air and heat energy.
- The global hydrologic cycle can be represented as a system containing three subsystems: the atmospheric water system, the surface water system, and the subsurface water system.
Catchment and Basin
- A Catchment is a portion of the earth’s surface that collects runoff and concentrates it at its furthest downstream point, referred to as the catchment outlet.
- The runoff concentrated by a catchment flows either into a larger catchment or into the ocean.
- The place where a stream enters a larger stream or body of water is referred to as the mouth.
- The terms watershed and basin are commonly used to refer to catchments. Generally, watershed is used to describe a small catchment (stream watershed), whereas basinis reserved for large catchments (river basins).
Watershed and Stream order
- The watershed or basin is defined by the surrounding topography, the perimeter of which is called a divide. It is the highest elevation surrounding the watershed. All of the water that falls on the inside of the divided has the potential to be shed into the streams of the basin encompassed by the divide. Water falling outside of the divide is shed to another basin.
- The water flowing in streams is called stream flow
Horton suggested a classification of stream order as a measure of the amount of branching within a basin. A first order stream is a small, un-branched tributary. A second order stream has only first order tributaries. A third order stream has only first and second order tributaries and so on. When a channel of lower order joins a channel of higher order, the channel downstream retains the higher of the two orders.
Water Balance Problem
In a given year, a catchment with an area of 2500 km2 received 1.3 m of precipitation. The average rate of flow measured in a river draining the catchment was 30 m3s-1.
- How much total river runoff occurred in the year (in m3)?
- What is the runoff coefficient?
- How much water is lost due to the combined effects of evaporation, transpiration, and infiltration? (Expressed in m).
Solution
Total runoff volume
= number of seconds in a year ´ average flow rate
= 31 536 000 ´ 30
= 9.4608´108 m3
Runoff coefficient
= Runoff volume/ precipitation volume
= (9.4608´108) / (1.3 ´ 2500 ´ 106)
= 0.29 (29 %)
The water balance equation can be arranged to produce:
ET+F= P - R - ΔS
Where: P = (1.3 ´ 2500´106)
= 3.25´109 m3
R = 9.4608´108 m3 (from Total runoff volume)
ΔS = 0 (i.e. no change in storage)
So,
ET + F = 3.25´109 - 9.4608´108
= 2.30392´109 m3
= (2.30392´109) / (2500´106)
= 0.92 m
Precipitation
What is Precipitation? Defined:
Water falling in solid or liquid form e.g. rain, snow, and hail.
Uses of Precipitation Data
- Runoff estimation analysis
- Groundwater recharge analysis
- Water balance studies of catchments
- Flood analysis for design of hydraulic structures
- Real-time flood forecasting
- low flow studies
Mechanism Producing Precipitation
Three mechanisms are needed for formation of precipitation.
1. Lifting and Cooling - Lifting of air mass to higher altitudes causes cooling of air.
2. Condensation - conversion of water vapor into liquid droplets.
3. Droplet Formation - Growth of droplets is required if the liquid water present in a cloud is to reach ground against the lifting mechanism of air.
Types of Precipitation
Depending upon the way in which the air is lifted and cooled so as to cause precipitation, we have three types of precipitation, as given below:
- Cyclonic Precipitation
- Convective Precipitation
- Orographic Precipitation
Cyclonic Precipitation:
Cyclonic precipitation is caused by lifting of an air mass due to the pressure difference. Cyclonic precipitation may be either frontal or non-frontal cyclonic precipitation.
Frontal precipitation:
It results from the lifting of warm and moist air on one side of a frontal surface over colder, denser air on the other side. A front may be warm front or cold front depending upon whether there is active or passive accent of warm air mass over cold air mass.
Non-frontal precipitation:
If low pressure occurs in an area (called cyclone), air will flow horizontally from the surrounding area (high pressure), causing the air in the low-pressure area to lift. When the lifted warm-air cools down at higher attitude, non-frontal cyclonic precipitation will occur.
In the case of a cold front, a colder, denser air mass lifts the warm, moist air ahead of it. As the air rises, it cools and its moisture condenses to produce clouds and precipitation. Due to the steep slope of a cold front, forceful rising motion is often produced, leading to the development of showers and occasionally severe thunderstorms.
In the case of a warm front, the warm, less dense air rises up and over the colder air ahead of the front. Again, the air cools as it rises and its moisture condenses to produce clouds and precipitation. Warm fronts have a gentler slope and generally move more slowly than cold fronts, so the rising motion along warm fronts is much more gradual. Precipitation that develops in advance of a surface warm front is typically steady and more widespread than precipitation associated with a cold front. Warm front precipitation is generally light to moderate.
Convective Precipitation
Convective precipitation is caused by natural rising of warmer, lighter air in colder, denser surroundings. Generally, this kind of precipitation occurs in tropics, where on a hot day, the ground surface gets heated unequally, causing the warmer air to lift up as the colder air comes to take its place. The vertical air currents develop tremendous velocities. Convective precipitation occurs in the form of showers of high intensity and short duration.
Orographic Precipitation
Orographic precipitation is caused by air masses which strike some natural topographic barriers like mountains, and cannot move forward and hence rise up, causing condensation and precipitation. All the precipitation we have in Himalayan region is because of this nature. It is rich in moisture because of their long travel over oceans.
Definitions of some useful Technical terms
Depth: depth of rainfall at a point or over an area (mm)
Duration: the period of time during which rain fell (hours)
Intensity: Depth of rainfall per unit time i.e. depth/duration (mm/hr)
Time distribution: Rainfall hyetographs are plots of rainfall depth or intensity as a function of time. Cumulative rainfall hyetographs are also called rainfall mass curve.
Isohyets (contours of constant rainfall) can be drawn to develop isohyetal maps of rainfall depth.
Normal Annual Precipitation (mean of 30 years annual ppt)
Measurement of Precipitation
Point Data-Gauges
Non-recording rain gauges
They are known as non-recording because they do not record the rain but collect the rain.
Recording rain gauges
These are used to determine rates of rainfall over periods of time. Three types in common use are:
- Weighing Type
- Tipping Bucket Type
- Float Type
