Hydrologic Losses - Advanced Hydrology - Lecture Slides, Slides of Aeronautical Engineering

Download Hydrologic Losses - Advanced Hydrology - Lecture Slides and more Slides Aeronautical Engineering in PDF only on Docsity!

Lecture 3: Hydrologic losses

Module 1

Docsity.com

 In engineering hydrology, runoff is the main area of interest. So, evaporation and transpiration phases are treated as “losses”.

 If precipitation not available for surface runoff is considered as “loss”, then the following processes are also “losses”:  Interception  Depression storage  Infiltration

 In terms of groundwater, infiltration process is a “gain”.

Hydrologic losses

Lecture 2 Docsity.com^ Module 1

 Depression storage is the natural depressions within a catchment area which store runoff. Generally, after the depression storage is filled, runoff starts.

 A paved surface will not detain as much water as a recently furrowed field.

 The relative importance of depression storage in determining the runoff from a given storm depends on the amount and intensity of precipitation in the storm.

Depression storage

Lecture 2 Docsity.com^ Module 1

Infiltration

The process by which water on the ground surface enters the soil. The rate of infiltration is affected by soil characteristics including ease of entry, storage capacity, and transmission rate through the soil.

The soil texture and structure, vegetation types and cover, water content of the soli, soil temperature, and rainfall intensity all play a role in controlling infiltration rate and capacity.

Lecture 2 Docsity.com^ Module 1

Soil Type : Sand with high porosity will have greater infiltration than clay soil with low porosity.

Surface of Entry : If soil pores are already filled with water, capacity of the soil to infiltrate will greatly reduce. Also, if the surface is covered by leaves or impervious materials like plastic, cement then seepage of water will be blocked.

Fluid Characteristics : Water with high turbidity or suspended solids will face resistance during infiltration as the pores of the soil may be blocked by the dissolved solids. Increase in temperature can influence viscosity of water which will again impact on the movement of water through the surface.

Infiltration

Module 1

Factors affecting infiltration Contd…

Lecture 2 Docsity.com

Infiltration

Infiltration capacity : The maximum rate at which, soil at a given time can absorb water.

f = f c when i ≥ fc

f = when i < f c

where fc = infiltration capacity (cm/hr) i = intensity of rainfall (cm/hr) f = rate of infiltration (cm/hr)

Module 1

Infiltration rate

Lecture 2 Docsity.com

f 0

f t = f c+(f 0 -f c)e -kt

f f c

infiltration

time t

Infiltration

Graphical representation of Horton formula

Measurement of infiltration

1. Flooding type infiltrometer
2. Rainfall simulator

Module 1

Infiltration rate Contd…

Lecture 2 Docsity.com

Infiltration

Infiltration indices

The average value of infiltration is called infiltration index.

Two types of infiltration indices

 φ - index

 w –index

Module 1

Measurement of infiltration

Lecture 2 Docsity.com

Example Problem

Infiltration

A 12-hour storm rainfall with the following depths in cm occurred over a basin: 2.0, 2.5, 7.6, 3.8, 10.6, 5.0, 7.0, 10.0, 6.4, 3.8, 1.4 and 1.4. The surface runoff resulting from the above storm is equivalent to 25.5 cm of depth over the basin. Determine the average infiltration index (Φ-index) for the basin.

Total rainfall in 12 hours = 61.5 cm Total runoff in 12 hours = 25.5 cm Total infiltration in 12 hours = 36 cm Average infiltration = 3.0 cm/hr Average rate of infiltration during the central 8 hours 8 Φ +2.0+2.5+1.4+1.4 = 36 Φ = 3.6cm/hr

Lecture 2 Docsity.com^ Module 1

 In this process, water changes from its liquid state to gaseous state.  Water is transferred from the surface to the atmosphere through evaporation

Evaporation

Evaporation is directly proportional to :  Vapor pressure (e (^) w),  Atmospheric temperature (T),  Wind speed (W) and  Heat storage in the water body (A)

Lecture 2 Docsity.com^ Module 1

Temperature: The rate of evaporation increase if the water temperature is increased. The rate of evaporation also increase with the air temperature.

Heat Storage in water body: Deep bodies can store more heat energy than shallow water bodies. Which causes more evaporation in winter than summer for deep lakes.

Evaporation

Module 1

Factors affecting evaporation Contd…

Lecture 2 Docsity.com

 Soil evaporation: Evaporation from water stored in the pores of the soil i.e., soil moisture.

 Canopy evaporation: Evaporation from tree canopy.

 Total evaporation from a catchment or an area is the summation of both soil and canopy evaporation.

Evaporation

Module 1

Types of Evaporation

Lecture 2 Docsity.com

Evaporation

Demerits of Evaporation pan:

1. Pan differs in the heat-storing capacity and heat transfer from the sides and bottom. Result: reduces the efficiency (sunken pan and floating pan eliminates this problem)
2. The height of the rim in an evaporation pan affects the wind action over the surface.
3. The heat-transfer characteristics of the pan material is different from that of the reservoir.

Module 1

Measurement of evaporation Contd…

1. Evaporimeters

Lecture 2 Docsity.com

Evaporation

Pan Coefficient (Cp) For accurate measurements from evaporation pan a coefficient is introduce, known as pan coefficient ( Cp ). Lake evaporation = Cp x pan evaporation Type of pan Range of Cp Average value Cp Class A land pan 0.60-0.80 0. ISI pan 0.65-1.10 0. Colorado sunken pan 0.75-0.86 0. USGS Floating pan 0.70-0.82 0. Source: Subramanya, 1994 Module 1

Measurement of evaporation Contd…

Lecture 2 Docsity.com