Questions and Answers
Which of the following is a part of engineering hydrology?
What is the branch of science that deals with the occurrence, circulation, and distribution of water on Earth?
Hydrology
A watershed is an area that drains to multiple outlets.
False
What is the process called where precipitation is absorbed by plants before reaching the ground?
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What is the water balance equation regarding inflow and outflow?
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Name one factor that is determined by engineering hydrology.
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The calculation of inputs and outputs in the water budget is represented by the equation: ______ = E + I - R - G.
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What type of event does the study of engineering hydrology help predict?
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What are the main components involved in the hydrologic cycle?
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What is evaporation?
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Which factors affect the rate of evaporation? (Select all that apply)
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What is an evaporimeter used for?
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Which of the following is not a type of evaporimeter?
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Lake evaporation can be calculated using the formula: Lake evaporation = CP X __________
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What is the Dalton-type equation used for?
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What does 'EL' represent in evaporation equations?
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In the energy balance method, what does 'Hn' represent?
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Lake evaporation is directly proportional to atmospheric pressure.
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Study Notes
Definition of Hydrology
- Hydrology is a branch of science focused on the origin, occurrence, circulation, and distribution of water on Earth and in the atmosphere.
Engineering Hydrology
- Involves estimation of water resources and studying processes like precipitation, runoff, and evapotranspiration.
- Analyzes floods and droughts to aid in water management.
Importance of Engineering Hydrology
- Determines the necessity for managing local water resources.
- Identifies water sources, including their quantity and quality.
- Assesses data completeness and consistency.
- Calculates average rainfall in a region.
- Estimates stream flow resulting from rainfall in catchment areas.
- Defines necessary storage capacity for reservoirs.
- Establishes safe limits for groundwater withdrawal.
- Anticipates extreme hydrological events and their probabilities.
Watershed/Catchment Basin
- A watershed is an area that drains to a common outlet, collecting water into streams.
- The drainage divide separates adjacent watersheds, which can be delineated on topographic maps.
Watershed Water Balance
- Water balance equation: P - R - G - E - T = ΔS (where P is precipitation, R is runoff, G is groundwater flow, E is evaporation, T is transpiration, and ΔS represents changes in storage).
Hydrologic Cycle
- The cycle involves atmospheric moisture, precipitation, evaporation, runoff, infiltration, and groundwater flow.
- Includes various reservoirs: surface, subsurface, and atmospheric.
Aspects of Hydrologic Cycle
- Transportation of water occurs through precipitation, evaporation, transpiration, runoff, and infiltration.
- Temporary storage in depressions, lakes, reservoirs, soil moisture, and groundwater.
- Involves changes of state in water during processes.
Interception
- Interception loss refers to precipitation absorbed by plants or evaporated from leaf surfaces before it reaches the ground.
Interflow and Base Flow
- Interflow may surface before reaching the stream channel, while base flow represents groundwater that contributes to stream flow even during dry spells.
Water Balance Equation
- The equation for water balance: Inflow – Outflow = Change of Storage, expressed as I - Q = dS/dt.
Water Budget Equation
- Gains (Inputs):
- P = precipitation
- I = inflow
- Losses (Outputs):
- E = evaporation
- T = transpiration, often combined as ET
- R = surface runoff
- G = groundwater flow
World Water Quantities
- Fresh water constitutes about 30.3% of Earth's liquid water.
Evaporation Process
- Evaporation transforms liquid to gas below boiling point via heat energy transfer.
- Rate of evaporation influenced by:
- Vapour pressure at the water surface and in the air above
- Air and water temperatures
- Wind speed
- Atmospheric pressure
- Quality of water
- Size of the water body
Evaporimeters
- Devices estimate water evaporation from a surface.
- Methods include:
- Evaporimeter data collection
- Empirical evaporation equations
- Analytical estimation methods
Types of Evaporimeters
- Class A Evaporation Pan
- ISI Standard Pan (IS 5973-1970)
- Colorado Sunken Pan
- USGS Floating Pan
Challenges of Evaporimeters
- Difficulties in leak detection.
- Maintenance of clean surrounding areas is crucial.
- Installation costs can be high.
Evaporation Pan Limitations
- Variation in heat storing and transfer capacity affects results.
- Larger pans (e.g., 3 m diameter) produce values similar to large lakes; smaller pans (1 m diameter) may overestimate evaporation by 20%.
- Rim height impacts wind action and sunlight exposure on the water surface.
- Different heat transfer characteristics compared to natural reservoirs.
Energy Balance and Wind Impact
- Lake evaporation = CP x Pan evaporation (CP = pan coefficient).
- WMO recommends evaporation stations in specific densities based on region:
- Arid zones: One per 30,000 km²
- Humid temperate climates: One per 50,000 km²
- Cold regions: One per 100,000 km²
Empirical Evaporation Equations
-
Dalton-Type Equation:
( E_L = K_f (u)(e_w - e_a) )- ( e_w ): saturated vapour pressure at water temperature
- ( e_a ): actual vapour pressure of air
- ( f(u) ): wind speed correction function
-
Meyer’s Formula:
( E_L = K_M (e_w - e_a) \left(1 + \frac{u^9}{16}\right) )- ( u^9 ): monthly mean wind velocity
- ( K_M ): coefficient (0.36 for deep waters, 0.5 for shallow).
-
Rohwer’s Formula:
( E_L = 0.771(1.465 - 0.000732 p_a)(0.44 + 0.0733u_0)(e_w - e_a) )- ( p_a ): mean barometric reading
- ( u_0 ): mean wind velocity at ground level.
Analytical Methods of Evaporation Estimation
-
Water Budget Method:
( P + V_{IS} + V_{IG} = V_{OS} + V_{OG} + E_L + DS + T_L )- ( P ): daily precipitation
- ( E_L ): daily lake evaporation
- ( DS ): change in lake storage
- ( T_L ): daily transpiration loss.
-
Energy Balance Method:
( H_n = H_a + H_e + H_g + H_s + H_i )- ( H_n ): net energy received
- ( H_e ): heat used in evaporation
- Other terms represent energy transfer, including solar radiation and back radiation.
Bowen’s Ratio
- Ratio estimation for sensible heat transfer:
( b = \frac{H_a}{rLE_L} = \frac{T_w - T_a}{6.1 \cdot 10^{-4} p_a} )- Contextualizes atmospheric pressure and vapor pressures into evaporation assessment.
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Description
Explore the fundamental concepts of Engineering Hydrology, including the estimation of water resources and the study of critical processes such as precipitation and runoff. This quiz delves into the importance of understanding floods and droughts and the necessity of effective water resource management.
