Post on 07-Apr-2018
transcript
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Engineering Hydrology Class 5:
Rainfall Abstractions Evaporation
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Be able to: Define “abstractions” Distinguish between different types of
abstractions Use a mass-balance approach to quantify
an abstraction
Objectives
Objectives and Outline ET Overview Evaporation:
Mass Balance Rainfall Abstractions
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What are “abstractions”?
Objectives and Outline ET Overview Evaporation:
Mass Balance Rainfall Abstractions
Pidwirny, M. (2006). "Intercep7on, Stemflow, Canopy Drip, and Throughfall". Fundamentals of Physical Geography, 2nd Edi7on. Aug. 25, 2009. hLp://www.physicalgeography.net/fundamentals/8k.html
Objectives and Outline ET Overview Evaporation:
Mass BalanceRainfall
Abstractions
Objectives and Outline ET Overview Evaporation:
Mass BalanceRainfall
Abstractions
• Interception: Rainfall which is intercepted by and evaporated from plant canopies or plant residue
• Depression Storage
• Evaporation
• Transpiration
Thurow, T.L., W.H. Blackburn, S.D. Warren, and C.A. Taylor, Jr. 1987. Rainfall intercep7on by midgrass, shortgrass, and live oak moLes. Journal of Range Management 40:455-‐460.
Objectives and Outline ET Overview Evaporation:
Mass BalanceRainfall
Abstractions
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Free water Lake Bare Soil Transpiration Interception Loss Potential ET --- This is one of our major goals! Actual ET
ET Types
Estimating ET from Lakes Water Balance Mass Transfer (covered in textbook nicely) Direct Measurement (Pan, Eddy Correlation) Energy Balance (covered in textbook nicely) Combination – Energy and Mass Transfer (Penman)
Objectives and Outline ET Overview Evaporation:
Mass Balance Rainfall Abstractions
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Example Problem: Mass Balance Approach
Objectives and Outline ET Overview Evaporation:
Mass Balance Rainfall Abstractions
Given a watershed with area =3000 mi2with average annual precipitation (P)=44 inand average annual dischage = 2000 ft3/s,what is the annual evaporation?
Hint: consider all mass fluxes in and out, includingevaporation (E), inflow (I), precipitation (P), outflow (O), seepage to groundwater (GW), and change in water stored in the watershed (storage, or ∆S). Now, simply balance these components: E=I+P-O-Gw-∆S. Given what information is provided, what assumptions must you make?
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Interception Depression Storage Infiltration Evaporation Transpiration
Rainfall Abstractions
Interception�
Interception Demo Materials: Two separate plants – preferably broadleaf and non-broadleaf Water spray bottle (fan may help!)
• Portion of precipitation that wets above-ground objects• function of: storm size, species age and density, season
Depression Storage• Portion of precipitation that is captured in surface depressions• function of: surface slope and other characteristics