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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