Engineering Hydrology
Class 5:
Rainfall Abstractions
Evaporation
1
Objectives
Rainfall
ET Overview
and Outline Abstractions
Evaporation:
Mass Balance
Objectives
Be able to:
 Define “abstractions”
 Distinguish between different types of
abstractions
 Use a mass-balance approach to quantify
an abstraction
2
Objectives
Rainfall
ET Overview
and Outline Abstractions
Evaporation:
Mass Balance
What are “abstractions”?
3
Objectives
Rainfall
ET Overview
and Outline Abstractions
Evaporation:
Mass Balance
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
Rainfall
ET Overview
and Outline Abstractions
Evaporation:
Mass Balance
• Interception: Rainfall which is intercepted by and
evaporated from plant canopies or plant residue
• Depression Storage
• Evaporation
• Transpiration
Objectives
Rainfall
ET Overview
and Outline Abstractions
Evaporation:
Mass Balance
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
Rainfall
ET Overview
and Outline Abstractions
Evaporation:
Mass Balance
ET Types







Free water
Lake
Bare Soil
Transpiration
Interception Loss
Potential ET --- This is one of our major goals!
Actual ET
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)
7
Objectives
Rainfall
ET Overview
and Outline Abstractions
Evaporation:
Mass Balance
Example Problem:
Mass Balance Approach
Given a watershed with area =3000 mi2
with average annual precipitation (P)=44 in
and average annual dischage = 2000 ft3/s,
what is the annual evaporation?
Hint: consider all mass fluxes in and out, including
evaporation (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-OGw-∆S. Given what information is provided, what assumptions must
you make?
8
Rainfall Abstractions





Interception
Depression Storage
Infiltration
Evaporation
Transpiration
Interception Demo
Interception
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
9