Engineering Hydrology Class 5: Rainfall Abstractions Evaporation
1 Objectives Rainfall Evaporation: ET Overview and Outline Abstractions 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 Evaporation: ET Overview and Outline Abstractions Mass Balance
What are “abstractions”?
3 Objectives Rainfall Evaporation: ET Overview and Outline Abstractions Mass Balance
Pidwirny, M. (2006). "Intercep on, Stemflow, Canopy Drip, and Throughfall". Fundamentals of Physical Geography, 2nd Edi on. Aug. 25, 2009. h p://www.physicalgeography.net/fundamentals/8k.html Objectives Rainfall Evaporation: ET Overview and Outline Abstractions Mass Balance • Interception: Rainfall which is intercepted by and evaporated from plant canopies or plant residue
• Depression Storage
• Evaporation
• Transpiration Objectives Rainfall Evaporation: ET Overview and Outline Abstractions Mass Balance
Thurow, T.L., W.H. Blackburn, S.D. Warren, and C.A. Taylor, Jr. 1987. Rainfall intercep on by midgrass, shortgrass, and live oak mo es. Journal of Range Management 40:455-460. Objectives Rainfall Evaporation: ET Overview and Outline Abstractions 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 Evaporation: ET Overview and Outline Abstractions 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-O- Gw-∆S. Given what information is provided, what assumptions must you make?
8 Rainfall Abstractions
. Interception . Depression Storage . Infiltration . Evaporation . Transpiration
Interception Demo
Materials: Two separate plants – preferably broadleaf and non-broadleaf Water spray bottle (fan may help!) Interception • 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
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