Geol_127_Lecture_2_Transpiration, ET

Geol_127_Lecture_2_Transpiration, ET - Geology 127...

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Geology 127 (Hydrogeology) Lecture 2: Hydrologic budget (continued) Transpiration, evapotranspiration, overland flow, interflow - Last time: discussed the hydrologic equation, began to quantify components of the hydrologic cycle. - Today: Will pick up where we left off. 4) Quantifying transpiration: - Measure transpiration with a lysimeter Note: The term lysimeter is also used to refer to a probe that sucks tightly bound pore water out of the unsaturated zone. To avoid confusion, we will call the soil tool a “soil lysimeter”. See handouts and overheads: soil lysimeters - A procedure that is done by outfits like: U.S. Soil Conservation Service, Forest Service. .. (big government agencies) - Lysimeter is difficult, expensive, time-consuming to install - Lysimeter: a large pot that holds plants and soil - Is set outdoors - Is installed on a BIG scale (balance) Draw picture on board - Is designed to accurately reproduce soil type, soil profile, vegetation and soil moisture in the region studied - Need a new lysimeter each time these conditions change - Water is added to the lysimeter (at a rate to keep the soil moist) - Volume (weight) water added is recorded - Entire container is weighed periodically - Weight loss due to evaporation is subtracted (need a nearby evaporation pan (see above) - Remaining weight loss is from transpiration - Apparatus is large, expensive, difficult to operate - A problem: in nature, soil may not be completely saturated with moisture. - One solution: study evaporation and transpiration together, rather than trying to separate the two factors. 5) Quantifying Evapotranspiration - A different approach: a graphic representation of precipitation, evaporation, etc. - Provides different information about water budgets and water use in the shallow unsaturated zone. - Doesn’t try to separate evaporation and transpiration.
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Geol_127_Lecture_2_Transpiration, ET - Geology 127...

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