University of Central Florida Civil and Environmental Engineering CWR4101 Hydrology Laboratory 4A Consumptive Use

DISCUSSION

The terms evapotranspiration and consumptive use are frequently used interchangeably. However, they are synonymous only if evapotranspiration is used to indicate the amount of water consumed (used up and lost by change from a liquid state to a vapor state) in evaporation and transpiration in raising plants.

Evaporation is the process by which water accumulated on the land surfaces (including that held in surface depressions, water bodies, vegetation, etc.) is converted into the vapor state and returned to the atmosphere.

Transpiration is the process by which plants transfer water from the root zone to the leaf surface, where it eventually evaporates into the atmosphere.

Evapotranspiration is the process by which water in the land surface, soil, and vegetation is converted into the vapor state and returned to the atmosphere. Evapotranspiration is usually taken to be the sum of evaporation and transpiration.

Case I
During a precipitation event we are not normally concerned with evaporation and transpiration, but initial abstractions (which later become lost to evaporation) and infiltration (which may later become a part of evaporation and transpiration) in determining rainfall excess.

Case II
During a precipitation event with temporary storage (e.g. retention or detention ponds) evaporation may become an important item in cases where it is desired to maintain the pond in the dry state insofar as possible.

Case III
Inversely, during a normal stream flow condition, we are not concerned with initial abstractions but with infiltration, evaporation and transpiration (if there is significant vegetation along the stream bank).

Case IV
In a situation involving "permanent storage" good reservoir management would require knowledge about rainfall, evaporation, infiltration, and possibly transpiration. Further, use of the "storage" for productive purposes (consumptive use) such as irrigation, water supply and similar uses would require careful analysis of the needs of humans, plants, and animals for the water... i.e. we must have good consumptive use information.

OBJECTIVE

To obtain reliable data on consumptive use for a variety of vegetation types.

EQUIPMENT

Library and internet connection.

PROCEDURE

• A. Review the various indices for reference material on evaporation, transpiration, evapotranspiration, and related topics.
• B. Review pertinent journals...i.e. ASCE Proceedings (esp. Hydraulics, and Irrigation and Drainage); Water Resources Research; Climatological Documents; ASCE and AGU Transactions,etc.
• C. Review appropriate books containing material related to required topics.
• D. Provide consumptive use data and references on the vegetative type of your choice. Include:
1. Values for k= crop coefficient (See Blaney-Criddle)
2. Values for the Transpiration Ratio ( ratio of the weight of water transpired by a plant during the growing season to the weight of dry matter produced by the plant, excluding roots).
3. Record how date was determined by the original investigator (field experiment, water budget calculations, laboratory procedures, etc.)
4. Record the location (latitude and longitude) and elevation if known/given.
5. Record other pertinent research data if given (e.g. insolation, wind movement, temperature, soil type, etc.)

CALCULATIONS

Using the Blaney-Criddle equation (See Chapter 4 of text) calculate an estimated consumptive use requirement for the vegetation type of your choice during a typical growing season. Assume an 80 acre plot. Please identify all assumptions about length of growing season, solar time, location, climatic zone,etc. pertinent to your calculation.