ET

ET is a cross-platform component containing routines to estimate daily and hourly values of evapotranspiration from the reference surface, according to alternative approaches. The reference surface for daily computation is, according to the FAO Expert Consultation on Revision of FAO Methodologies for Crop Water Requirements, "a hypothetical grass reference crop with an assumed crop height of 0.12 m, a fixed surface resistance of 70 s m^-1 and an albedo of 0.23". This reference surface closely resembles an extensive surface of green, well-watered grass of uniform height, actively growing and completely shading the ground. The clipped grass reference is considered to be a "cool-season" grass variety such as perennial fescue (Festuca arundinacea Schreb., var. Alta) or ryegrass (Lolium perenne L.). A second reference surface, given by a tall crop with an approximate height of 0.50 m (similar to alfalfa), is also recommended by the Evapotranspiration in Irrigation and Hydrology Commettee of the American Society of Civil Engineers. A standardized form of the FAO-56 implementation of the Penman-Monteith equation is used to estimate daily or hourly evapotranspiration for both reference surfaces. Other methods (Priestley-Taylor and Hargreaves) estimate daily evapotranspiration from limited inputs. A multi-layer canopy is considered to estimate hourly reference evapotranspiration in greenhouse environment (Stanghellini equation), using a well-developed tomato crop, grown in a single glass, Venlo-type greenhouse with hot-water pipe heating. Besides estimates of reference evapotranspiration, the component allows for estimating specific variables related to evapotranspiration.
The component can be freely used and distributed by modellers and developers in their own applications, if the application contains either a dedicated menu item or a dedicated button calling the info() method of the component. This method displays information about the component and a button which activates the help of the component (examples of how to do this are shown in the sample applications provided to illustrate the use of the component).
The component can be used in clients developed for .NET-Windows. The component design allows for expansions by the users who can add their own models. Two sample clients (C#) are provided inclusive of source code to demonstrate how to add models and to build a Win .NET application. A sample web service and a sample web application are available on line as further examples of possible use of the component ET.