Grass Canopy Resistance Response to Climatic Variables and Reference Evapotranspiration Estimate: Analysis by Means of the Penman-Monteith Equation
1993
Todorovic, M.
This thesis describes the crop canopy resistance (rc) as a function of weather variables, and discuss some modelling techniques that can be used to estimate reference evapotranspiration (ETo). The basic framework to make the various analysis is the Penman-Monteith equation. The study is limited to reference grass community and to daily meteorological data. These data, for a period of 8 years (1982-1989), are derived from the Agrometeorological Station of the Bari University experimental field "E. Pantanelli", located in the area of Policoro (Southern Italy). The results showed that in the process of evapotranspiration, scaling up from leaf to canopy -spatial scale- and from short time basis (few minutes to 1 hour) to 24 hours -temporal scale- the importance of climatic (macroscale) variables arises and they are typically defined as the variables that drive the system. Introducing non dimensional concepts in the Penman-Monteith approach, it is possible to derive two critical rc values corresponding to two specific values of the ratio between available energy (A) and latent heat flux of evapotranspiration (LET). These two critical values of the ratio (A/LET) properly define two boundaries and three ranges within which rc differently responds to environment. The models for evapotranspiration (ET) estimate, which use critical resistance values of rc, have the advantage of adjusting ET estimates to the local environment, as opposed to models which use fixed rc value. In the present analysis, the critical canopy resistance corresponding to the ratio A/LET=1, improves the ET estimates on daily basis, as compared to all the other models tested.
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