Step 1: Review the historical evolution of evapotranspiration (ET) estimation techniques.
Methods progressed from simple temperature-based empirical formulas to advanced models incorporating radiation, wind, and plant physiology.
Step 2: Position each method within its historical timeline.
- A. Blaney and Criddle (1950): An early, simple empirical method relying mainly on temperature and daylight hours.
- D. Modified Penman (1963 by FAO, based on Penman 1948): Howard Penman's 1948 combination method was a significant advancement. The "Modified Penman" typically refers to the FAO-24 version, which featured adjusted coefficients and became a widely adopted standard. It emerged after Blaney-Criddle but predated the broad implementation of specific radiation-based methods or the Penman-Monteith refinement.
- B. Radiation (e.g., Jensen-Haise 1963, Priestley-Taylor 1972): Methods primarily driven by solar radiation as the main ET factor were developed as an alternative to the more data-intensive Penman method. These generally followed the original Penman work.
- C. Penman-Monteith (1965): A further development of the Penman equation, it introduced a surface resistance term to better represent the physiological control of transpiration by plants. This method is now recognized as the global standard (FAO-56 Penman-Monteith).
The chronological order is: Blaney and Criddle \( \rightarrow \) Modified Penman \( \rightarrow \) Radiation \( \rightarrow \) Penman-Monteith. This corresponds to A, D, B, C.