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Evapotranspiration Estimation: The FAO-56 Penman-Monteith Combination Method

 Accurate estimation of reference crop evapotranspiration $(ET_0)$ is essential for computing net irrigation requirements $(ET_c = K_c \cdot ET_0$, where $K_c$ is crop coefficient). The FAO-56 Penman-Monteith Method integrates energy balance principles with aerodynamic transport mechanics, serving as the global standard equation:

$$ET_0 = \frac{0.408 \Delta (R_n - G) + \gamma \frac{900}{T + 273} u_2 (e_s - e_a)}{\Delta + \gamma (1 + 0.34 u_2)}$$

​Where $R_n$ is net radiation at crop surface, $G$ is soil heat flux density, $T$ is mean daily air temperature at $2\text{ m},$ $u_2$ is wind speed at $2\text{ m},$ $e_s - e_a$ is saturation vapor pressure deficit, $\Delta$ is slope of saturation vapor pressure curve, and $\gamma$ is psychrometric constant.

​In water-stressed agricultural command areas across India, relying on regional empirical evaporation pans leads to substantial water over-allocation.

​Under national smart farming initiatives, real-time Penman-Monteith $ET_0$ values are calculated using gridded automatic weather station (AWS) networks. Coupled with thermal infrared satellite imaging from ISRO sensors, these data feeds deliver localized crop-water stress indices directly to farmers' mobile applications to optimize micro-irrigation scheduling.

​Note: This technical content was curated and structured with AI assistance to support technical education.

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