Evapotranspiration Dynamics & Atmospheric Demand: Potential/Actual ET, Penman-Monteith, Dalton's Law & Transpiration Ratios

• Crop Coefficient Approaches in Evapotranspiration (FAO-56):
• Statement (I): Single crop coefficient approach is used for surface irrigation methods. $\implies$ Correct.
• In surface (border, furrow, basin) irrigation where the entire soil surface is wetted periodically, crop transpiration and soil evaporation are combined into a single time-averaged crop coefficient ($K_c$), yielding $ET_c = K_c \cdot ET_0$.
• Statement (II): Dual crop coefficient approach is used for micro irrigation methods. $\implies$ Correct.
• In drip/micro irrigation where only a small fraction of the soil surface is wetted frequently, $K_c$ is partitioned into basal crop transpiration ($K_{cb}$) and soil surface evaporation ($K_e$), yielding $ET_c = (K_{cb} + K_e) ET_0$.
• Conclusion: Both Statement (I) and Statement (II) are correct (Option 1).

Correct option: (1) Both Statement (I) and Statement (I) are correct

• Blaney-Criddle Evapotranspiration Method:
• Statement (I): Blaney-Criddle method uses mean temperature and monthly percentage day time hours. $\implies$ Correct.
• The empirical Blaney-Criddle formula computes consumptive use factor as $f = P(0.46 T + 8.13)$, where $T$ is mean monthly temperature (°C) and $P$ is annual percentage of daytime hours.
• Statement (II): Blaney-Criddle method was developed for the humid condition for consumptive use computation. $\implies$ Incorrect.
• The Blaney-Criddle method was developed specifically for the arid and semi-arid Western United States; it requires significant correction factors when applied to humid regions.
• Conclusion: Statement (I) is correct but Statement (II) is incorrect (Option 3).

• Hierarchy of Controlling Factors in Evapotranspiration:
• In classical micrometeorology and Penman-Monteith energy balance principles, evapotranspiration is hierarchically governed by:

• Moisture availability at surface: Without available water in soil and plant tissues, ET cannot proceed regardless of energy.
• Energy supply capability of atmosphere: Net solar radiation ($R_n$) supplying latent heat of vaporization.
• Vapor transport capability of surroundings: Vapor pressure deficit and aerodynamic wind speed removing humid boundary layers.
• Type and density of vegetation: Crop stomatal resistance and leaf area index.
  • Conclusion: (A), (B), (C), (D) (Option 1).

• Match Crops with Transpiration Ratios:
• Restored Reference Table:

| List-I (Crops) | List-II (Transpiration Ratios) |
| :--- | :--- |
| (A) Sorghum | (III) 250 |
| (B) Wheat | (I) 500 |
| (C) Potatoes | (II) 650 |
| (D) Maize | (IV) 350 |

• Physiological Rationale:
• Transpiration Ratio: Kilograms of water transpired per kilogram of dry matter produced.
• Sorghum ($C_4$): Possesses exceptional water use efficiency, requiring only $\approx 250\text{ kg water/kg dry matter}$ $\implies$ (A) $\rightarrow$ (III).
• Maize ($C_4$): Moderately efficient $C_4$ cereal with a ratio of $\approx 350$ $\implies$ (D) $\rightarrow$ (IV).
• Wheat ($C_3$): Standard temperate $C_3$ cereal with higher photorespiration, ratio $\approx 500$ $\implies$ (B) $\rightarrow$ (I).
• Potatoes ($C_3$): Succulent tuber crop with high transpiration ratio $\approx 650$ $\implies$ (C) $\rightarrow$ (II).
• Conclusion: (A) - (III), (B) - (I), (C) - (II), (D) - (IV) (Option 4).

Correct option: (4) (A) - (II), (B) - (), (C) - (I), (D) - (IV)