DefinePK

Research Objective

To estimate and verify Reference Evapotranspiration (ETO) values on multiple Land Use Land Cover (LULC) types using three different versions of the Hargreaves equation, and to calculate Land Surface Temperature (LST) and Solar Radiation (SR) using satellite data and empirical formulas.

Methodology

The study utilized three methods for estimating ETO: Hargreaves-Samani Calibrated (HSCAL), Hargreaves-Samani (HS), and Hargreaves. Solar Radiation (SR) was calculated using empirical formulas and the Shuttle Radar Topography Mission (SRTM) 30m Digital Elevation Model (DEM). Land Surface Temperature (LST) was derived from Landsat 8 thermal band data. The HSCAL method used SR calculated from empirical formulas, while HS and Hargreaves used SR calculated from SRTM 30m DEM. ETO obtained from HSCAL was compared with standard FAO-ETO values and then used as a standard for verifying the other two methods across various LULC types. Statistical indicators such as Root Mean Square Error (RMSE), Mean Bias Error (MBE), Mean Absolute Error (MAE), and Correlation Coefficient (R2) were used for evaluation.

Methodology Flowchart

                        graph TD
    A[Acquire Landsat 8 and SRTM DEM Data] --> B[Calculate LST from LS8];
    B --> C[Calculate SR from SRTM DEMfor HS & Hargreaves];
    A --> D[Calculate SR from Empirical Formulasfor HSCAL];
    D --> E[Calibrate Hargreaves-Samani EquationHSCAL];
    C --> F[Apply HS and Hargreaves Equations];
    E --> G[Estimate ETO using HSCAL];
    F --> H[Estimate ETO using HS and Hargreaves];
    G --> I[Compare HSCAL ETO with FAO-ETO];
    H --> J[Compare HS & Hargreaves ETO with HSCAL ETO];
    I --> K[Analyze results by LULC type];
    J --> K;
    K --> L[Evaluate performance using statistical indicatorsRMSE, MBE, MAE, R2];
    L --> M[Draw Conclusions];                    

Discussion

The study highlights the challenge of estimating Reference Evapotranspiration (ETO) when comprehensive meteorological data required for the FAO-56 Penman-Monteith (PM) equation is insufficient. The Hargreaves-based methods offer viable alternatives. The regional calibration of the Hargreaves-Samani equation (HSCAL) proved to be a valuable approach for improving ETO estimation accuracy in data-scarce regions. The comparison of HSCAL with FAO-ETO and the subsequent use of HSCAL as a standard for evaluating HS and Hargreaves methods provided insights into their performance under different LULC types and climatic conditions. The underestimation of ETO by HS and Hargreaves methods in the semi-arid climate zone suggests a need for further refinement or consideration of local conditions.

Key Findings

- The regionally calibrated Hargreaves-Samani (HSCAL) equation, which uses empirically derived solar radiation, showed values mostly within the range of standard FAO-ETO, though often on the lower side.
- Vegetation LULC was found to be the best case for ETO,CAL values, while built-up LULC was the worst.
- During the monsoon season, ETO,HSCAL values tended to fall into the lower class of ranges.
- Both the Hargreaves and HS equations tended to underestimate ETo in the semi-arid climate zone.
- The HS equation showed better performance (lower RMSE, MBE, MAE) compared to the Hargreaves equation when compared against the HSCAL standard, indicating it is closer to the HSCAL estimates.

Conclusion

In areas lacking comprehensive meteorological data, the regionally calibrated Hargreaves-Samani (HSCAL) equation is recommended for estimating ETO in the Punjab Rice Wheat Agroclimatic Zone (PRW-ACZ). The empirical ETO dataset generated by HSCAL facilitates the assessment of satellite-based ETO data at more locations and under diverse conditions than previously possible. The study confirms that while HS and Hargreaves methods can provide ETO estimates, they tend to underestimate values in semi-arid climates, with HS performing better than Hargreaves.

Fact Check

- The study area is 1121501 hectares.
- Landsat 8 (LS8) thermal band was used to calculate Land Surface Temperature (LST).
- Shuttle Radar Topography Mission (SRTM) 30m Digital Elevation Model (DEM) was used for Solar Radiation (SR) calculation in HS and Hargreaves methods.

Mind Map