Evaluation of CatBoost method for prediction of reference evapotranspiration in humid regions

Accurate estimation of reference evapotranspiration (ET0) is critical for water resource management and irrigation scheduling. This study evaluated the potential of a new machine learning algorithm using gradient boosting on decision trees with categorical features support (i.e., CatBoost) for accurately estimating daily ET0 with limited meteorological data in humid regions of China. Two other commonly used machine learning algorithms, Random Forests (RF) and Support Vector Machine (SVM), were also assessed for comparison. Eight input combinations of daily meteorological data [including both complete and incomplete combinations of solar radiation (R-s), maximum and minimum temperatures (T-max and T-min), relative humidity (H-r) and wind speed (U)] from five weather stations during 2001-2015 in South China were applied for model training and testing. The results showed that all the three algorithms could achieve satisfactory accuracy for ET0 estimation in subtropical China using R-s, T-max and T-min, or U, H-r, T-max, and T-min as inputs, under the circumstances of lacking complete meteorological parameters. The increases in testing RMSE and MAPE over training RMSE and MAPE showed positive correlations with the number of input parameters to the machine learning models. For the local models, among the three algorithms, SVM offered the best prediction accuracy and stability with incomplete combinations of meteorological parameters as inputs, while CatBoost performed best with the complete combination of parameters. Patterns of the generalized models were almost the same as the local models, but the former ones showed less than 10% decreases in RMSE or MAPE in comparison with the latter ones. In addition, the computing time and memory usage for data processing of CatBoost were much less than those of RF and SVM. Overall, as a tree-based algorithm, CatBoost made significant improvements in accuracy, stability and computational cost when compared to RF. Therefore, the CatBoost algorithm has a very high potential for ET0 estimation in humid regions of China, and even possibly in other parts of the world with similar humid climates.