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Relative humidity (RH) has an important effect on precipitation, especially in arid and semiarid regions. Prediction of RH has been the focus of attention of climate change researchers as well. In this investigation, the accuracy of six intelligent models, including an artificial neural network (ANN), a co-active neuro-fuzzy inference system (CANFIS), principal component analysis (PCA) combined with ANN (PCA–ANN) and three hybrid wavelet-artificial intelligence models, including WANN, WCANFIS and WPCA–ANN, was evaluated in daily RH prediction. Thirty weather stations located in different climates in Iran for the period 2000–2010 were selected for the evaluation and comparison of these models. The performance of each model was evaluated using correlation coefficient (r) and normal root mean square error (NRMSE). Based on the statistical evaluation criteria, the accuracy ranks of the six models were: WPCA–ANN, WCANFIS, WANN, PCA–ANN, ANN and CANFIS. The results indicated that the WPCA–ANN model was the optimal model for estimation of RH, and the range of NRMSE and r values were from 0.009 to 0.080 and from 0.996 to 0.999, respectively. Overall, WPCA–ANN is a new approach that can be successfully applied to predict RH with a high accuracy. [ABSTRACT FROM AUTHOR]

• A complementary intelligent model predicts soil temperature using air temperature. • Modeling is conducted over 12 stations located in six different climate regions. • The proposed model shows superior soil-temperature prediction capability. Soil temperature (ST) is an essential catchment property strongly influenced by air temperature (Ta). ST is also the key factor in sustainable agricultural developments, so researchers are still motivated to develop robust machine learning (ML) models to predict ST more reliably. Four different ML models, utilizing the standalone algorithms (i.e., artificial neural networks: 'ANN' and co-active neuro-fuzzy inference systems: 'CANFIS') and complementary algorithms (i.e., wavelet transformation combined with ANN: 'WANN' and wavelet transformation combined with CANFIS: 'WCANFIS') were developed to predict the ST at six meteorological stations incorporating a wide range of climatic features to improve the overall performance. The study has utilized data over the period 2000–2010, collected at 12 locations in Iran. In the first phase of this research, the effects of climate variability on the changes in ST at different depths (i.e., 5, 10, 20, 30, 50 and 100 cm) were explored using air temperature as the exploratory and ST as the response variable. Assessing the performance of the predictive models used in ST prediction, the results indicated good predictive capability of the WCANFIS model, thus, advocating its potential utility in ST prediction problems, especially over diverse climatic regions. This study has also ascertained that the minimum and the maximum predictive errors were encountered at a depth of about 20 cm and 100 cm, respectively. The assessment of climatic features based on air temperature datasets on the performance of the models indicated the highest efficacy demonstrated by the ANN model for the case A–C–W climate type (i.e., a moist climate regime: Arid, temperature regime in winter: Cool, and temperature regime in summer: Warm), in comparison with the PH–C–W climate type (moist regime: Per-humid) for the other best ML models (i.e., WANN, WCANFIS and CANFIS). The order of the model accuracies based on the root mean square error (RMSE) can be ranked with error values of as: WCANFIS = 0.43 °C, ANN = 0.69 °C, CANFIS = 2.16 °C and WANN = 2.31 °C, demonstrating the wavelet-based CANFIS model to exceed the performance of the counterpart comparative models. The present study provides evidence of successfully developing new ML models, improved through wavelet transform for effective feature extraction, and the importance of such hybrid models that have practical implications in studying soil temperature based on air temperature feature inputs in diverse climatic conditions. The outcomes of this study are expected to support key decisions in sustainable agriculture and other related areas where soil health, based on air temperature changes, needs to be monitored or predicted. [ABSTRACT FROM AUTHOR]