“…In recent years, machine-learning methods have been successfully applied to the estimation of meteorological and hydrological variables, such as air temperature (Cobaner, Citakoglu, Kisi, & Haktanir, 2014;Kisi & Shiri, 2014;Kisi & Sanikhani, 2015a;), solar radiation (Mohammadi, Shamshirband, Kamsin, Lai, & Mansor, 2016), dew point temperature (Kisi, Kim, & Shiri, 2013), soil temperature (Hadi, Deo, Mundher, Okan, & Ozgur, 2018;Kim & Singh, 2014), precipitation (Kisi & Sanikhani, 2015b), and evapotranspiration (Feng, Cui, Zhao, Hu, & Gong, 2016;Gavili, Sanikhani, Kisi, & Mahmoudi, 2017;Sanikhani, Kisi, Maroufpoor, & Yaseen, 2018). A few studies have aimed to predict SM using machine-learning methods.…”