Is spending money to dig an irrigation well, for which fees are paid, considered an ongoing charity (Sadaqa Jariyah) like digging a drinking well? And is it obligatory to fulfill the first promise despite the new details that have emerged?

The best of charities is providing water, and this is not limited to drinking but also includes watering plants, animals, for purification, and other benefits. Donating to dig a well for watering gardens falls under ongoing charity (sadaqah (definition) jariyah). According to the majority of jurists, it is not legally binding to fulfill a promise of charity, but it is recommended to carry out the charity you intended.