A single inlet/single outlet modular counterflow cooling tower having two heat transfer sections installed atop two cold water basin sections and below three fan sections, each heat transfer section having its own water distribution system and draining into its own distinct cold water basin section. The water distribution system can provide flow over both heat transfer sections or over only a single section. The center fan support section supports the mechanical drive system for the fan and has a sealing plate at its bottom for sealing the gap between the two heat transfer sections.