The very early settlers settled where water was actually available, or easily accessed. Along rivers small or large lakes, anywhere that offered or indicated water. Later settlements/settlers and ranchers could take the time, energy and technology to dig, but there are a lot of environmental clues on where that would be. Lower areas and low areas with an unusual amount of long term plants like trees were usually a pretty good place to sink a well and then build a windmill to pump the water out.