These plants are Cooperia drummondii in the family Liliaceae.
They are called rain lilies because they appear after an extended rain. These pictures were taken on an overcast days at 6pm one evening in early September and 11am the next morning. The blooms last for only 4 days before withering to a pinkish color, like the right-most bloom in the pictures. These were found in a large grassy area under large oak and Pecan trees. This species produces flower stalks about 12 inches long from underground bulbs. After flowering, they produce grasslike gray green leaves and become practically invisible in a short grass environment like the one pictured here. I was suprised to find these popping up all over the grassy parkland which was regularly mowed. An interesting life-cycle, and arid adaptations.