Small clay pot (good for that) with a growing plant in the summer with high evaporation means it dries out quicker. The lighter green on the plant usually means it is in less than enough light. Your Echinopsis can stand a decent amount of water in the summer. Mine are outside and enjoy the rains we get here in the Mid-Atlantic.

One more thing, the soil looks peat-y. This might mean that when you water the middle of the pot isn't getting wet and the water flows through without wetting the root mass. Peat dries slowly and is hard to re-wet.