Many of the cool season vegetable seeds planted at the beginning of the season are small and can't be planted deep. These include broccoli, onions, lettuce, kale and radish. Being near the surface leaves them vulnerable to drying out during the days-to-germination period. If seeds start germination and then dry before emergence, they often perish.
What to do? In addition to frequent, light watering, the use of straw mulch or germination blankets can help. A useful germination blanket is spun polyester floating row cover fabric tacked down with wire U-pins. These fabrics gerenally allow 85% light transmission as well as passage of moisture through to the soil.
Blanket removal immediately upon seedling emergence is not mandatory and can be delayed until several true leaves have formed and seedlings are well established - that is if fabric is anchored loose enough to "float".
Photo credit: Floating row cover seed germination blanket, Carl Wilson