Raindrops strike the exposed ground, pulverizing and separating the soil, just like you might find under the drip line of a house. The fine particles of silt, sand and clay dry to form a hard surface crust. Seeds cannot grow through the capped surface, and bare patches develop between the plants. Weeds, brush and grasshoppers thrive in the open patches. New moisture is lost as runoff and may cause floods.
Water bypasses the water table and old springs can dry up.
Freezing and thawing, plus wetting and drying can also cause the top inch of the soil to become so porous and fluffy that seeds dry out before they germinate. Seedling germination in seasonal rainfall environments is always a challenge, even under the best of circumstances. The seeds must be thoroughly mixed with the soil and covered with organic matter and manure fertilizer to hold the moisture and protect them from the sun. Otherwise the delicate seedlings dry out and die before they become established. The more extreme or brittle the climate is the more difficult it is for new seedlings to germinate.