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. People assume that removing the livestock would allow the land to recover, but in reality, the complete removal of livestock accelerates the process of desertification. We are losing the land right out from underneath our feet, yet few people have even noticed. For more details please read my on-line article The American Sahara . Allan Savory discovered, or possibly re-discovered, the important link between hooves and the soil.