Recovery from over-exploitation can take many years, even centuries. The result may be the land turning into desert. It is interesting to reflect that the Sahara Desert was once the grain basket of the Roman Empire. Irrigation Plants need water and sunlight to grow. Sunlight alone is not enough.
Farmers like to plant beside rivers, where water is abundant.
Farmers begin to water more land, and irrigation systems develop. This has happened many times in many different cultures. But this water, that once would have flowed to the sea, carries dissolved salts and minerals. As an irrigation system reaches its maximum potential, enough water is poured onto the soil to make the plants grow, but there is not enough extra water to flow through the soil and carry the salts and minerals away. Slowly the salts build up in the soil, and plants do not grow so well.