From Envirowiki

Irrigation is the act of artificially increasing the water on a plot of land, usually for agricultural purposes. Methods of irrigation vary widely, and include channel irrigation, flood irrigation, and spray watering.

Irrigation destroys the natural cycles of wet and dry, allowing farmers to farm the same crops year-round. This has numerous problems associated with it, such as increases in weeds and pests (which die off as the crops die off), and human disease, such as Bilharzia (See: Aswan dam)