Artificial supply of water to land, to maintain or increase yields of food crops, a critical element of modern agriculture. Irrigation can compensate for the naturally variable rate and volume of rain. Water is pumped from natural ponds, lakes, streams, and wells; basin systems and dams hold back larger streams and annual floods. Below the dam, gates to concrete-lined canals are opened, conveying the water over the land through gravity flow. More elaborate, expensive canals flow from huge constructed reservoirs, which hold a year-round water supply. Today portable irrigation systems of lightweight aluminum pipe are in wide use. Drip irrigation, a newer method, uses narrow tubing to supply water directly to the base of each plant. Agricultural irrigation, water towers, and machines invented to lift and distribute water are ancient innovations. Early Egyptians were irrigating with Nile River water by 5000 BC, and such other ancient civilizations as Babylon and China seem to have developed largely as a result of irrigation-based agriculture.