Rain water harvesting is contributed in reducing rainfall losses with evaporation and increasing production of yield, therefore it improved the efficiency water use under water scarcity. The water harvesting system is used to meet the water requirement of crop by increasing water storage in the soil of cropped area. This book explains the methods on the rain water harvesting systems in some countries of the world and Mosul area in North Iraq. The book also reviews the method calculated supplemental irrigation on concept gross water requirement in equations, and discussion the experiments conducted on water harvesting systems and the supplemental irrigation to wheat crop in season 2009/2010 at Mosul area.