Rain-fed agriculture with supplementary irrigation, usually referred to as ‘water harvesting on terraced fields’ is the traditional irrigation technique in the Ta'izz area, located in the southern uplands of the Yemen Mountain Massif. The non-traditional method of groundwater irrigation has recently led to overuse and depletion of aquifers. A new orientation towards the traditional agricultural irrigation techniques is necessary for the purpose of sustainability. This requires a better scientific understanding of these irrigation systems. In order to understand the design and management of water harvesting schemes, rainfall analysis and the identification of prevailing rainfall patterns is required. A statistical rainfall analysis was conducted to detect the probability of rainfall exceedance during the vegetation period from May until October. Rainfall supply was then measured against water requirements of the prevailing crop Sorghum bicolore. To explore the beneficial effect of the local runoff irrigation schemes, also referred to as water harvesting schemes, water-harvesting factors were introduced. The rainfall is supplemented by a water-harvesting factor to show the amplifying effect of those water-harvesting measures.
Analysis results show that a lack of rainfall during the end of the first rainy season and an intermediate dry period under pure rain-fed conditions can be mitigated by the local runoff irrigation schemes. A fairly reliable and sufficient rainfall characterizes the second rainy season. Copyright © 2005 John Wiley & Sons, Ltd.