Trends of Productivity of Water in Rain-fed Agriculture Proceedings of the East Africa Integrated River Basin Management Conference Sokoine University of Agriculture, Morogoro Tanzania 9 Trends of productivity of water in rain-fed agriculture Henry E. Igbadun 1 igbadun20@yahoo.com Henry F. Mahoo1 hmahoo10@yahoo.co.uk Andrew K.P.R. Tarimo1 andrewtarimo@yahoo.co.uk Baanda A. Salim1 basalim2000@yahoo.co.uk 1 Department of Agricultural Engineering and Land Planning Sokoine University of Agriculture, Morogoro, Tanzania Abstract In Sub-Saharan Africa, rain-fed agriculture is the dominant source of food production. It is likely to remain so for the foreseeable future. However, yields from rain-fed agriculture are often very low. But there is an enormous opportunity to raise crop yield of rain-fed agriculture especially by focusing on the aspect of increasing productivity of water. In order to formulate and adopt appropriate and adequate options for increasing productivity of water in rain-fed agriculture, there is a need to have an understanding of the trend of productivity of water in rain-fed agriculture. In this paper, an analysis of the trend of productivity of water (PW) for five crops cultivated under rain-fed conditions in Mbarali District, Mbeya Region, Tanzania, was carried out using secondary data. The crops included maize, sorghum, beans, potato, and groundnut. The PW (eta) for maize, sorghum, potato, beans and groundnut had peak values of 0.49kg/m 3 in 1993/94, 0.47 kg/m 3 in 1994/95, 3.07kg/m 3 in 1993/94, 0.33kg/m 3 in 1996/97, and 0.20kg/m 3 in 1994/95 cropping seasons, respectively. Evapotranspiration deficit caused by either mid cropping-season dry spell or early cessation of rainfall and low rainfall utilization efficiency were the primary drivers of the PW in rain-fed agriculture in the area. Key words: Productivity of water, Crop yield, Crop water requirement, Evapotranspiration deficit Introduction About 95% of current world population growth occurs in tropical developing countries with rural economies based on rain-fed agriculture (Rockstrom et al., 2003). In Sub-Saharan Africa, rain-fed agriculture has been the dominant source of food production. It is likely to remain so for the foreseeable future, since more than 95% of the agricultural farmland is under rain-fed agriculture (Parr et al., 1990; Rosengrant et al., 2000). The common characteristics of rain-fed agriculture, especially in the tropical and the semi-arid agroecosystems are low crop yields far below potential yields attainable in the regions, and high on-farm water losses. For example, in tropical and semi-arid Sub-Saharan Africa, cereal yields from rain-fed cultivation have been reported to be generally around 1 t ha -1 (Rockstrom et al., 2003) as against potential yields attainable in the region, which are reported as 3-5 t ha -1 (Barron, 2004). This wide yield gap suggests that there is an enormous opportunity to raise crop yields of rain-fed agriculture. According to McCalla (1994) and Young (1999), new lands that can be put under agriculture are limited, contrary to the last three decades, where the bulk of food production in Sub- Saharan Africa came from expansion of agricultural lands. The opportunities to increase crop yields under rain-fed agriculture strongly rest on focusing our attention on maximizing yield per unit of water applied. In order to formulate and adopt appropriate and adequate options for increasing productivity of water in rain-fed agriculture, it is worthwhile to have an understanding into the performance of this sector from trends analysis of the productivity of water of crops cultivated under rainfall. Such insight will enable us to identify possible factors that dictate productivity of water in rain-fed agriculture and their magnitude. The primary objective of this paper therefore is to show the trends of productivity of water (PW) for some selected crops commonly cultivated under rain-fed agriculture and identify the forces dictating PW. The crops include: maize, sorghum, beans, potato, and groundnut. The case study is that of Mbarali District of Mbeya Region, Tanzania. Methodology The location of the study area