Vol.2, No.3, 238-247 (2011) doi:10.4236/as.2011.23032 Copyright © 2011 SciRes. Openly accessible at http://www.scirp.org/journal/AS/ Agricultural Sciences Soil fertility effect on water productivity of maize in the upper blue nile basin, Ethiopia Teklu Erkossa * , Seleshi Bekele Awulachew, Denekew Aster International Water Management Institute, Addis Ababa, Ethiopia; * Corresponding Author: t.erkossa@cgiar.org Received 1 July 2011; revised 28 July 2011; accepted 5 August 2011. ABSTRACT Maize (Zea mays) is among the major cereals grown in the high rainfall areas of the sub- Saharan Africa’s (SSA) such as the Ethiopian part of the Blue Nile basin. However, its pro- ductivity is severely constrained by poor soil, water and crop management practices. This study simulated water productivity of the crop under varying soil fertility scenarios (poor, near optimal and non limiting) using hybrid seeds under rainfed conditions using the FAO Aqua- Crop model. The result indicated that grain yield of maize increased from 2.5 tons·ha –1 under poor to 6.4 and 9.2 tons·ha –1 with near optimal and non-limiting soil fertility conditions. Corres- pondingly, soil evaporation decreased from 446 mm to 285 and 204 mm, while transpiration increased from 146 to 268 and 355 mm. Conse- quently, grain water productivity was increased by 48% and 54%, respectively, with the near optimal and non-limiting soil fertility conditions. The water productivity gain mainly comes from reduced evaporation and increased transpi- ration without significantly affecting water left for downstream ecosystem services. Therefore, this has a huge implication for a basin scale water management planning for various pur- poses. Keywords: AquaCrop; Simulation; Water Productivity; Soil Fertility; Nitisols 1. INTRODUCTION Agricultural water management for food and liveli- hood security is a major concern in the face of persistent poverty and rampant environmental degradation in the Sub Saharan Africa (SSA). About 97% of agricultural land in SSA is under rainfed system [1], which will re- main the dominant source of food production in the near future [2]. However, crop yield from rainfed agriculture in the region remains meager (around 1 t·ha –1 ) [3]. This suboptimal performance is due to management problems rather than low potential of the agro-ecosystem [4,5]. In the tropical environment, various abiotic and biotic fac- tors including climatic conditions such as temperature, rainfall, season length and fertility affect crop productiv- ity [6]. There are evidences showing that rainfed agri- culture generates among the world’s highest yields in several regions of the world [7]. Yields in commercial rainfed agriculture in the sub-humid and humid tropical regions may exceed 5 - 6 tons·ha –1 [5]). However, due to the widespread nutrient depletion in agricultural soils exacerbated by improper land use, yield and water pro- ductivity in the rainfed systems in many SSA countries is decreasing or stagnating [7]. Drechsel [8] suggests that nutrient depletion is the chief biophysical factor limiting small-scale production in Africa. In the upper part of the Blue Nile basin, sever land degradation, exacerbated by lack of external inputs such as improved seeds and fertilizers lead to low agricultural productivity. Hitherto, expansion of cultivated land has been the major strategy to cope with the low productivity, population expansion and increased demand for food. However, this strategy is challenged as the agriculturally suitable lands are almost used up, especially in the high- lands. Therefore, technological interventions are indis- pensable to overcome the biophysical constraints and enhance land and water productivity in the area. With its total annual production and productivity ex- ceeding all other cereals (23.24% of 13.7 Million tons), and second after tef (Eragrostis tef) in area coverage (16.12% of the 8.7 million hectares), maize (Zea mays) is one of the most important crops grown in Ethiopia [9]; [10]. It is the most extensively cultivated food crops and main source of calorie in the Ethiopian part of the Blue Nile basin [11]. With the introduction of the hybrid seeds and the high yielding open pollinated varieties, and the increasing local demand, the importance of the crop may increase even further. However, the current national av-