On-farm green water estimates as a tool for increased food production in water scarce regions

Rainfed agriculture covers > 95 % of the crop lands in water scarce tropica l regions, and will in a forseeable future be the dominating source of food for growing populations in the tropics. Despite this fact, focus has until recently been on water used for irrigated agriculture, industry and househ olds - the so called "blue" water recharging rivers, lakes and aquifers. Yi eld levels have to increase substantially in order to guarantee a minimum o f livelihood security in dry tropical regions. This paper focuses on the po tential of improving the efficient use of rainfall through integrated soil nutrient and water management. Results are presented from an on-farm water balance research project on pearl miller in the Sahel (Niger). The findings indicate that only a very small fraction of the rainfall, 4 - 9 %, takes t he productive water flow path as "green" transpiration water. Surface runof f was substantial, amounting to 25 - 50 % of rainfall for intensive events, despite sandy soils. Soil evaporation amounted to around 50 % of annual ra infall. Drainage was significant in this dryland farming system, despite th e presence of periods of severe water scarcity during critical growth phase s. Very low water use efficiencies were observed, with a range of 4000 - 80 00 m(3) of evapotranspiration water needed to produce 1 ton of grain yield, indicating a large potential for increased yield and productive "green" wa ter flow. An option discussed is water harvesting techniques for supplement ary irrigation. (C) 1999 Elsevier Science Ltd. All rights reserved.