15 © Springer Nature Switzerland AG 2020 S. Ouda et al., Defcit Irrigation, https://doi.org/10.1007/978-3-030-35586-9_2 Chapter 2 Defcit Irrigation and Water Conservation Samiha Ouda and Tahany Noreldin Abstract In this chapter, we tackled some of the concepts and defnitions used in application of defcit irrigation instead of full irrigation as a technology aims at conserving irrigation water. Both water use effciency and water productivity are important estimators in the assessment of the effect of defcit irrigation. Water use effciency serves as a key variable in the assessment of plant responses to water stress induced by defcit irrigation. On the other hand, water productivity is a quan- titative term used to defne the relationship between crop produced and the amount of water involved in crop production. This chapter also reviewed defcit irrigation strategies, which consist of sustain defcit irrigation, regulated defcit irrigation (stage-based and partial root zone irrigation), and subsurface drip irrigation. The hidden role of intercropping systems in irrigation water conservation was also discussed. Keywords Water use effciency · Water productivity · Sustain defcit irrigation · Regulated defcit irrigation · Stage-based and partial root zone irrigation · Subsurface drip irrigation · Intercropping systems 2.1 Introduction Recently, water shortages is prevailing in many parts of the world and threaten its population, where nearly 800 million people lack access to safe drinking water and 2.5 billion have no proper sanitation (Schiermeier 2014). It is expected that this situ- ation will get worse in coming decades, as world’s population is anticipated to increase by 30% in 2050 (Godfray et al. 2010). Thus, providing food for the grow- ing population is expected to be more diffcult in light of the situation of water shortage. Globally, more than 40% of annual food production comes from irrigated S. Ouda (*) · T. Noreldin Water Requirements and Field Irrigation Research Department, Soils, Water and Environment Research Institute, Agricultural Research Center, Giza, Egypt