The Replenishment of underground water is essential for combating water scarcity as global water sources face growing pressure. Arid and semi-arid regions worldwide face severe water scarcity due to low rainfall and increasing population, resulting in higher demand for water resources. Groundwater resources, often secondary sources, are in a vulnerable phase due to the actions of both natural variability and anthropogenic interventions. The main aim of this chapter is to devise a method for accurately determining the rate at which underground water is replenished in dry regions experiencing rapid groundwater depletion. This chapter covers (1) how to assess the zone where replenishment is needed and find water sources using geological, climatic, and remote sensing data; (2) how to investigate an aquifer property through the analysis of groundwater isotopes; and (3) evaluation of water resources sustainability by quantifying replenishment volume using distinct hydrological/hydraulic models. This chapter, it is also discussing how changes in local water extraction practices, shifts in rainfall patterns, and the expansion of irrigated areas influence water storage, depletion, and replenishment dynamics over time and highlights the valuable insights gained from understanding the complex interactions that affect the underground water behaves, essential for managing groundwater sustainably in arid and semi-arid regions.