Review on soil water isotope-based groundwater recharge estimations

Groundwater recharge estimations are necessary for sustainable management of groundwater resources. Among various hydraulic, hydrogeological and hydrological approaches, tracer methods are proposed to improve groundwater recharge estimations. Soil water isotope-based estimates of groundwater recharge rates were developed in the 1960s and have been applied in more than 40 studies reviewed within this manuscript. This comprehensive review research distinguishes three fundamental research approaches: (1) labelling method with tritium or deuterium, which can be used to trace unsaturated zone water movement, (2) tracing soil water movement using a seasonal signal of stable isotopes in precipitation, mainly applied in humid zones, and (3) interpretation of an evaporation signal that impacts soil water stable isotope composition, mainly applicable in arid and semiarid regions. More recently, advances in soil water extraction and analytical methods allow a higher sampling resolution and in situ measurements of soil water stable isotopes. Analytical advances may strengthen and improve the applicability of stable isotope methods for quantitative recharge. We present, compare and discuss studies of unsaturated zone isotope profiles with an emphasis on necessary prerequisites, such as a substantial thick porous sedimentary layer, or negative influences through high clay and silt contents or dry soil conditions. Copyright © 2015 John Wiley & Sons, Ltd.