The relationship between the isotopic composition of precipitation, surface runoff and groundwater for semiarid and arid zones

During the transition from precipitation to surface water or groundwater, and while water moves within such systems, selection and isotope fractionation processes modify the isotope composition of the precipitation. Such modifications result from isotope fractionation that accompanies evaporative processes, and also from selective utilization of rainfall of different duration and intensity and during different seasons in the processes of runoff generation and of groundwater recharge. These modifications are controlled by ecological, morphological and climatic factors. The correlation between the isotopic parameters of the precipitation and those of the surface water and groundwater systems acts on a wide spectrum of temporal and spatial scales; they can characterize the hydrological system and any change informs about anthropogenic or climatic changes in its structure. In the arid zone, evaporation processes from the ground and from within a shallow soil or sand cover dominate. There is an almost momentary response within a very small time and space interval. In this case the system is extremely sensitive to details of the surface structure and to changes due to development and urbanization. In the semiarid zone, in contrast, the degree and the nature of the vegetation cover controls the water balance, and the main factor to change the isotope composition is the seasonal pattern of water utilization by the plants (transpiration). Evaporative fractionation of isotopes occurs mainly on the canopy.