This paper reports the results of a one-year exploratory study of the distribution of deuterium in rainfall, snow, stream runoff, and groundwater in and near the city of Tucson in southern Arizona. The original impetus to make the study arose from a desire to find a way to distinguish between recharge to groundwater in basin alluvium that occurred as a result of summer precipitation from that resulting from winter precipitation. The regional weather pattern is such that most summer moisture is advocated from the Gulf of Mexico and sub-tropical Atlantic Ocean, whereas most winter moisture is advocated from the Pacific Ocean. It was suspected that the deuterium content of summer precipitation is significantly different from that of winter precipitation. Useful information can be obtained on snowmelt-runoff relationships and on the ageing of snow.