Isotopic, chemical, and crowdsourcing studies of selected water cycle components in arid environments

Abstract

This cumulative thesis comprises five studies (four published, one submitted). The investigations either deal with specific aspects of Saudi Arabia’s water cycle or have a methodological character and the developed methods are particularly useful for arid environments. The first study addresses Riyadh rainfall. Besides major ion analyses, isotope analyses were conducted to calculate a precipitation-weighted Local Meteoric Water Line – a crucial, but hitherto missing reference for past and future groundwater isotope studies. The second investigation focuses on the next element of the water cycle – the unsaturated zone. Stable isotope fingerprints of sand dune pore waters and meteorological data were used in an isotope model to constrain rain events leading to deep infiltration into dune sands. Interestingly, the derived original (i.e., pre-evaporation) isotopic signatures were not represented in the data set on rainfall (see above) – probably due to the temporally limited nature of the latter. This observation calls for a long-term monitoring effort targeting the isotopic signature of precipitation in Riyadh and elsewhere in Saudi Arabia. To go beyond the somewhat generic call for more data, concrete advice on which type of cumulative rain sampler to use for such an endeavor is needed. As post-sampling evaporation has to be minimized, several established and new rain collector designs were tested in a laboratory oven. In this experiment, the Tube-dip-in-water collector with pressure equilibration tube proved to be a reliable and contamination-free option. Building on this knowledge, an automatic counterpart was designed in a fourth study. The microcontroller-based device enables timer-actuated integral rain sampling. The simple low-cost collector is robust and effectively minimizes post-sampling evaporation from the bottles, and the associated isotope fractionation. The excellent performance of the device during an extensive evaporation experiment in a laboratory oven suggests that even multi-week field deployments in warm climates are feasible. The fifth study deals with shallow groundwater. To reconstruct an unexpected water level rise in a Saudi Arabian cave, a crowdsourcing approach was applied. Chronologically sorted YouTube videos recorded in the cave were screened for suitable reference points (e.g., cave graffiti) that appear in several videos. Then, the distances between these points and the water level were visually estimated and their changes were traced over time. The acquired data helped to identify the likely cause of the piezometric changes – two nearby lakes formed from treated sewage effluent. Hopefully, the outlined findings can contribute to a better understanding of the hydrogeological system in Saudi Arabia and can help to identify promising paths for future research. The utilized methods might also be of value for other arid areas.