Water isotopic composition traces source and dynamics of water supply in a semi-arid agricultural landscape

Changes in seasonality and form of precipitation alter the structure and function of grassland and steppe ecosystems and pose challenges for land management and crop production in regions like the Northern Great Plains, North America. This research uses isotopic composition of water (δ¹⁸O and δ²H) to explore the sources and fate of soil water in lower-elevation agricultural areas of the Judith River watershed, in the headwaters of the Missouri River, USA. Extensive non-irrigated cereal crop production in this area occurs on well-drained soils and depends on careful water management. Our observations indicate that colder precipitation contributes isotopically distinct water to cultivated terrace soils relative to downgradient groundwaters and streams. Riparian waters also exhibit a higher fraction of contributions from colder precipitation relative to terrace groundwaters and streams. Apparent contributions from colder precipitation in terrace and riparian soil waters suggest that snowmelt is a key component of the water supply to these systems. Riparian waters also show evidence of evaporation suggesting that water spends sufficient time in some ponds and open channels in the riparian corridor to reflect fractionation by evaporation. The evolution of water isotopic composition from soils to shallow aquifers to stream corridors indicates source water partitioning as precipitation moves through this semi-arid agricultural landscape. The apparent mixing processes evident in this evolution reveal source water dynamics that are necessary to understand plant transpiration, solute processing, and contaminant leaching processes.