Stable Isotopes in Precipitation and Meteoric Water: Sourcing and Tracing the North American Monsoon in Arizona, New Mexico, and Utah

The North American Monsoon (NAM) is an important source of precipitation across the southwestern United States. The approximate northern boundary of this feature crosses the Navajo Nation (NN), in the Four Corners region, where NAM rains have long been important to the livelihoods of Native Americans. Relatively little is known about the characteristics and hydrological significance of the NAM in this region. Here, we report a new 4 yr record of stable H and O isotope ratios in monsoon‐season rainfall and water resources across the NN. Monthly precipitation samples collected at 39 sites document a characteristic pattern of 2H‐ and 18O‐enrichment associated with monsoonal precipitation. These changes are weakly correlated with local precipitation amount, however, and the correlation that does exist is dominated by sub‐cloud evaporation effects. In contrast to precipitation amount, monsoon‐season isotopic values exhibited little spatial variability across the region, and after correction for sub‐cloud evaporation NN values were similar to those from a site in southern Arizona. Airmass back trajectory analysis suggests that the uniformly high NAM precipitation water isotope ratios across the region may reflect (a) a region‐wide shift from mid‐latitude to low‐latitude moisture sources at the onset of the peak monsoon, and (b) substantial land‐surface recycling of NAM moisture in upwind regions. Comparison of precipitation water isotope data with surface and groundwater values implies that, despite its hydroclimatic significance, monsoon rainfall contributes little to subsurface water resources. This highlights the monsoon's importance for warm‐season land‐surface ecology and hydrology critical to residents in the Four Corners region. Plain Language Summary Climate change will impact water resources in many parts of the world. These impacts may be especially severe in dry regions, places with limited infrastructure, and locations where individuals live traditional lifestyles. In order to understand the possible impacts of climate change in such places, we need to understand how different types of weather systems produce precipitation and how precipitation from those systems contributes to water resources. We studied one of two major precipitation‐producing systems in the Navajo Nation: the North American Summer Monsoon. By measuring the isotopic chemistry of rainwater and comparing it with other data from the region, we show that evaporation from the land surfa