Introducing Water Tracers in the Noah-MP Land Surface Model

Water vapor tracers (WVTs) have been incorporated in global and regional climate models, mainly used to track the atmospheric branch of the hydrologic cycle by tagging the moisture evaporated/transpirated from a specific region. However, the subsequent movement of moisture when precipitation falls onto the land surface is not tracked. Understanding the fate of precipitation on the surface and subsurface is important because it can help us better understand the terrestrial branch of the hydrologic cycle and close the water cycle. In this study, we introduce water tracers into the Noah-MP land surface model. The stocks and fluxes of the water from one specific precipitation event can thus be tracked through different land surface processes. It helps decompose the total water input from precipitation into different components in the land surface. It also helps differentiate the impact of the current event from the effect of antecedent conditions (e.g. soil moisture, snow accumulation). For instance, we applied it to an extreme flooding event over the Chehalis river basin (western Washington) due to an atmospheric river (AR) event. Precipitation over the Cascade Mountains mostly contributes to snow accumulation while that over lower elevated area to the west of the mountains is mostly stored as soil moisture. However, the largest snowmelt event happened during this event happens mostly due to antecedent snow accumulation instead of the snowfall from this AR events.