The Science

Lateral water movement from ridges to valleys plays a key role in organizing water and energy at the watershed scale. But it has been long neglected in traditional land models. In response, scientists developed a water tracer tool in an advanced hydrologic model, WRF-Hydro, to understand the importance of modeling lateral flow in hydrologic simulations. The water tracer tool allows scientists to track precipitation-event water traveling in a watershed until it drains as streamflow at the watershed outlet. By running two sets of simulations with and without lateral flow, the tracer tool revealed different pathways and timescales that event-water travels within a watershed. Specifically, in two contrasting watersheds, scientists found that accounting for lateral flow lengthens the time water spends in the dry watershed, but shortens that in the wet watershed.

The Impact

The new water tracer tool can reveal how water travels through watershed pathways. Here, it is used to understand the effect of incorporating lateral flow on modeling streamflow, but it can also be used by the broader scientific community to examine the responses of water movement to different perturbations. In addition, the water tracer tool applied to a wet watershed suggests substantial underestimation of the time water spends in the watershed compared with the time inferred from isotopic measurements. Testing of the new tool helped scientists identify several aspects for future model improvements.

Summary

Most current land models neglect lateral water movements from ridges to valleys, despite their fundamental role in organizing water and energy at watershed scale. To better understand the importance of modeling lateral flow in hydrologic models, researchers developed a water tracer tool within an advanced hydrologic model WRF-Hydro, or the Weather Research and Forecasting Model Hydrological modeling system, an open-source community model that links atmosphere and terrestrial hydrology models. This tool allows scientists to tag rainwater or snowmelt and track how it travels through a watershed differently if lateral flow is enabled and disabled in the simulations. By testing over two watersheds, researchers found that accounting for lateral flow lengthens the time that water spends in a dry watershed but shortens the time in a wet watershed, indicating different responses to modeling lateral flow in different regions. This new tool can be useful for the broader community to gain a process-level understanding of watershed responses to different forcings. In locations where tracer measurements are available, the modeled results compared with the measured ones can indicate model deficiencies that require future improvements.

PNNL Contact

L. Ruby Leung, Pacific Northwest National Laboratory, ruby.leung@pnnl.gov

Funding

The Department of Energy Office of Science, Biological and Environmental Research supported this research as part of the Regional and Global Model Analysis program area.