Evaluation of Hydrologic Impacts from Ensemble Cloud Seeding Experiments over Southern Wyoming Using the WRF-Hydro Modeling System

The community WRF-Hydro system is a fully distributed hydrologic modeling system that simulates land surface processes, surface and shallow subsurface flow, along with channel flow within river networks. In addition to forecasting vital surface hydrologic states, the WRF-Hydro system can be used in a research framework to study various hydrologic impacts from anthropogenic processes. This study evaluates the impacts on hydrologic states from an ensemble of randomized cloud seeding experiments generated by the WRF model.

For this study, the WRF-Hydro model was setup over a domain that covers the Sierra Madre and Medicine Bow mountain ranges in Southern Wyoming. This region was the focus area for the Wyoming Weather Modification Pilot Program ground-based seeding activities. An ongoing hypothesis is that these seeding activities will enhance snowpack, and subsequent streamflow, in headwater regions, increasing available water resources. As an initial step, key forcing variables from a high resolution (4 km) WRF simulation were processed to the WRF-Hydro domain for several years prior to the cloud seeding experiment period. This was done in order to provide WRF-Hydro with a historical period of forcings and observations to calibrate the model over 300 iterations. An ensemble of cloud seeding experiments was run for the 2009-2013 water years using permutations in both WRF physics and the cloud-seeding parameterization that was developed at the National Center for Atmospheric Research. The changes in precipitation attributed to seeding from each ensemble member were applied to the processed WRF simulation to generate an ensemble of WRF-Hydro forcings that account for the change from each seeding ensemble experiment. WRF-Hydro was then run using each set of ensemble forcings for each water year to generate an ensemble of hydrologic outputs for each ensemble member.

Several variables were evaluated for each ensemble member to study the impact of the cloud seeding experiments on the overall water balance for the target seeding regions. Most of the seeding experiments took place during the cold season in high elevation areas in order to enhance frozen precipitation. This enhanced precipitation is stored as additional snowpack in high elevations. During springtime, this additional snow melts and leads to enhanced streamflow for key headwater regions. Additionally, some of this additional water is lost to enhanced soil moisture and evapotranspiration. This study looks at snow water equivalent, changes in soil moisture, evapotranspiration, and streamflow to evaluate impacts on the overall water budget for the region. This overall framework for simulating cloud seeding experiments in the WRF model, while forcing the outputs through the WRF-Hydro system helps evaluate the downstream impacts of potential cloud seeding scenarios. In addition, the use of ensembles helps quantify the uncertainty and range of impacts from the various seeding scenarios. This information can be critical for agencies planning various seeding activities over the course of a season.