Drought to Deluge: Ryan Ferrell Presents at AGU Conference in San Francisco

Each year, American Geophysical Union’s (AGU) annual meeting the Advancing Earth and Space Sciences Conference convenes more than 25,000 attendees from over 100 different countries to share research and connect. The goal behind bringing this many scientists, educators, policymakers, journalists, and communicators together is to collaborate and build a better collective understanding of our planet and environment – opening pathways to discovery, greater awareness around climate change and how to address it, and strengthening collaborations to lead to solutions. It’s an incredible gathering of collaboration and action, and this year, Pepperwood’s very own Research Scientist, Ryan Ferrell is presenting!

Ryan presented Pepperwood’s findings around drought and flooding, based on research funded by the Wildlife Conservation Board 2019 Streamflow Enhancement Program and conducted in partnership with the USGS.

Download the Presentation Poster

Drought to Deluge: the Influence of Antecedent Soil Moisture Conditions on Soil Recharge and Runoff in Pacific Coastal Headwaters

Ryan Ferrell (presenter at AGU conference), Tosha Comendant, Michelle Stern, Elisabeth Micheli
Pepperwood Foundation & United States Geologic Survey (USGS)


Antecedent soil moisture has a direct effect on the partitioning of precipitation between runoff and recharge across the landscape. Few sites have adequate instrumentation to close this water balance and accurately measure differences across sites, time, and management regimes. Runoff efficiency (runoff/Precipitation) provides an indicator of antecedent conditions that can be calibrated using a combination of measurements and modeling. Understanding these relationships is critically important for forecasting flood events, adapting to drought events, and managing our forest and water resources.

Study Site & Methods

Pepperwood’s Sentinel Site sits on the southern end of the Mayacamas Mountains in eastern Sonoma County California. The preserve is characterized by a Mediterranean climate with warm dry summers (mean 19.6C), mild wet winters (mean 10.1C), and a mean annual precipitation of 33”. Pepperwood’s Redwood and Weimar watersheds represent two typical headwater catchments of the Russian River, characterized by little to no summer flow and dynamic, responsive wintertime flows (fig. 1,4). Draining towards the northeast these adjacent watersheds range in elevation from ~1500 to 660’ at the outlets and are dominated by northern aspects. Both watersheds are characterized by moderately steep topography- slopes range from 0-40˚with a mean of 17˚. While dominated by mixed hardwood forests both watersheds also have substantial areas of grassland, chaparral, mature conifer forest, and mixed conifer/hardwood environments. Both watersheds burned in the 2017 Tubbs fire.

Pepperwood has developed a comprehensive water balance monitoring network to measure distributed rainfall, soil moisture storage, and streamflow in two headwater systems (fig. 1,2). Leveraging this system, we track water through the hydrologic cycle in both space and time. Tracking the latency between precipitation, increases in soil moisture, and streamflow under differing antecedent soil moisture conditions we develop a relationship between these conditions and runoff efficiency (Runoff/Precipitation) (fig.3). Antecedent soil moisture was calculated for 0-50cm as a watershed average across multiple stations.


  • The correlation between runoff efficiency (RE) and antecedent soil moisture is best explained by an exponential relationship. R2= 0.86. (fig. )
  • Runoff efficiency ranged from 1.4% to 99% corresponding to an increase in antecedent soil moisture from 0.29 to 0.37m3/m3
  • A watershed average soil moisture content in the upper 50cm of soil better explained the variation in runoff efficiency than any individual station or depth.
  • Hydrologic latency from the onset of precipitation to streamflow response ranged from 6.5hours to 0.25hours corresponding to an increase in antecedent soil moisture from 0.29 to 0.37m3/m3 (fig 3)

Conclusions & Future Research

Antecedent soil moisture is a strong predictor of runoff efficiency within the study area. We suggest that these findings are applicable and scalable to the coastal ranges of Northern California. The results of this study in combination with our real-time monitoring network can be leveraged by regional regulators and agencies to inform flood risk lower in the Russian River watershed. Future research efforts will be conducted to bolster this limited data set and explore the role of deep soil moisture (>50cm) in influencing streamflow and runoff efficiency.

Funding for this work and continued efforts provided by the California Wildlife Conservation Board 2019 Streamflow Enhancement Program. Grant no. WC-2000AB

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