• Deaths
  • Floods and storms resulted in 257,400 deaths
    globally from 1988 to 1997
• Economic Loss
  • $434 billion in damages were caused by floods
    and storms globally from 1988 to 1997
• Water Quality
  • Floods and storms can overload sewage systems
    leading to bacterial contamination in water bodies

• Rainfall
  • A majority of the storms
    occur in the winter months
  • These storms coincide with
    landfalling atmospheric rivers
  • Atmospheric rivers have a strong
    correlation with increased flooding
    (Young et al. 2017)
• Urbanization (Sheng & Wilson 2009)
  • Urban watersheds in Los Angeles County
    lose 90% of rainfall from storms to runoff
  • Impervious surface cover is greater
    than 45% in five Los Angeles watersheds
  • Four of the five watersheds
    have shown significant increases in
    peak stream discharge from 1921 to 2000

• Location
  • Riverside and Orange County is to the north, Imperial
    County is to the east, Mexico is to the south, and
    the Pacific Ocean is to the west
  • Roughly 30 percent is classified as developed land use
  • Urban development is seen in the western third,
    rural areas are seen in the eastern two-thirds,
    and deserts are seen beyond that
• Recent Rain Events and Droughts
  • Tropical Storm Hilary led to $900 million in damages
    in the United States
  • Up to three inches of precipitation fell within a span
    of three hours on January 22, 2024 in San Diego
    • This event had a return period of every 1,000
      years and resulted in severe flash flooding
  • From 2012 to 2014 California experienced an
    unprecedented drought so severe that the return
    interval was estimated at once every 10,000 years

1. Determine if climate change has had a significant
   impact on streamflow characteristics and flood
   behavior in San Diego County
   
   – North and South County sites were
      analyzed to answer this
   
   
   
   
   

2. Determine if watershed urbanization has had a
   significant impact on streamflow characteristics
   and flood behavior in San Diego County
   
   – South and Upstream South County sites
      were analyzed to answer this

• North County
  • USGS Site 11042000:
    San Luis Rey River
    at Oceanside
    
• South County
  • USGS Site 11023000:
    San Diego River at
    Fashion Valley
    
• Upstream South County
  • USGS Site 11022480:
    San Diego River near
    Mast Road at Santee

• Precipitation & Discharge
  • The Parameter-elevation Regressions on Independent Slopes Model (PRISM)
    spatial dataset was used to collect daily precipitation data
  • The dataRetrieval package in R was used to collect daily stream discharge data
    • The grwat package in R was used to separate quickflow from baseflow
  • Time frame: October 1, 1986 to September 30, 2023
  • The Sens slope package in R was used to determine trends over time
• Watershed Characteristics
  • Site locations and watershed delineations were derived from USGS StreamStats
  • Land cover data was collected from USGS LCMAP for both 1985 and 2021
• Flood Days
  • Top 1% rainfall event
    OR
    Top 1% streamflow event

• Top 1% Precipitation
  • Greater in North County than South County for the same return period
  • More events are observed from 2005 to 2023 for both sites
  • Increasing at 2, 4, and 8 year return periods for both sites
    • But not significantly increasing for all return periods
• Top 1% Stream Discharge
  • Greater in South County than North County for the same return period
  • More events are observed from 2005 to 2023 in North County
  • Increasing at 2, 4, and 8 year return periods in North County
    • But remains mostly unchanged for all return periods

• North County
  • Flashiness is not significantly increasing
  • Upstream diversions from 2013 to 2016 led to zero flow
    • Water rights issues were exacerbated due to the drought
    • This could explain why the P-value was not significant
• South County
  • Flashiness is not significantly decreasing
    • Stormwater management improvements could be responsible
      for the observed decrease in flashiness

• Peak Q vs. Peak P
  • South County has a moderate linear correlation: Adjusted R Squared = 0.74
  • Upstream South County also has a moderate linear correlation: Adjusted R Squared = 0.76
  • Both watersheds experience a step change when precipitation > 40 mm/d
• Events: Step Change
  • Initial peak precipitation slightly increases stream discharge (solid line)
  • Final peak precipitation greatly increases stream discharge
    • Soil moisture percentage and dams above Upstream South County are influencing peak discharge during storms
• El Capitan Dam
  • In poor condition and could fail during an earthquake or heavy rainfall event so water levels are kept low (NPR 2022)
  • Short-term repairs are being made (Garrick 2023)
  • Reservoir release in addition to soil saturation may be the cause of the flashiness
  • Future analyses should investigate the relationship between flood frequency and reservoir release

• Upstream South County
  • Flashiness is significantly increasing
    • This alone is not enough to show an increase in flooding
    • Hourly or 15 minute precipitation and stream discharge data is necessary
    • Forest cover loss and its impact on flashiness in San Diego County should
      be explored in future analyses

• North County
  • Flood magnitude is increasing at 2, 4, and 8 year return periods, but I cannot
    definitively say that flood frequency is increasing
• South County
  • Flood magnitude and frequency have not been significantly impacted due to
    climate change and/or watershed urbanization
• Upstream South County
  • Flashiness has been significantly increasing but this alone is not enough to
    demonstrate urbanization’s impacts on flood magnitude and frequency
• Limitations
  • Very few USGS sites have current stream discharge data in San Diego County
  • Hourly and 15 minute data is not current or easily accessible and high resolution
    spatial imagery is not open to the public
• Next Steps
  • Even though significance was not demonstrated in my results, they should prove useful to
    water utility agencies and support further research on flood behavior in Southern California
  • The impacts of forest cover loss, reservoir release, and peak flow timing on flood behavior
    in San Diego County should be explored in future analyses

Cosgrove, William & Rijsberman, Frank & Water, Council. (2000). World Water Vision: Making Water Everybody’s Business. https://doi.org/10.4324/9781315071763.

Young, A. M., Skelly, K. T., & Cordeira, J. M. (2017). High-impact hydrologic events and atmospheric rivers in California: An investigation using the NCEI Storm Events Database. Geophysical Research Letters, 44(7), 3393–3401. https://doi.org/10.1002/2017GL073077

Sheng, J., & Wilson, J. P. (2009). Watershed urbanization and changing flood behavior across the Los Angeles metropolitan region. Natural Hazards, 48(1), 41–57. https://doi.org/10.1007/s11069-008-9241-7

NPR. (2022, May 5). More than 2,200 dams are in poor condition, posing danger to communities. NPR. https://www.npr.org/2022/05/05/1096940224/dams-poor-condition-hazardous-dangerous-infrastructure

Garrick, D. (2023, September 12). San Diego spending $9M on repairs to structurally vulnerable dams—El Capitan, Lake Morena. The San Diego Union-Tribune. https://www.sandiegouniontribune.com/news/politics/story/2023-09-12/san-diego-spending-9m-on-repairs-to-structurally-vulnerable-dams-el-capitan-lake-morena