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Long-Term Trends in Water Supply Forecast Skill: Understanding the Shifts and Implications

Explore the shifts in April 1st water supply forecast skill over time. Skill peaked around 1965-1985 for the Western region, then declined. Recent anomalies highlight Oregon, Colorado, AZ/NM as areas with worsening skill. Spring precipitation irregularity is observed, particularly in PNW and Southwest. The broader implications of climate non-stationarity on water supply forecasting and management are discussed, emphasizing the need to reassess assumptions regarding soil moisture and regression forecasts in a warming climate.

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Long-Term Trends in Water Supply Forecast Skill: Understanding the Shifts and Implications

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  1. Tom.Pagano@por.usda.gov Long-term Trends in Water Supply Forecast Skill Are there any long-term trends in April 1st water supply forecast skill? If so, where, when and why?

  2. Average skill of all sites over 20-yr moving window For the West, as a whole, skill peaked in ~1965-1985 then slumped afterwards Good Bad

  3. Average skill of all sites over 20-yr moving window For the West, as a whole, skill peaked in ~1965-1985 then slumped afterwards Good Bad Good Bad “Synthetic” hindcasts reproduce this feature almost exactly Therefore it’s not the human forecaster

  4. Recent skill anomalies Where has skill been going down? Mostly in Oregon, Colorado, AZ/NM Doing fine in CA/NV/UT Better than usual +.15 0.0 -.15 -.30 Worse than usual

  5. 20-year moving window Spring precipitation “irregularity” More than 1 = more extreme than usual Less than 1 = Calm, reliably near-normal Westwide average of 29 basins Calm Extreme

  6. Calm Typical Extreme 1961-80 1981-00 Where is spring precip more irregular? Now, especially in PNW and Southwest, whereas before it was very calm This matches decline in forecast skill

  7. Of course, the $100,000 question: Will trend continue or return to normal?

  8. Issues of climate non-stationarity Are rising temperatures a problem for water supply forecasts? Or just a problem for water managers?

  9. Summary Skill trends evident, trends dominated by climate Flows more variable, persistent Spring precip is more extreme Need to reconsider assumptions about climate stationarity, in context of “soil moisture” Need to know how a warmed climate affects regression forecasts

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