1. A Process-Based “Bottom-Up” Approach for Addressing Changing Flood-Climate Relationships COHS Workshop National Academy of Sciences Global Change and Extreme Hydrology: Testing Conventional Wisdom January 5-6, 2010 Katie Hirschboeck

2. How can water managers deal with events in the “tails” of streamflow probability distributions, both floods and droughts,...by moving beyond conventional wisdom and approaches.... “GLOBAL CHANGE & EXTREME HYDROLOGY”

3. are planning for future extreme LOW FLOW conditions using : -- tree-ring reconstructions -- simulations -- scenario-building -- climate projection modeling WESTERN WATER MANAGERS : Water supply simulation based on extreme low flow sequences in the paleo-record STREAMFLOW RECONSTRUCTION for 1330-2005 2002 & 1996 = lowest annual flows in the entire record

4. In contrast... FLOOD HAZARD MANAGERS: have been more constrained in developing ways to incorporate climate change information operationally due to: -- existing flood management policy and practices -- the short-term, localized, and weather-based nature of the flooding process itself

5. Information presented in an operationally useful format for flood managers which describes how changes in the large-scale climatic “drivers” of hydrometeorological extremes will affect flooding variability in SPECIFIC WATERSHEDS  What’s Needed:

6. This presentation argues:... that attention to some very basic elements at the local and regional watershed scale -- such as basin size, storm type, seasonality, atmospheric circulation patterns, and geographic setting... can provide a basis for a cross-scale approach to linking GLOBAL climate variability with LOCAL hydrologic variations...

7. ... let the rivers “speak for themselves” about how they respond to climate ! In other words we will.... Santa Cruz River at Tucson, Arizona

8. I.RE-EXAMINING CONVENTIONAL WISDOM & ASSUMPTIONS: The Standard iid Assumption for FFA II.RE-THINKING New Insights from “Flood Hydroclimatology” III.THE “BOTTOM-UP” APPROACH Complementary Upscaling IV.FINAL THOUGHTS OUTLINE

9. I. RE-EXAMINING

10. http://acwi.gov/hydrology/Frequency/B17bFAQ.html#mixed http://acwi.gov/hydrology/Frequency/B17bFAQ.html#mixed “Flood magnitudes are determined by many factors, in unpredictable combinations. It is conceptually useful to think of the various factors as "populations" and to think of each year's flood as being the result of random selection of a "population”, followed by random drawing of a particular flood magnitude from the selected population.”

11. “ iid ” assumption: independently, identically distributed The standard approach to Flood Frequency Analysis (FFA) assumes stationarity in the time series & “iid” The Standard iid Assumption for FFA

12. Summer convective event Synoptic- scale winter event Tropical storm or other extreme event The type of storm influences the shape of a hydrograph and the magnitude & persistence of the flood peak This can vary with basin size (e.g. convective events are more important flood producers in small drainage basins) Storm type  hydrograph

13. The way in which rainfall is delivered in both space (e.g., storm movement, direction) and time (e.g., rainfall rate, intensity) over drainage basins of different sizes & orographies In addition, extreme flow events can emerge from synergism in: from Doswell et al. (1996)

14. Meteorological & climatological flood-producing mechanisms operate at varying temporal and spatial scales FLOOD-CAUSING MECHANISMS

15. HYDROCLIMATOLOGY  Weather, short time scales  Local / regional spatial scales  Forecasts, real-time warnings vs.  Seasonal / long-term perspective  Site-specific and regional synthesis of flood-causing weather scenarios  Regional linkages/differences identified  Entire flood history context  benchmarks for future events HYDROMETEOROLOGY

16. It all started with a newspaper ad.... Re-Examining the “iid” Assumption

17. THE FFA “FLOOD PROCESSOR” With expanded feed tube – for entering all kinds of flood data including steel chopping, slicing & grating blades – for removing unique physical characteristics, climatic information, and outliers plus plastic mixing blade – to mix the populations together

18. Alternative Conceptual Framework: Time- varying means Time- varying variances Both SOURCE: Hirschboeck, 1988 Mixed frequency distributions may arise from: storm types synoptic patterns ENSO, etc. teleconnections multi-decadal circulation regimes

19. II. RE-THINKING

20. FLOOD HYDROCLIMATOLOGY is the analysis of flood events within the context of their history of variation - in magnitude, frequency, seasonality - over a relatively long period of time - analyzed within the spatial framework of changing combinations of meteorological causative mechanisms SOURCE: Hirschboeck, 1988

21. Flood Hydroclimatology Approach  Meteorological / Mechanistic / Circulation-Linked  Flood Hydroclimatology Framework / Link to Flood Distribution  “ Bottom–Up ” Approach (surface-to-atmosphere)  Observed Gage Record

22. 3 EXAMPLES: Flood Hydroclimatology in AZ

23. Sample Distributions of Peaks-above- Base (Partial Duration Series) events: Are there climatically controlled mixed populations within?

24. Santa Cruz River at Tucson Peak flows separated into 3 hydroclimatic subgroups Hirschboeck et.al. 2000 Tropical storm Sumer Convective Winter Synoptic All Peaks

25. What does this time series look like when classified hydroclimatically? What kinds of storms produced the biggest floods?

26. Hydroclimatically classified time series...

27. Hirschboeck et.al. 2000 Verde River below Tangle Creek Peak flows separated into 3 hydroclimatic subgroups Tropical storm Sumer Convective Winter Synoptic All Peaks

28. Historical Flood

29. Annual flood peaks only: Empirical plotting positions computed separately for each hydroclimatic type Sample frequency curve defined by plotting observed flood magnitudes vs their empirical probability plotting positions, separated by flood type Alila & Mtiraoui 2002 Probability analysis based on hydroclimatically separated flood series

30. Thinking Beyond the Standard iid Assumption for FFA.... Based on these results we can re- envision the underlying probability distribution function for Arizona floods to be not this....

31. Alternative Model to Explain How Flood Magnitudes Vary over Time Schematic for Arizona floods based on different storm types Varying mean and standard deviations due to different causal mechanisms... but this:

32. HOW MIGHT CLIMATE CHANGE AFFECT THESE DISTRIBUTIONS?

33. Some Important Flood- Generating Tropical Storms Tropical storm Octave Oct 1983 Change in Frequency or Intensity of Tropical Storms? Latitudinal Shifts in Winter Storm Track? Roosevelt Dam Jan 1993 Winter flooding on Rillito in Tucson More Intense Summer Monsoon? Sabino Creek July 2006

34. When the dominance of different types of flood- producing circulation patterns changes over time, the probability distributions of potential flooding at any given time (t) may be altered. Conceptual Framework for Circulation Pattern Changes El Nino year La Nina year Blocking Regime Zonal Regime... or this:

35. Conceptual Framework for Low-Frequency Variations and/or Regime Shifts:... or this: A shift in circulation or SST regime (or anomalous persistence of a given regime) will lead to different theoretical frequency / probability distributions over time. Hirschboeck 1988

36. Flood Hydroclimatology for Floods of Record after Costa (1985)

37. Extreme Floods of Record evolved from: uncommon (or unseasonable) locations of typical circulation features (a future manifestation of climate change?) unusual combinations of atmospheric processes rare configurations in circulation patterns (e.g. extreme blocking) exceptional persistence of a specific circulation pattern.

38. Lane Canyon flash flood EXAMPLE: Rare configurations in circulation patterns (extreme blocking)

39. Jimmy Camp Creek flood of 1965 EXAMPLE: exceptional persistence of a specific circulation pattern.

40. OVERALL: Unusually large floods in drainage basins of all sizes are likely to be associated with circulation anomalies involving quasi-stationary patterns such as blocking ridges and cutoff lows in the middle-level flow.

41. III. THE BOTTOM-UP APPROACH

42. Hirschboeck 2003 “Respecting the Drainage Divide” Water Resources Update UCOWR Interpolation of GCM results computed at large spatial scale fields to higher resolution, smaller spatial scale fields, and eventually to watershed processes at the surface. DOWNSCALING

43. “Scaling up from local data is as important as scaling down from globally forced regional models.” — Pulwarty, 2003

44. PROPOSED COMPLEMENTARY APPROACH:

45. RATIONALE FOR PROCESS-SENSITIVE UPSCALING: Attention to climatic driving forces & causes: -- storm type seasonality -- atmospheric circulation patterns with respect to: -- basin size -- watershed boundary / drainage divide -- geographic setting (moisture sources, etc.)... can provide a basis for a cross-scale linkage of GLOBAL climate variability with LOCAL hydrologic variations at the individual basin scale...

46. Process-sensitive upscaling... can define relationships that may not be detected via precipitation downscaling Allows the imprint of a drainage basin’s characteristic mode of interacting with precipitation in a given storm type to be incorporated into the statistics of the flow event’s probability distribution as it is “scaled up” and linked to model output and /or a larger scale flow-generating circulation pattern

47. IV. FINAL THOUGHTS

48. Is this evidence of climate change?

49. Extreme events have a legacy of confounding us!

50. Overall Recommendation: A systematic compilation of watershed- specific information about spatially and temporally varying hydroclimatic extremes is proposed as a starting place for making operationally useful decisions about prospective climatic changes.

51. 1. The impact of climate change on a flood distribution is likely to be more complex than a simple shift in mean or variance 2. Climatic changes can be conceptualized as time-varying atmospheric circulation regimes that generate a mix of shifting streamflow probability distributions over time Recommendation: We need new and evolving statistical tools that can address this behavior.

52. 3. The interactions between storm properties and drainage basin properties also play an important role in the occurrence and magnitude of large floods both regionally and seasonally. Recommendation: Watershed–based hydrometeorology studies should be a key component of watershed and flood management practice.

53. 4. Shifts in storm track locations and other anomalous circulation behavior are clearly linked to unusual flood (and drought ) behavior. They are likely to be the factors most directly responsible for projected increases in hydrologic extremes under a changing climate. Recommendation: Use process-sensitive upscaling to link circulation patterns directly to flood–producing mechanisms and to complement downscaling