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A Personal History of Evapotranspiration: 10 years in 45 minutes (or fewer)

This presentation by Prof. Jorge Ramírez and Dr. Tom Brown explores the Complementary Relationship (CR) in Evapotranspiration (ET), modeling CR across the continental US, national water supply, trends in modeled and observed ET, evaporation paradox, drought trends, and more.

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A Personal History of Evapotranspiration: 10 years in 45 minutes (or fewer)

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  1. Prof. Jorge Ramírez Dr. Tom Brown Prof. Graham Farquhar Dr. Michael Roderick Dr. Leon Rotstayn Dr. Luc Claessens Dr. Aiguo Dai Prof. Janos Jozsa Prof. Jozsef Szilagyi A Personal History of Evapotranspiration: 10 years in 45 minutes (or fewer) Mike Hobbins

  2. Outline • Complementary Relationship (CR) in Evapotranspiration (ET) • Modeling CR across the continental US • National Water Supply • Trends in modeled ET and its components • Observing CR across the continental US • Trends in observed ET and its components • The Evaporation Paradox • Trends in pan evaporation (Epan) • Dimming and stilling • Drought trends, and how (not) to screw them up • Confusing messages • Demonstrating the power of a simple (bad) assumption • What am I doing here?

  3. Modeling ET with the Complementary Relationship

  4. Complementary relationship models Modeling the Complementary Relationship across CONUS Evapotranspiration (ET) estimation techniques • Ep-based: • Epan observations • Ep = f(T) – Thornthwaite, Hamon, Blaney-Criddle • Ep = f(T, humidity, U2, Rn, surface parameters) – Penman-Monteith • Water budget: • ET = Prcp – Runoff – … • Energy budget-Bowen ratio: • similarity of humidity and T profiles • Flux towers: • direct observations of vapor motion • integrate over very small areas • Satellite-based: • regional scales ET = actual evapotranspiration Ep = potential evaporation Epan = pan evaporation T = surface air temperature U2 = 2-m wind speed Rn = net solar radiation

  5. Modeling the Complementary Relationship across CONUS Complementary relationship hypothesis Bouchet [1963] For uniform surfaces of a regional scale under conditions of constant energy input, when the evapotranspiration rate is limited by the moisture available at the surface, the potential evaporation rate (Ep) increases by the same amount that the actual evapotranspiration rate (ET) is decreased. Bouchet, Proc. IAHS symp., 1963]

  6. Modeling the Complementary Relationship across CONUS Energy budget at an evaporating surface Short-wave radiation balance Long-wave radiation balance Net available energy RG aRG LW↓ LW↑ λET H W t G Ground heat flux

  7. Modeling the Complementary Relationship across CONUS Energy budget at an evaporating surface assumed constant RD = incident direct normal radiation RH = incident diffuse radiation RG = incident global radiation Qn = net available energy

  8. ETwWet Environment Evaporation - rate under conditions where the only limitation is the availability of energy; • ETActual Evapotranspiration - occurs under conditions of limited moisture availability; • EpPotential Evaporation - theoretical rate under conditions of limited moisture availability if the resulting excess in surface energy is used to evaporate further moisture. –q2 +q1 Ep q1 ETw ET rates q2 q1=q2 ET Increasing moisture availability Modeling the Complementary Relationship across CONUS Complementary relationship hypothesis Complementary relationship considers three measures of evaporation:

  9. Modeling the Complementary Relationship across CONUS Advection-aridity (AA) model Simplified wind function Wind function [Brutsaert and Stricker, Water Resources Research, 1979]

  10. ET* = basin-derived ET Modeling the Complementary Relationship across CONUS Calibrating the AA model Calibrating the wind function, f(Ur) Evaluating a and b: 1. ET*-based 2. Epan-based

  11. Modeling the Complementary Relationship across CONUS Estimating ET with the AA model Mean annual modeled ET depths, WY 1953-1994 ET*-based ET Epan-based ET • ~5 km spatial resolution • monthly • WY 1953-1994 • Similar latitudinal trends in eastern US • Similar elevational trends in western US • Highest ET near Gulf of Mexico • ET*-based ET >> Epan-based ET

  12. Modeling the Complementary Relationship across CONUS Product of improved AA model Climatological national water supply (Prcp – ET) depth, WY 1953-1994 • Forest: 53% of U.S. water supply, 66% in West and South. • National forests and grasslands: 18% of water supply nationwide, 51% in West. • All other federal lands contribute another 6% (U.S.) and 15% (West). [Brown et al., Journal of the American Water Resources Association, 2008]

  13. advective forcing (regional) ¶ ¶ radiative forcing (local) dQ dET ET ET dE l = + n A ¶ ¶ dt Q dt E dt n A Traditional paradigm: ET = f(Ep) Complementary relationship: ET = 2ETw - Ep Modeling the Complementary Relationship across CONUS Trends and the complementary relationship hypothesis

  14. Modeling the Complementary Relationship across CONUS Trends and the complementary relationship hypothesis ΔQn> 0 Ep ET rates ETw ΔEA > 0 ΔEA < 0 ET ΔQn < 0 Increasing moisture availability ΔQn = change in local radiative budget ΔEA = change in regional advective budget

  15. Modeling the Complementary Relationship across CONUS Product of improved AA model Trend in annual ET, WY 1953-1994 • Decreasing across 65% of lower-48 states • Spatial mean decrease of 0.57 mm/yr2 [Hobbins et al., Proc. AGU Hydrology Days, 2003]

  16. Modeling the Complementary Relationship across CONUS Trends in components of ET, WY 1953-1994 • Radiative driver, Qn • down 7.5% • Advective driver, EA • down 7.4 mm/yr2 • Annual mean VPD • down 0.52 mb • down across 75% of CONUS • Annual mean U2 • down 0.5 m/sec • down across 95% of CONUS [Hobbins et al., Proc. AGU Hydrology Days, 2003]

  17. Observing the Complementary Relationship

  18. Observing the Complementary Relationship across CONUS Comparing observations of annual Ep and ET Epan ET = Prcp - Runoff [Hobbins et al., Geophysical Research Letters, 2004]

  19. Observing the Complementary Relationship across CONUS Trends in ET components at different scales DET > 0 DEpan > 0 Trends in the radiative budget Qn (mm/yr2) DET < 0 DEpan < 0 Trends in the vapor transfer budget EA (mm/yr2) [Hobbins et al., Geophysical Research Letters, 2004]

  20. Observing the Complementary Relationship across CONUS Explaining trends in basin-derived ET* ET* = Prcp - Runoff

  21. Observing the Complementary Relationship across CONUS Summary and conclusions • Observed long-theorized Epan / ET behavior as a complementary relationship. • Difficulties arise in using Epan observations in annual, continental-scale ET estimates. • Estimating ET in homogenous areas, CR models are preferred over traditional models: • they predict ET directly, • input data reflect surface conditions regardless of water origins or anthropogenic disturbance, • they can be calibrated accurately to predict ET over regional extents, • they implicitly account for the soil moisture-dependence of Ep. • Examined long-term, large-scale trends in ET and its components: • technique allows examination of regional sensitivities to climate change and variability at a variety of spatial breakdowns, • dimming - observed distinct Qn decrease, • stilling - observed non-zero DEA, arising from both DVPD and DU2. • Complementary relationship is an important dynamic in trends in ET.

  22. Observing the Complementary Relationship across CONUS Take-home messages • The Complementary Relationship is important. • Ep and ET are not the same. They rarely scale. • ET drives Ep, not vice-versa.

  23. Pan evaporation paradox

  24. higher VPD greater drought exposure drying Warming Epan is on average decreasing globally, 10-20% in ~50 years: more evaporation hence more Prcp Global warming lower drought exposure Region: USA Canada NW fmr Soviet Union China Tibetan plateau India Eurasia Australia New Zealand Turkey Italy Thailand U.K. Ireland According to: [Peterson et al., Science, 1995; Chattopadhyay and Hulme, AgForMet, 1997; Lawrimore and Peterson, JHydroMet, 2000; Thomas, IntJClim, 2000; Groisman et al., JHydromet, 2004; Golubev et al., GRL, 2001; Moonen et al., AgForMet, 2002; Hobbins et al., GRL, 2004; Liu et al., JGR, 2004; Liu and Zeng, WaterInt, 2004; Ozdogan and Salvucci, WRR, 2004; Roderick and Farquhar, IntJClim, 2004; Roderick and Farquhar, IntJClim, 2005; Tebakari et al., JHydrEng, 2005; Qian et al., GRL, 2006; Wu et al., IntJClim, 2006; Xu et al., JHydrology, 2006; Burn and Hesch, JHydrology, 2007; Kirono and Jones, IntJClim, 2007; Wang et al., TheoAppClim,2007; Zhang et al., JGR, 2007; Jovanovic et al., ClimChange, 2008.] ? wetting The Pan Evaporation Paradox Declining Epan BUT…

  25. The Pan Evaporation Paradox Numerous expressions of the “paradox” • Epan down, BUT: • T rising[Roderick and Farquhar, Science, 2002] • Prcp and cloudiness increasing[Brutsaert and Parlange, Nature, 1998] • GCM-predicted ET increases[Brutsaert and Parlange, Nature, 1998] In the face of climate change, what is driving Epandown?

  26. The Pan Evaporation Paradox Evolution of VPD across CONUS “Warming = drying” paradigm, 1950-1999 • “Increased evaporative demand of the airdue to increasing T will lead to increased exposure to drought, or drying, in continental interiors: • observed over past half-century, • predicted over most mid-latitude continental interiors during the 21st century.” • [IPCC Third Assessment Report, Summary for Policymakers, 2001] Tdew = dewpoint temperature

  27. The Pan Evaporation Paradox Evolution of VPD across CONUS Allowing for Tdew trends, 1950-1999 • Declining VPD in the face of warming! • down 0.52 mb over 42 years • declining down across 75% of CONUS

  28. The Pan Evaporation Paradox Modeling Epan using the “Penpan” model [Roderick et al. Geophysical Research Letters, 2007]

  29. dEp/dt = -2.0 mm/yr2 dEp,Advective/dt = -2.6 mm/yr2 dEp,Radiative/dt = +0.6 mm/yr2 dVPD/dt = -0.2 Pa/yr dEp,VPD/dt = 0.0 mm/yr2 dU2/dt = -0.01 m/sec/yr dEp,U2/dt = -2.7 mm/yr2 The Pan Evaporation Paradox What drives trends in Epan? [Roderick et al. Geophysical Research Letters, 2007]

  30. The Pan Evaporation Paradox Stilling

  31. The Pan Evaporation Paradox Dimming dRn/dt, WY 1953-1994 • dRn/dt = 0.298 watts/m2/yr • 42-year decrease of 12.52 watts/m2, or 14.4% [Hobbins et al. Geophysical Research Letters, 2004]

  32. The Pan Evaporation Paradox Take-home messages • Ep, Epan declining globally, in the face of warming. • Of the drivers of long-term variability of Ep, Epan: • T is not the most significant driver, • most significant is surface wind speed, U2..

  33. Debunking the “Brownhouse” scenario

  34. Debunking the Brownhouse Scenario The Coming Catastrophe “Dustbowl: as temperatures rise and more water evaporates, some regions will become even more arid.” [Good Weekend, Sydney Morning Herald Magazine, 2004]

  35. Debunking the Brownhouse Scenario Message from Australian policymakers to public

  36. Debunking the Brownhouse Scenario Message from science to Australian policymakers Potential evaporation % change, 1990-2030 • Mid-range emissions scenario • Warmer, drier climate expected: warming of ~1 K over 1990 by 2030 • Exceptional Circumstances (EC) triggered by droughts equivalent to current 1-in-20 to -25 year event • By 2010-2040: EC likely 2 X often, over 4 X area [Hennessy et al., CSIRO-BoM,2008]

  37. Debunking the Brownhouse Scenario Global message from science to policymakers • Increased evaporative demand of the airdue to warming => increased exposure to drought, or drying, in continental interiors: • observed over past half-century, • predicted over most mid-latitude continental interiors during the 21st century. • [Summary for Policymakers, Climate Change 2001: The Scientific Basis. • The IPCC WG1 Third Assessment Report, 2001] More intense, longer droughts … observed over wider areas, particularly in the tropics and subtropics since the 1970s. Increased drying due to higher temperatures and decreased land precipitation have contributed to these changes. • [Technical Summary, Climate Change 2007: The Physical Science Basis. • The IPCC WG1 Fourth Assessment Report, 2007]

  38. Debunking the Brownhouse Scenario PDSI values: -3.0 = “very dry” +3.0 = “very wet” PDSI-trends under warming “Increased risk of droughts as anthropogenic warming progresses and produces both increased temperatures and increased drying.” [Dai et al., Journal of Climate, 2004]

  39. Debunking the Brownhouse Scenario Conflicting predictions: increasing terrestrial Prcp, 1925-1999 OBS MODEL + + + + + - + - - - - - - - + + + + + + + - • Models show global Prcp increasing at +2.2% /K • more warming => more Prcp [Zhang et al., Nature, 2007; Held and Soden, Journal of Climate, 2006]

  40. Debunking the Brownhouse Scenario Conflicting predictions: increase in SM and/or Prcp [Sheffield and Wood, Journal of Climate, 2008]

  41. Debunking the Brownhouse Scenario Difference between paradigm and observations/modeling? Question If Epan is declining, why are we saying that the trends in most measures of drought are increasing in continental mid-latitudes? Our hypothesis It is the temperature-based Ep component in the PDSI that leads to the drying “observation” whereas a more-realistic, Epan-based estimate will produce mixed results: drying and wetting.

  42. Debunking the Brownhouse Scenario PDSI bucket model and two Ep parameterizations 2 runs of the water balance component of the PDSI: Pan – observations of Epan = f(T, Tdew, SW and LW radiation, U2) PDSI – T-based Ep= f(T, solar declination, latitude, heat factors) Pan PDSI Prcp Ep ET from root zone ET from plow layer Recharge to root zone Recharge to plow layer Runoff Plow layer Soil moisture in plow layer = 25.4mm max Wetting: Prcp partitions: recharge to plow layer > root zone > ET demand > runoff Drying: 2-stage process: plow layer: ET = Ep root zone: ETi = a.(SMi-1) Root zone Soil moisture in root zone = f(soil texture) [Palmer, U.S. Dept of Commerce, 1965]

  43. EpPan Trend in Ep (mm/year2) Monthly Ep (mm) EpPDSI Trend in T (oC/year) Monthly mean T (oC) Debunking the Brownhouse Scenario Potential evaporation estimators: Ep-T relations compared

  44. 2 runs of PDSI water bucket model EpPDSI ~ f(T, solar declination) EpPan = k.Epan ~ f(T, Tdew, SW and LW radiation, U2) Prcp + dETPDSI = -1.17 mm/yr2 dETPan = -1.11 mm/yr2 Output: annual water-balance trends dSMPDSI= -1.01 mm/yr dSMPan = +0.05 mm/yr Debunking the Brownhouse Scenario Water balances at 35 stations across Australia/New Zealand: Example: Cobar, NSW Prcp Input: monthly P, Ep time-series EpPan and EpPDSI

  45. Debunking the Brownhouse Scenario Secular trends in PDSI model input (T and Prcp) across Australia/New Zealand (1975-2004) T trends (°C/year) 29 (9) warming 6 (0) cooling Aus: +0.01 NZ: +0.02 Prcp trends (mm/yr2) 17 (1) increasing 18 (5) decreasing Aus: +0.10 NZ: -3.24 [Hobbins et al., Geophysical Research Letters, 2008]

  46. Wetness Index: Annual Prcp/Ep EpPDSI-forced trends EpPan-forced trends EpPDSI-forced trends EpPan-forced trends Debunking the Brownhouse Scenario Secular trends in Ep, ET, SM across Australia/New Zealand (1975-2004) Ep trends (mm/yr2) Aus: +1.1Aus: -2.0 NZ: +1.0 NZ: -1.2 Soil moisture trends (mm/yr) Aus: -0.27 Aus: -0.05 NZ: -0.19 NZ: -0.05 ET trends (mm/yr2) Aus: +0.5 Aus: +0.2 NZ: +0.5 NZ: -0.9 No significant relation between SMPan and SMPDSI 7 sites show opposite SM-trends: 5 in SW WA, NSW and Vic. [Hobbins et al., Geophysical Research Letters, 2008]

  47. Budyko curve 1.2 Energy limit 1.0 New Zealand Water limit 0.8 ET / Ep 0.6 0.4 Australia 0.2 0.0 0 1 2 3 4 5 Prcp / Ep Debunking the Brownhouse Scenario General forcing of actual ET: sensitivity of ET to DEp and DPrcp Prcp-forcing (constant Ep) Ep-forcing (constant Prcp) ETPan model Annual ET response (mm) ETPDSI model Annual Prcp anomaly (mm) Annual Ep anomaly (mm)

  48. Debunking the Brownhouse Scenario It’s an old story: Ep-trends under modeled warming [McKenney and Rosenberg, Agricultural and Forest Meteorology, 1993]

  49. Debunking the Brownhouse Scenario It’s not just an Australian story • CONUS - drought trends: • Variable Infiltration Capacity model • 1915-2003 • Prcp, T, U2, Vegetation and Topography • findings: • US generally wetting (exception is Southwest) • SM - 44% of US is wetting, 3% drying (95% sig.) • Runoff - 28% stations increasing, 3% decreasing • droughts shorter, less severe, less frequent, less widespread Annual soil moisture trends [Andreadis and Lettenmaier, Geophysical Research Letters, 2006] Ukraine - SM averaged across 150 stations (JJA): dPrcp/dt = -0.031 mm/yr2 dT/dt = +0.015 K/yr yet: dSM/dt > 0. [Robock et al., Geophysical Research Letters, 2005] China - comparing Ep trends: T increasing in all river basins, SW and U2 decreasing All basins, dEp[Thorn.]dt > 0 80% basins, dEp[Thorn.]/dt opposite to dEp[Pan]/dt [Chen et al., Climate Research, 2005]

  50. Debunking the Brownhouse Scenario Summary and conclusions • Is warming drying? It depends where you’re looking • water- vs. energy-limits on ET • global vs. local • If you’re going to do it, do it right! All driving variables must be considered • Ep is not well characterized by T alone • adding remaining available drivers greatly affects predictions • Hypothesis supported • “drying” forced by temperature-based Ep • pan-based Ep produces drying and wetting • Good Science vs. Simplicity • “warming = drying” is over-simplistic • science permits far more accurate estimates of drought • but may be harder to sell to public/policy-makers

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