Improving ET/Ep treatment at the CBRFC

1. Improving ET/Ep treatment at the CBRFC

2. ET/Ep project goal • Improve the physics of the treatment of ET (and hence Ep?) in the CBRFC operations. • Improve the water supply forecast skill. • Inter-compare: • ET and Ep from CBRFC operations, • ET and Ep from NLDAS models, • ET and Ep from AA model, • ET from in-region flux tower observations. • Compare ET = f(Prcp,T) relations between CBRFC basins and NLDAS. ET = actual evapotranspiration Ep = potential evaporation Prcp = Precipitation T = surface air temperature

3. Evapotranspiration (ET) estimation techniques • Ep-based: • Epan observations • Ep = f(T) • e.g., Thornthwaite, Hamon, Hargreaves, Blaney-Criddle • Ep = f(T, humidity, U2, Rn, surface parameters) • Penman-related combination equations • Complementary relationship models • 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 Epan = pan evaporation U2 = 2-m wind speed Rn = net SW radiation

4. How is evapotranspiration best estimated operationally? • Observations: • Actual evapotranspiration ET • only as a residual • Pan evaporation Epan • advantages • problems • - RFCs’ usage in SAC-SMA • Ideal fluxes: • Potential evaporation Ep • Reference crop evapotranspiration ETrc • Parameterizations: • Temperature-based • Physically based • - combine advective and radiative drivers • Articles of faith: • Ep is not ET ! • - ET is the supply of water from land to atmosphere (or “actual ET”). • - Ep is the atmospheric demand for water from the surface (or “evaporative demand”). • Ep and ET are not the same • - Ep is the maximal rate of ET. • they relate differently at different time-steps • complementary at annual time-scales. • Ep is driven by ET • - through land-atmosphere feedback mechanisms. ETrc = reference crop evapotranspiration

5. Evaporative flux-types • Actual evapotranspiration (ET) • the actual evaporation rate from any surface under prevailing conditions of moisture availability and radiative input. • Reference crop evapotranspiration (ETrc) • meters atmospheric evaporative demand as the ET that would take place under strictly prescribed biologic and surface moisture conditions: • well-watered grass, 0.12 m high, canopy resistance (rs) of 70 s/m, albedo of 0.23, actively growing, completely shading the ground. • only climatic factors effect changes in ETrc. • Potential evaporation (Ep) • the theoretical evaporation rate from any surface sufficiently moist that air in contact with it is saturated, or the maximal rate at which overpassing air can extract moisture from the terrestrial surface. • Pan evaporation (Epan) • evaporation from a US Class-A evaporation pan defines an Ep rate when scaled to account for the additional energy intercepted by pan walls.

6. Ep rate = f(Epan) ET rate LTZWC UZTWC % of capacity Treatment of ET in NWSRFS: ET from soil moisture • SAC-SMA ET from two sources: • upper zone tension water, UZTW, • lower zone tension water, LZTW. • Ep driven by Epan climatology: • seasonal disaggregated to daily, • missing seasons filled in, • adjustment factor PEADJ calibrated to water balance. • Wetter years in southern CBRFC region (e.g., San Juan), or when/where Prcp drives Runoff, we under-forecast Runoff, or over-estimate ET. • Option to use time-series in SAC-SMA, down to 6-hourly: • best option is to use observed Ep, • cannot use Epan at a daily or sub-daily scale, • best modeling option is Penman-Monteith, • heavy on data inputs/parameters.

7. Treatment of ET in NWSRFS: ET from snow • Sublimation considered in Snow-17 model… • Energy flux accounted for in snowpack – drives snowmelt. • Mass flux NOT passed to SAC-SMA.

8. Other ET work in NWS: WFOs, RFCs, WR-SSD, ABRFC… • Calls for standardized ET, Ep, or ETrc output on www from forecasters, RFCs, extension agents, and within WR: • Colorado Basin RFC: • goal to improve RFC simulations and forecasts. • WFOs’ goals are often to provide ET grids to satisfy AG users: • Great Falls WFO (MT) experimenting with Penman Ep. • Pendleton WFO (ne OR) publishing Kimberly Penman ETrc, • Hanford WFO (SJ valley, CA) publishing Penman-Monteith ETrc. • Sacramento WFO (int. n. CA) publishing Penman-Monteith ETrc, • working with UC Davis and California DWR. • Arizona WFOs All AZ MICs agreed to host ET estimates on their websites • Western Region: • regionwide team from various WFOs to create standardized ET webpage, primarily for ag users, • wants ET knowledge-base in-house, • MSD wants standardized procedure by spring 2010, • WR wants to coordinate their efforts with CBRFC ET work and goals. • Arkansas Basin RFC: • calculates observed Ep with the MAPE preprocessor and its associated SYNTRAN program (see C:\home\mth\Ep_ET\ABRFC\SYNTRAN_program_doc.pdf). Values calculated for selected METAR sites, then each basin's calculation is related to one of these sites. Then modified by Ep adjustment curve.

9. Other ET work in NWS: Office of Hydrologic Development • Reformulating SAC-HT: • Three sources of moisture: • direct evaporation from top soil layer • direct evaporation from canopy interception • transpiration through roots and canopy • Actual ET flux driven by a maximal rate, Ep: • “prefer a simple Ep”, due to RFC constraints, suggested a Hamon Ep • can use dynamic, fully physical Ep • How does our work interact with theirs? • distributed model • operational in GFE ~4 km

10. Who are the users? • agriculture? Irrigators? Water resource managers? City/state/federal agencies? • Are the users educated as to the various products we can offer? • How will they use them? • What products do the users want and/or expect? • ET / Ep / ETrc, do they understand the difference? • time-periods, • spatial resolution, • spatial distinction between ETrc and Ep, • how often updated? • How much of the above can we offer? • Do our own people understand the basics? • e.g., differences between ET, Ep, and ETrc? Research questions: User-related questions

11. Motivation: • why examine ET or Ep? • Data: • input variables: • limited to Prcp and T? • availability of Tdew, SW, LW, U2? • if Uz were used – signal vs. noise in output? • snow cover: daily gridded operational snow cover map from NOHRSC satellite data sets. • can Penman-Monteith ETra-values be derived from land cover? • map ra to NLCD (through d or h)? • can load in a static grid of land cover but the big question is: is there literature that would allow us to change the resistances in the actual Penman equation or use some sort of "crop or vegetation/land use coefficient" to adjust the ETrc based on the land cover grid? Research questions: Science questions

12. Modeling: • do we adjust the treatment of ET within the SAC-SMA? • combine moisture availability from lower and upper zones into one moisture source? • complementarity between Ep and ET : • Do we observe complementarity on a daily scale in current operations? • Do we enforce complementarity in future operations? • Verification: • does comparing observed vs. synthetic Epan offer a check on the radiative parameterization? • check climatology of observed (atlases) vs. synthetic (PenPan) Epan. • matching Ep ~ Epan anomalies • what do we want to check our forecast skill against? • ET ~ hydrology – ET vs. SAC-SMA output • – closing a water balance • - difficult to isolate ET skill from the rest of the modeling in this case. • Other questions: • connection between inter-annual variabilities of Ep and Runoff? • north vs. south, or snow- vs. rain-dominated? • do we need ET from snow? How do we check? • do we need interception model to kick in after Prcp events?0 Research questions: Science questions

13. Ten potential ways forward: Ep, Epan, ET, or ETrc Ep(t) ET(t) Epan(t) T(t) Prcp(t) ET(t) ETrc(t) E[Epan] complementary relationship PEADJ crop coefficient, Kc water stress coefficient, Ks(t) pan coefficient, Kp PE(t) PE(t) ET(t) Existing SAC-SMA model Existing SAC-SMA model Adjusted SAC-SMA model Q(t) Assumption: dynamic series of evaporative driver to SAC-SMA, whether it be ET,Ep, Epan, or ETrc, is preferred over climatology Skill measures?