Estimating reservoir Evaporation: Current practice and state-of-the-art Justin Huntington, Associate Research Professor, DRI Agencies Contributing to This Work & Lake Tahoe, NV/CA Workshop Collaborators: Robert Grossman, Katjia Friedrich, Peter Blanken, Ben Livneh

Introduction Open water evaporation is one of the most difficult surface energy/water fluxes to quantify, and is rarely directly measured in the natural environment Reservoir operations and the development of new storage and water accounting strategies require estimates of evaporation and net evaporation (E minus PPT) Changes in open water evaporation under future climate are uncertain, especially if we don’t know current evaporation rates Boca Reservoir, CA – Truckee River Basin

Introduction Primary factors that govern open water evaporation include net radiation heat storage air temperature water surface “skin” temperature humidity wind speed stability of the atmosphere advection of water and heat in and out of the water body salinity All of these factors are important to consider when deciding which technique is most appropriate given the application and data requirements Washoe Lake, NV – Truckee River Basin Windiest valley in Nevada!!

Introduction – Heat Storage Heat storage can alter both the rate and timing of evaporation depending on the volume, geometry, clarity, and the surrounding environment For shallow water bodies, the heat storage impact on seasonal evaporation is minor, however for deep water bodies it can be significant Recent research at Lake Tahoe has found that the peak evaporation is in September-November, rather than summer months Heat storage causes evaporation to be NEGATIVE in the summer over the Great Lakes!! Lake Tahoe temperature profile Figure modified from UC Davis

Introduction – Heat Storage Evaporation is a surface process Heat storage occurs from penetration of solar radiation beneath the water surface Energy not readily available at the surface for immediate consumption by evaporation Heat storage is only available to the surface when transferred by conduction or convection Large amount of energy is transferred to the surface during fall in winter when Tsurface > Tair Significant amount of stored energy can be partitioned into heating the air or long-waver emission rather than to evaporation Important to consider heat storage with any evaporation method..

Introduction – Common Indirect Techniques Common indirect techniques for estimating evaporation include: Pan evaporation and pan coefficients Water budget Energy budget Bulk mass transfer Combination of energy and mass transfer techniques The water budget technique is considered the most accurate indirect approach in arid environments where inflows are minimal and evaporation is a large component of the water budget Folsom Reservoir, CA - American River Funded by Reclamation and Cal. DWR

Introduction – Common Direct Techniques The eddy covariance technique is a direct technique, and considered the most accurate if environmental conditions, physical setting, and experimental design is ideal Eddy Covariance, EC (Enough Corrections!!) Hard to collect data on shore due to fetch issues Hard to collect data over water with float/buoy due moving horizontal plane Subject to energy balance blues… (Rn – G) = (LE + H) Energy fluxes = Turbulent fluxes… but never do.. Eddy Covariance Station at American Falls Reservoir, ID - Snake River Funded by Reclamation

Pan Evaporation Historically, evaporation for operations has been estimated using average pan evaporation data Pan data are widely known to have significant uncertainty both in magnitude and timing Freezing conditions limit use of the pans No heat storage in a pan Often poorly sited and maintained Must determine the pan Coef.. Rye Patch Reservoir, Nevada - Humboldt River Basin

Typical Operations Table – Truckee River

Pan Evaporation Proper pan siting is extremely important Floating Pans? Proper pan siting is extremely important But how do you accurately represent open water and fetch with pans? Floating pans have issues too… Lake Tahoe Pan and NWS COOP Station

Pan Evaporation Example for Tahoe Old Pan Example of pan differences due to pan siting Pier location is double that of the NWS pan station New Pan.. But discontinued.. Figures from Trask (2007)

Energy Balance Evaporation is estimated as a residual of the water body energy balance Most widely used in research Most data intensive and complex approach due the need to consider the entire water body as a control volume rather than just the surface in the case of a land surface energy balance LE is the latent energy consumed for evaporation Rn is the net radiation Qv is the net advected energy to the water body from surface and groundwater inflows and outflows, and direct precipitation (dependent on temperature and amount of flux…linked to water budget!) Qb is the energy exchange from bottom sediments to the water body, H is the energy convected and conducted between the air and water as sensible heat Qx is the energy that is stored in the water body Qw is the energy advected by evaporating water (dependent on temperature and amount of evaporating water) Qp is the energy of precipitation Figure modified from Stannard (2014)

Energy Balance Lake Mead Example Modified from Moreo and Swancar, 2013

Energy Balance Energy Balance - required measurements Net radiometer to measure net radiation Air temperature and vapor gradient over the water Temperature profile to measure heat storage per layer Bathymetry to estimate volume weighted heat storage per layer Inflow and outflow volumes and water temperature Precipitation volume and temperature (usually ignored in arid environments) Sediment heat storage (usually ignored in deep reservoirs) Lahontan Reservoir, NV – Carson River Basin Funded by Reclamation

Energy Balance Example - Walker Lake Walker Lake – USGS energy balance Bowen Ratio station Walker Lake Bowen Ratio Energy Balance Station, Photo by Allander , 2009 (USGS) Funded by Reclamation

Water Balance Inflows and outflows must all be measured or modeled Evaporation can be expressed as a residual of the water budget volume or depth per unit time following the continuity equation for a generalized water body as P is direct precipitation on the water surface SWin and GWin are surface and groundwater inflows SWout and GWout are surface and groundwater outflows B is bank storage DS is the change in storage Inflows and outflows must all be measured or modeled Using modeled inflows, and measured outflows and lake stages can serve as a reality check for other independent methods Figure modified from Robertson et al. (2003)

Aerodynamic / Bulk Mass Transfer Aerodynamic / Bulk Mass Transfer is probably the most widely and commonly used approach for estimating evaporation Function of: surface temperature humidity wind speed atmospheric stability surface roughness thermally induced turbulence barometric pressure and the density and viscosity of the air Stampede Reservoir, CA – Truckee River Basin Funded by Reclamation and DRI

Aerodynamic / Bulk Mass Transfer Dalton’s (1802) general form of the mass transfer equation can be expressed as E is the evaporation rate, es is the saturation vapor pressure at the temperature of the water surface ea is the actual vapor pressure of the air M is the mass transfer coefficient and is a function of wind speed, atmospheric stability, surface roughness, thermally induced turbulence, barometric pressure, and the density and viscosity of the air

Aerodynamic / Bulk Mass Transfer The direct aerodynamic equation: The merit of using the aerodynamic method: Surface temperature can be estimated relatively accurately to estimate qsat qact and windspeed can be measured and/or estimated fairly easily Ce can be estimated using Monin-Obukhov similarity theory or computed (calibrated) from evaporation estimates using Energy Balance or Eddy Covariance approaches Accounts for heat storage by using surface temperature (i.e. seasonal surface temperature variations is the result of heat storage..) Can be applied at the sub-hourly or daily time steps for near real-time operational monitoring.. Energy balance and water balance can only be applied at longer time steps (weekly, monthly, annual..) 𝐸= 𝜌 𝑎 𝐶 𝑒 𝑢 𝑞 𝑠 − 𝑞 a , ra = density of the air u = windspeed qs is specific humidity at saturation, estimated from water surface temperature qa is actual specific humidity, estimated from relative humidity and air temperature measured over water Ce is the exchange coefficient – a function of atmospheric stability, surface roughness, thermally induced turbulence, barometric pressure, and the density and viscosity of the air

Aerodynamic / Bulk Mass Transfer Pros: Probably the most cost effective approach for operational monitoring due to low instrument maintenance needs, low instrument cost, and daily time steps Cons: Theory is not ideal for reservoirs with limited fetch and located in highly advective environments.. But no other approach works well in these situations either.. Data needs to be collected over water

Notable Aerodynamic Study Harbeck (1962) USGS Study

Combination Approaches Combination energy-aerodynamic mass transfer methods are commonly used for land applications and are typically based on Penman (1948; 1956) and Penman-Monteith formulations Radiative component: SW radiation LW radiation Ground heat flux Advective component: Temperature Wind Speed Humidity   λE= Δ Δ+γ 𝑅 𝑛 −𝑄 𝑥 + γ Δ+γ 𝐸 𝐴 Rn = net radiation (shortwave + longwave) Qx = heat storage of water λ = latent heat of vaporization Δ = slope of saturation vapor pressure-temperature curve γ = psychrometric constant EA = drying power of air term f (uz)(es – ea))

Combination Approaches Morton modified Penman’s combination approach for practical and operational estimates of evaporation (Morton, 1983) Morton’s approach is based on Penman’s combination equation with a simple heat storage procedure, and relies on feedbacks between the over passing air and the evaporating surface termed the complementary relationship lake evaporation model (CRLE) (Morton, 1983a,b; Morton et al., 1985; Morton, 1986) Because CRLE requires limited input data (monthly solar radiation, temperature, humidity) the approach has been fairly popular Also, the CRLE is not very sensitive to differences in temperature, humidity, and windspeed from land to water, therefore it overcomes shortcomings of the mass transfer method and combination approach, and instead relies on the strength of the Priestly-Taylor available energy approach in which wind speed and vapor pressure are not used

CRLE Model

CRLE Model

CRLE Model

Approach Fortran program and well documented Recently converted to Python by DRI funded by Reclamation

Annual Evaluation of CRLE Evaluated CRLE model for historical periods - compares well to previous estimates of evaporation (water budget, mass transfer, Bowen ratio energy balance) Average Annual Comparison - Stannard, 2013 Ratio of Average Annual CRLE Estimated to Research E: Range = 0.9 -1.13 Average = 1.02 STD = 0.08 WWCRA - Reclamation, 2015

Seasonal Evaluation of CRLE Compared CRLE to USGS bi-weekly energy balance estimates of E at Upper Klamath Lake (Stannard, 2014) funded by Reclamation WWCRA - Reclamation, 2015

Other CRLE Comparison Studies Most studies found CRLE to be fairly accurate at the annual time step, and less accurate at the seasonal Very useful for water bodies with limited weather and hydrologic data

Limitations of CRLE Advected heat from inflows and outflows should be considered Does not account for advection of heat from surrounding water limited environment (negative H) Does not account for freezing conditions Heat storage function is based on mean water body depth (area weighted depth) and a simple lag function so seasonal evaporation estimates are more uncertain than annual estimates

Seasonal Variation Lake Superior Lake Tahoe Heat storage is a significant source of uncertainty in most models Aerodynamic approaches relies on surface temperature… so it must be observed or modeled… Evaporation can be negative in summer!! Like condensation on a cold beer glass on a hot humid day Lake Superior Lake Tahoe Trask (2007) – UC Davis Dissertation Blanken et al. (2015) – Journal of Great Lakes Research

Heat storage variations for different lakes in Japan Lakes in Japan - Kondo (1994)

What Model To Use? Depends on the situation and data available… Don’t pick a highly empirical model that is known to have issues.. be smart about it.. For a model to be good it must have the right ingredients..

Reality of Uncertainties – Example for Lake Tahoe 3.6 ft 2.6 ft Figure from Trask (2007)

Uncertainties Uncertainties are important to consider… Skin temperature and windspeed (aerodynamic) Heat storage (energy balance) Inflows outflows (water balance / energy balance) Solar radiation / Net radiation (combination energy / aerodynamic) Humidity and temperature (combination / aerodynamic)

Future Directions Combining remote sensing of water surface temperature with gridded weather data and physically based models of evaporation Model has to have the right type and with the right ingredients to be useful for operations Model Samani et al. (2009)

Use Satellite Imagery to Track Freezing Conditions From Huntington and McEvoy (2011)

Landsat TIRS Based Open Water Evaporation Initial tests of Landsat /MODIS TIRS aerodynamic evaporation from Lake Mead compared to USGS eddy flux measurements Photo by M. Moreo - USGS Initial results suggest that a TIRS based aerodynamic approach can simulate open water evaporation fairly well, while capturing the lag in evaporation due to the heat storage effect… Looking into Nov, 2010 and 2011..

How Good is the Gridded Weather Data for Lakes and Reservoirs?? NLDAS/METDATA (Abatzoglou, 2011) comparison to in-situ eddy station weather data at Lake Mead Huntington et al. (2015) – in prep.

How Good is the Gridded Weather Data for Lakes and Reservoirs?? NLDAS/METDATA (Abatzoglou, 2011) comparison to in-situ buoy station weather data at Lake Tahoe -- Bias Correction / Conditioning?? Huntington et al. (2015) – in prep.

Net Evaporation; Net E = E - PPT Ultimately we need to estimate net evaporation for operations and for predicting lake stage Precipitation varies significantly across many lakes We need to measure / model precipitation!! Quality precipitation measurements or modeling is often forgotten about during our rigorous and expensive flux measurement campaigns..

Future Projections of Evaporation Try to understand the past before we understand the future… How do we estimate evaporation in the future using a defensible approach (water budget, energy budget, aerodynamic methods) Water budget requires estimating all future inflows, outflows, and storage changes.. Energy budget requires estimating lots of future variables (net radiation, heat storage, and sensible heat flux… hard ones..), and water inflows and outflows Aerodynamic requires future surface temperature, windspeed, and humidity

Future Projections of Lake Tahoe Evaporation CRLE forced with 112 climate model projections (many estimated variables) Lake Tahoe ensemble median and 5th and 95th percentile annual precipitation, temperature, reservoir evaporation, and net evaporation. WWCRA - Reclamation (2015)

Future Projections of Lake Tahoe Evaporation Lake Tahoe mean monthly ensemble median and 5th and 95th percentile reservoir evaporation and net evaporation. WWCRA - Reclamation (2015)

Need for Representative Data!! In talking about the lack of data to estimate evaporation Morton (1994) says…. “After all, if physicists can discuss such esoteric subjects as quarks and black holes, why can't the environmental scientists know what is happening right under their noses?” “The tragedy is that this misapprehension causes the financial resources and scientific skills to be directed toward the development of techniques that can only provide trivial or dubious answers, at the expense of the kind of long-term original research that is needed before worthwhile answers can be expected”

Recap Common indirect techniques include: Pan evaporation and pan coefficients Proper sitting important Must select pan coefficients based on lake/reservoir characteristics.. Not really reliable.. Water budget Inflows outflows and stages must be measured.. Energy budget Net radiation, heat storage, inflows / ouflows, humidity & temperature gradients must be measured Aerodynamic Surface temp, over water wind, and humidity Combination of energy and mass transfer techniques (all of the above…or use model that estimates.. like CRLE..) DRI / Reclamation / California Department of Water Resources buoy weather station for energy balance and bulk mass transfer evaporation estimates, Folsom Reservoir, American River, CA.

Summary Estimating open water evaporation is not easy!! True measurements of it are rare.. Important that models be chosen wisely based on environmental conditions and available data Weather models and gridded weather data have biases and need to be known Remote sensing of surface temperature can help constrain models or be used directly Perhaps multiple models should be used… but only those that have “good” ingredients..

Thanks – Questions/Suggestions? Justin Huntington, Desert Research Institute, justinh@dri.edu, 775-673-7670