idwr metric applications panel 1 how it works

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HOW METRIC WORKS: 

Applications of the METRIC Evapotranspiration Model at the Idaho Department of Water Resources Richard G. Allen University of Idaho Kimberly, Idaho HOW METRIC WORKS Anthony Morse and William J. Kramber Idaho Department of Water Resources Boise, Idaho General Principle METRIC (Mapping Evapotranspiration at high Resolution and with Internalized Calibration) is an image-processing tool for calculating evapotranspiration (ET) as a residual of the energy balance at the earth’s surface using the equation ET = Rn-G-H where ET is evapotrans- piration, Rn is net radiation, G is sensible heat flux conducted into the ground, and H is sensible heat flux convected into the air as illustrated by Fig. 1. The fundamental principle underlying METRIC is that evaporating liquids absorb heat METRIC is a variant of SEBAL, an energy balance model developed in the Netherlands and applied worldwide by Bastiaanssen (1995, 1998 a,b, 2000, 2004). METRIC has been extended to provide tighter integration with ground-based reference ET and has been applied with Landsat images in southern Idaho to predict 24-hour, monthly, and seasonal ET for a variety of applications as described in panels 2 and 3. n Internal Calibration Internal calibration of the sensible heat computation within METRIC eliminates the need for atmospheric correction of temperature or albedo. The internal calibration also reduces impacts of any biases in estimation of aerodynamic stability correction or surface roughness. The calibration is done by manually picking a hot and a cold pixel to define the range of ET (Fig. 2). Cold Pixel Hot Pixel 0 Maximum ET Figure 2. Thermal histogram and the hot and cold pixels for an image. Figure 1. 24-Hour ET METRIC computes 24-hour ET from the essentially instantaneous ET calculated at the time of the satellite image using reference ET fraction (ETrF). ETrF is the ratio of ET to ETr, which is alfalfa reference ET. ETr is essentially the same as the well-known crop coefficient, Kc. The assumption of constant ETrF during a day captures impacts of advection and changing wind and humidity conditions during the day, as illustrated by Figure 3. Accuracy Energy-balance ET models have proven to be robust tools for computing and mapping ET. Bastiaanssen, et al. (2007) summarize the results of 19 studies that include verification of results from METRIC and/or SEBAL. The 19 studies have an average of 4.5% disagreement with other, ground-based methods of measuring ET. Interpolation to Monthly and Seasonal ET The 24-hour ET can be interpolated to a cumulative ET for monthly or seasonal periods, or for any arbitrary period within a season. For any given period, the ETrF is computed by Formula 1, where m and n are the starting and ending dates, respectively, which are usually halfway between two Landsat overpass dates. For the same period, the cumulative ET is computed using Formula 2. Formula 1. Computation for reference ET fraction for any given period. Formula 2. Computation for ET for a given period. Figure 3. The diurnal relationship between ET and ETrF for sugar beets. ETrF ETr Sugar Beets ET References Bastiaanssen W.G.M. (1995) Regionalization of surface flux densities and moisture indicators in composite terrain: A remote sensing approach under clear skies in Mediterranean climates. Ph.D. dissertation, CIP Data Koninklijke Bibliotheek, Den Haag, The Netherlands. 273 p. Bastiaanssen, W.G.M., M. Menenti, R.A. Feddes, and A.A.M. Holtslag. (1998a.) A remote sensing surface energy balance algorithm for land (SEBAL): 1. Formulation. Journal of Hydrology, 212-213, p. 198-212.   Bastiaanssen, W.G.M. et al (1998b) The Surface Energy Balance Algorithm for Land (SEBAL): Part 2 validation, Journal of Hydrology , 212-213: 213-229 Bastiaanssen, W.G.M. ( 2000) SEBAL-based sensible and latent heat fluxes in the irrigated Gediz Basin, Turkey. Journal of Hydrology, 229:87-100. Bastiaanssen, W.G.M., et al. (2005) SEBAL Model with Remotely Sensed Data to Improve Water Resources Management Under Actual Field Conditions, ASCE J. of Irrigation and Drainage Engineering. Vol. 131 Bastiaanssen, W.G.M., et al. (2007) Estimation of groundwater extraction on irrigated lands in arid zones from thermal infrared satellites. J. GeoHydrology. (in press)

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Bastiaanssen W.G.M. (1995) Regionalization of surface flux densities and moisture indicators in composite terrain: A remote sensing approach under clear skies in Mediterranean climates. Ph.D. dissertation, CIP Data Koninklijke Bibliotheek, Den Haag, The Netherlands. 273 p. Bastiaanssen, W.G.M., M. Menenti, R.A. Feddes, and A.A.M. Holtslag. (1998a.) A remote sensing surface energy balance algorithm for land (SEBAL): 1. Formulation. J. Hydrology, 212-213, p. 198-212.   Bastiaanssen, W.G.M. et al (1998b) The Surface Energy Balance Algorithm for Land (SEBAL): Part 2 validation, J. Hydrology , 212-213: 213-229 Bastiaanssen, W.G.M. ( 2000) SEBAL-based sensible and latent heat fluxes in the irrigated Gediz Basin, Turkey. J. Hydrology, 229:87-100. Bastiaanssen W.G.M. et al. (2004) SEBAL for spatially distributed ET under actual management and growing conditions, ASCE J. of Irrigation and Drainage Engineering (in press) Bastiaanssen, W.G., R. Chavez, A. Alsulaimain and M. Ahmad. 2007. Estimation of groundwater extraction on irrigated lands in arid zones from thermal infrared satellites. J. GeoHydrology. (in press)

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