OpenET CONUS SIMS Monthly Evapotranspiration v2.0

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conus evapotranspiration gridmet-derived landsat-derived monthly openet water


Satellite Irrigation Management Support

The NASA Satellite Irrigation Management Support (SIMS) model was originally developed to support satellite mapping of crop coefficients and evapotranspiration (ET) from irrigated lands and to increase access to this data to support use in irrigation scheduling and regional assessment of agricultural water needs (Melton et al., 2012). SIMS uses a reflectance based approach and incorporates the density coefficient described by Allen and Pereira (2009) and Pereira et al. (2020) to compute basal crop coefficients for each 30 x 30 m pixel. The primary change from the most recent SIMS publication (Pereira et al., 2020) for implementation in OpenET is the integration of a gridded soil water balance model to account for soil evaporation following precipitation events. Results of the OpenET Phase I intercomparison and accuracy assessment (Melton et al., 2022) showed that SIMS generally performed well for cropland sites during the growing season, but had a persistent low bias during the winter months or other time periods with frequent precipitation. This result was anticipated, since the reflectance-based approach used by SIMS is not sensitive to soil evaporation. To correct for this underestimation, a soil water balance model based on FAO-56 (Allen et al., 1998) was implemented on Google Earth Engine and driven with gridded precipitation data from gridMET to estimate soil evaporation coefficients. These coefficients were then combined with the basal crop coefficients calculated by SIMS to calculate total crop evapotranspiration using the dual crop coefficient approach. In addition, a modest positive bias was observed in the SIMS data for periods with low or sparse vegetative cover. To correct for this bias, updates were made to the equations that calculate the minimum basal crop coefficient to allow lower minimum basal crop coefficient values to be achieved. Full documentation of the SIMS model, current algorithms, and details and equations used in the soil water balance model are included in the SIMS user manual.

The SIMS model calculates ET under well-watered conditions for the current crop growth stage and condition as measured by the satellite data, and SIMS is generally expected to have a positive bias for deficit irrigated crops and croplands with short-term or intermittent crop water stress. At present, SIMS is only implemented for croplands, and non-agricultural lands are masked out in this data collection. Future research will extend the vegetation density-crop coefficient approach used within SIMS to other land cover types. Additional information


30 meters


Name Units Description
et mm

SIMS ET value

count count

Number of cloud free values

Image Properties

Image Properties

Name Type Description
build_date STRING

Date assets were built

cloud_cover_max DOUBLE

Maximum CLOUD_COVER_LAND percent value for Landsat images included in interpolation

collections STRING

List of Landsat collections for Landsat images included in the interpolation

core_version STRING

OpenET core library version

end_date STRING

End date of month

et_reference_band STRING

Band in et_reference_source that contains the daily reference ET data

et_reference_resample STRING

Spatial interpolation mode to resample daily reference ET data

et_reference_source STRING

Collection ID for the daily reference ET data

interp_days DOUBLE

Maximum number of days before and after each image date to include in interpolation

interp_method STRING

Method used to interpolate between Landsat model estimates

interp_source_count DOUBLE

Number of available images in the interpolation source image collection for the target month

mgrs_tile STRING

MGRS grid zone ID

model_name STRING

OpenET model name

model_version STRING

OpenET model version

scale_factor_count DOUBLE

Scaling factor that should be applied to the count band

scale_factor_et DOUBLE

Scaling factor that should be applied to the et band

start_date STRING

Start date of month

Terms of Use

Terms of Use



  • Melton, F., Huntington, J., Grimm, R., Herring, J., Hall, M., Rollison, D., Erickson, T., Allen, R., Anderson, M., Fisher, J., Kilic, A., Senay, G., volk, J., Hain, C., Johnson, L., Ruhoff, A., Blanenau, P., Bromley, M., Carrara, W., Daudert, B., Doherty, C., Dunkerly, C., Friedrichs, M., Guzman, A., Halverson, G., Hansen, J., Harding, J., Kang, Y., Ketchum, D., Minor, B., Morton, C., Revelle, P., Ortega-Salazar, S., Ott, T., Ozdogon, M., Schull, M., Wang, T., Yang, Y., Anderson, R., 2021. "OpenET: Filling a Critical Data Gap in Water Management for the Western United States. "Journal of the American Water Resources Association, 58(6), pp.971-994. doi:10.1111/1752-1688.12956

  • Pereira, L.S., P. Paredes, F.S. Melton, L.F. Johnson, R. López-Urrea, J. Cancela, and R.G. Allen. 2020. "Prediction of Basal Crop Coefficients from Fraction of Ground Cover and Height." Agricultural Water Management, Special Issue on Updates to the FAO56 Crop Water Requirements Method 241, 106197. doi:10.1016/j.agwat.2020.106197

  • Melton, F.S., L.F. Johnson, C.P. Lund, L.L. Pierce, A.R. Michaelis, S.H. Hiatt, A. Guzman et al. 2012. "Satellite Irrigation Management Support with the Terrestrial Observation and Prediction System: A Framework for Integration of Satellite and Surface Observations to Support Improvements in Agricultural Water Resource Management.IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing 5 (6): 1709–21. doi:10.1109/JSTARS.2012.2214474

  • Allen, R.G. and Pereira, L.S., 2009. Estimating crop coefficients from fraction of ground cover and height. Irrigation Science, 28, pp.17-34. doi:10.1007/s00271-009-0182-z

  • Allen, R.G., Pereira, L.S., Raes, D. and Smith, M., 1998. Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. Fao, Rome, 300 (9), p.D05109.


Explore with Earth Engine

Code Editor (JavaScript)

var dataset = ee.ImageCollection('OpenET/SIMS/CONUS/GRIDMET/MONTHLY/v2_0')
  .filterDate('2020-01-01', '2021-01-01');

// Compute the annual evapotranspiration (ET) as the sum of the monthly ET
// images for the year.
var et ='et').sum();

var visualization = {
  min: 0,
  max: 1400,
  palette: [
    '9e6212', 'ac7d1d', 'ba9829', 'c8b434', 'd6cf40', 'bed44b', '9fcb51',
    '80c256', '61b95c', '42b062', '45b677', '49bc8d', '4dc2a2', '51c8b8',
    '55cece', '4db4ba', '459aa7', '3d8094', '356681', '2d4c6e',

Map.setCenter(-100, 38, 5);

Map.addLayer(et, visualization, 'OpenET SIMS Annual ET');
Open in Code Editor