ET-IDWR

The ET-IDWR website provides estimates of evapotranspiration (ET), net irrigation requirements (NIR), and effective precipitation supporting ET (PEff). These data and this web site were produced in 2007 and were updated in 2009, 2012, 2017, and 2023. ET calculation procedures employ a modern reference equation (ASCE standardized Penman-Monteith method) and a modern procedure to calculate crop coefficients that considers the impact of surface wetting by irrigation and precipitation on total evaporation from the soil surface. ET is provided for daily, monthly, and annual timesteps for 208 weather station locations across Idaho and adjoining states. In addition to ET and NIR estimates for agricultural crops grown in Idaho, ET estimates are available for a number of native plant systems (wetlands, rangelands, and riparian areas) and open water surfaces. The ET and NIR estimates are intended for use in design and management of irrigation systems, for water rights management and consumptive water right transfers, and for hydrologic studies. ET estimates are available for all times during the calendar year to provide information for land application design, operation, and management of waste streams from agriculture, food processing and other sources during the nongrowing periods.

These web pages allow you to access historical ET data for Idaho and adjoining states at various locations. Currently, the data are based on 208 stations associated with five meterological networks. Each station in the list of stations below ends with an abreviation that indicates the meterological network. The five networks are:

NWS -- National Weather Service Cooperative Observer Network.
PN-AM -- USBR Pacific Northwest Region AgriMet Network
GP-AM -- USBR Great Plains Region AgriMet Network.
AM/INL -- USBR Columbia-Pacific Northwest Region AgriMet Network and NOAA INL Center Cooperative Stations
RAWS -- National Interagency Fire Center Remote Automatice Weather Station Network

Find and explore the summary data for stations and land covers:

History

Dr. Rick Allen and Clarence Robison, researchers from the University of Idaho Kimberly Research and Extension Center, authored the 2007 dataset and report under contract with the Idaho Department of Water Resources (IDWR). Work was supported by funding from IDWR, the Idaho Agricultural Experiment Station, and the Idaho Engineering Experiment Station. IDWR periodically contracted with the University of Idaho to extend the time series. Allen and Robison made updates in 2009, 2012, 2017, and most recently in 2023. Each update provided opportunities for adding weather stations and improving the code. The entire time series was reanalyzed with each update, meaning that each update supersedes all previous updates. A summary of the updates is provided below. In 2020, with the retirement of Dr. Rick Allen on the horizon, IDWR contracted once more with the University to obtain an updated dataset and to in-house the knowledge to make independent updates in the future. The 2023 update is the result of that work. The 2023 update also adapted the United States Bureau of Reclamation's (USBR) Python ET-Demands model to match the Visual Basic 6 ET_Idaho model. The switch to ET-Demands was necessary because Microsoft no longer supports Visual Basic 6.

The ET_Idaho website was initially developed and maintained by Clarence Robison at the University of Idaho. To ensure the continued availability of the ET_Idaho-like data, IDWR created this website.

Technical Reports and other information

A pdf copy of the 2007 report submitted to IDWR, Evapotranspiration and Consumptive Irrigation Water Requirements for Idaho, Research Technical Completion Report, including the 2009 supplement, is available for download here. The 2009 supplemental report covers the modifications, changes, and enhancements in computing evapotranspiration and is located at the end of the report pdf. The 2016 update to ET_Idaho is covered in a supplemental report that is available for download here.

Changes and updates to datasets

In 2009, the University of Idaho updated evapotranspiration and net irrigation water requirement estimates through 2008. These estimates were updated in 2012 through December 2010. In 2017, the estimates were again recomputed and updated through 2016. In 2023, the estimates were updated through 2021. The 2023 version provides data for daily, monthly, and annual timesteps for 208 weather stations across Idaho. Thirty-year normals have been updated to the end of 2021 (if 2021 was an active year of data collection for the station).

2009
Minor coding errors discovered in the calculation routines were corrected in 2009. These coding errors included a correction to the statistics program to prevent calculated deep percolation from irrigation from going negative but to allow effective precipitation to be negative on some days to account for delayed deep percolation of precipitation. In the primary ET calculation code, the crop startup was modified to consistently calculate the estimated date of greenup or planting. Some resetting of variables at the start of a new crop was done, and an error in estimating deep percolation from very shallow rooting depths in coarse soils was corrected (this did not occur often).

2012
Slight changes in the logic of computing precipitation deficit were implemented in the 2012 update. The partitioning of evaporation into that supported by precipitation and by irrigation was modified to distribute evaporation better when both irrigation and a concurrent and/or recent rain event occurred. Additionally, effective precipitation for supporting transpiration and evaporation was partitioned for growing season, non-growing season, and annual time periods. For the annual time series, additional parameters were included in the output to assist in quantifying effective precipitation used in evapotranspiration during the growing and non-growing seasons. Also, estimates of non-growing season precipitation stored in the root zone and later used in the growing season were added.

2017
For the 2017 version, to support calculations of ET, the complete National Weather Service Cooperative station database for Idaho stations was re-downloaded from the National Climate Data Center (NCDC) site so that some previous 'holes' in daily time series were filled in for some stations. The 2017 update also added additional stations surrounding Idaho in Oregon, Washington, Montana, Wyoming, Utah, and Nevada. Due to the abandonment of some COOP stations, some stations found in the earlier releases were not included in this release. Some COOP stations were supplemented with nearby stations (coop or automatic) to provide a more complete record.

Missing air temperature and wind speed data were filled with the long-term DOY means in 2017 but were filled with the long-term monthly means in 2023.
Missing snow data were filled with the long-term DOY mean in 2017, but in 2023, the snow depth was based on a minimum of 11 years of record for the DOY from which the median depth was selected. The median depth was set to zero if less than 26 mm. The median depths were smoothed with a 7-day running average. The running average is set to zero if less than 26 mm. If all seven days were missing snow cover information, the snow depth was set to the long-term mean snow depth for the day. If there were fewer than 11 years of data, the snow depth was filled with 0.
Missing rain data were filled with the long-term DOY mean in 2017, but in 2023, they were set to 0 if the mean was less than 0.3 mm
Winter wheat was not initialized during the first growing season in ET_Idaho, resulting in very low ET for the first year of the time series. Winter wheat was correctly initialized in the 2023 version.
Alfalfa uses three different Kc curves: one for the pre-cutting period, another for mid-season cuttings, and a third leading up to the final cutting. The 2017 version incorrectly applied the final curve to mid-season cuttings. The 2023 data fix this error.
Canola and Mustard were treated as rainfed in southern Idaho in the 2017 version, but they were modified to be irrigated in the 2023 version.

The future of crop coefficient-based ET and irrigation requirements

The time series data will continue to be updated periodically by IDWR. Future versions will possibly use gridded weather data instead of weather station data. Gridded data offers several advantages, including complete spatial coverage of Idaho and temporal coverage back to 1979. GridMET is an example of a gridded weather dataset.

Use Constraints: The Idaho Department of Water Resources is making this data available as a public service. The Idaho Department of Water Resources strives to ensure that all technical data and other information made available to the public is accurate, complete and in conformance with the Idaho Public Records Act. Neither the Department of Water Resources nor the State of Idaho, however, assumes any legal responsibility for the accuracy or completeness of the information contained on this site. Persons using information from this site for official purposes, or other purposes, for which accuracy and completeness are required, are hereby notified that they should first verify the information with the public records or other primary sources from which the information was obtained.

--> Questions regarding the data should be addressed to the IDWR Geospatial Technology Section Idaho Department of Water Resources, Idaho Water Center, 322 E Front Street STE 648, Boise, ID 83702-7371. Telephone (208)-287-4800