St Mary-Milk Rivers Consumptive Use Study – Remote Sensing Component

The primary goal of this project was to develop a historical database representing annual or sub-annual evapotranspiration (ET) totals in the Milk and St. Mary River basins from 1982-2018 using remote sensing. This database laid the foundation for the establishment of a remote sensing tool to objectively estimate ET in the upper Milk and St. Mary River basins on a 2-4 week interval using high spatial resolution (30- to 100-meter) satellite imagery. In addition, historical trends in consumptive water use in the basins can be analyzed and compared to historical estimates that were made using alternative methods. 

The completion of the project occurred in three phases over the Fiscal Years (FYs) 19-23. The phases are as follows:

• Phase 1 (FY 19-20, complete): ‘Development of Historical Database of Evapotranspiration Using Remote Sensing’. Google Earth Engine, an online platform for image processing, was used to download, mask, and preprocess all of the necessary remotely sensed imagery and gridded climate data. A portion of Phase 1 was completed using funding provided to the Wyoming-Montana Water Science Center in FY18. The next steps of Phase 1 included compiling all necessary and relevant gridded climate data, as well as local meteorological data that was used to optimize the ET algorithms specifically for the Milk and St. Mary River basins, and align them with the remotely sensed data. Together, these data served as the primary inputs to the ET algorithms developed in Phase 3 to estimate actual evapotranspiration in the Milk and St. Mary River basins.
• Phase 2a and 2b (FY 21-23, complete): Field work was conducted to verify the location and extent of irrigated land and any information available regarding typical levels of service (e.g., amount of water applied, time period of irrigation). These extents were digitized, and then used to ground-truth locations to improve the ET associated with irrigation in these years and used as a guide to associate the ET estimates with irrigated land in previous years.
• Phase 3 (FY 23, complete): Using the compiled databases of remotely sensed imagery and gridded climate data developed in Phase 1, a locally optimized algorithm was developed using published USGS methods to estimate actual evapotranspiration in the Milk and St. Mary River basins from 1985-present (or, most recent growing season). The actual ET estimates were then used to estimate consumptive use in the study area by comparing the actual ET estimates on irrigated and non-irrigated land.