Assessment of Water Availability for Irrigation within the Elevational Gradient of a Mountainous Catchment in Southwestern Montana

In snowmelt dominated regions, droughts are projected to increase in frequency, severity, and duration. Agricultural professionals and resource managers will need more efficient and accurate methods of characterizing the snowmelt cycle to forecast water availability for irrigation. Often, hydrological studies are conducted at the watershed scale which is generally too broad to be relevant to agricultural producers. A paired understanding of the source of irrigation water within a mountainous watershed, and of the temporal characteristics of snowpack runoff can equip agricultural producers with information for water planning at a smaller, more meaningful scale. To predict water supply from mountainous snowpack, we utilize stable water isotopes to trace the source of irrigation water to various elevation bands in the snow-dominated Mill Creek sub-watershed in Paradise Valley, Montana, USA. Plant tissue samples collected from a hayfield irrigated with water diverted from Mill Creek provide stable isotope ratios (H2 and O18) from water extracted via cryogenic distillation. Adjacent to this field we collect precipitation event samples throughout the irrigation season using a Palmex liquid water collector. To trace the source of irrigation water within the watershed during the runoff season, weekly snow and stream water samples were collected at 12 locations along an elevational gradient between 1,463 m and 2,256 m for analysis. To provide temporal context, analysis of backscatter data from Sentinel-1 Synthetic Aperture Radar (SAR) provides approximate dates of snowpack moistening, ripening, and importantly, runoff onset at each elevational band in the watershed. Combined, these data and analyses provide estimates of where and when snowmelt-derived water supply originated for distinct fields in the Paradise Valley that rely on water from Mill Creek. While our work focuses on a local region in Montana, the efforts and approaches used are potentially applicable globally for agricultural regions that rely on snowmelt for irrigation.