Idaho’s Current Irrigation Water Supply; A Deep Dive into Idaho Irrigation Supply | Part II

A few weeks back we posted a graphic detailing the historical water inventory in the Upper Snake River reservoir system vs. what the inventory as of that time looked like under the current drought conditions. The reality as of that time was that although water inventories were certainly tightening, the region was (is) coming off 5 continuous years of above average inventories as compared to the period since the last reservoirs in the system came online in the early 1960’s. Dry yes, unprecedented, no. The same methodology can be applied virtually (to or on) any basin or district within the Pacific Northwest or other western states.

The updated chart below provides a different look than the previous, with the daily inventory shown in blue and a 365-day moving average in orange. As of October 4th, the reservoir inventory totaled to a little over 600,000 acre feet, the lowest it has been since 2013. Inventory lows typically occur in the fall of the year, after crop irrigation for the growing season has ended and before winter snow and rains begin to accumulate. During the time period covered, we have seen the Upper Snake River reservoir system with inventories at or below the current figure in nine different years (see red arrows in the chart), the most recent being 2013.

By this metric, we are still not in record territory. That said, in each of the 9 years we have seen inventories this low, the 365-day moving average inventory (orange) has moved below its long term average (dotted orange), implying at least one year of tight inventories. In 7 of those 9 years, it has taken at least 2 years for the moving average to return to average or greater inventories.

Bottom line: Water available for irrigation in this region is likely to remain tight through 2022. That outcome, the time length of irrigation water constraints, and its ultimate severity will remain dependent on the 2021 -2022 snowfalls.