Hydrologists, climatologists, and other water experts and users are paying close attention to one question: How much water will get stored away in the state’s snowpack and soil this winter. This math can be spelled out in an equation (see above.) This natural storage is “delta S.”
Snowpack, of course, releases water into the state’s rivers and streams throughout the spring and summer. It also recharges the aquifer beneath these systems. Fall and winter moisture also recharges the soil, which is especially important in central and eastern Montana. So measuring snowpack and soil moisture are key factors in determining the state’s water supply and occurrence of drought.
But it is not a simple calculation of measuring rainfall or precipitation (“P” in above). The combination of evaporation (“E”) and transpiration (“T”) subtract from a basin’s water storage.* And runoff (“Q”) also removes water downstream and away.
At the Governor’s Drought and Water Supply Advisory Committee meeting last month, experts summed up the 2021 water year like this:
- precipitation was low
- evapotranspiration was high
- soil moisture and groundwater became depleted
- streamflows quickly declined
The initial indicators for 2022 are not good: fall moisture has been underwhelming. But La Nina conditions originating in the Pacific Ocean feed hopes of above-average precipitation and snowfall. So the ultimate value of “delta S” remains to be seen.
Next year’s water supply or drought–and how Montana may be better prepared for it–are ongoing issues tracked by the Water Policy Interim Committee. Visit leg.mt.gov/water for more information.
* Evaporation is water lost on any surface; transpiration is water lost from plant cells. Together this is referred to as evapotranspiration.
