The fact that a moderate chunk of the Upper Basin snowpack has already melted but the rivers have not shown a major hydrological response has been puzzling to me as well. There could be several reasons for this, all of which might be additive:
1) Unseasonably warm temperatures in March and April, coupled with some high wind events could have created an atmosphere with a high vapor pressure deficit and evaporative demand, so that some of the snowpack simply sublimated and got taken up by the air. A recent NOAA study looking at the ongoing Southwest drought warned about this phenomenon. See the following reference, available online, which makes sobering reading:
Mankin, J. S., Simpson, I., Hoell, A., Fu, R., Lisonbee, J., Sheffield, A. & Barrie, D. 2021. NOAA Drought Task Force Report on the 2020–2021
Southwestern U.S. Drought. NOAA Drought Task Force, MAPP, and NIDIS.
2) Because we carried a soil moisture deficit over from last year’s very dry November of 2021, and because quite a bit of the Upper Basin vegetation was drought stressed when it went dormant at the start of the cold winter season, there may have been a fair bit of latent moisture demand in the forests and shrublands, which immediately utilized melting snowpack when it became available. From what I recall in USDA briefs from a few months ago, the soil moisture deficit, although not as bad as we had coming out of the winter of 2020-2021, was still going to be enough to reduce potential runoff by 15 percent. And that was just to replenish water in the soil pore spaces, and did not figure in additional demand from stressed vegetation.
3) Trans-basin diversions in the river headwaters, particularly in Colorado, almost certainly pulled off a moderate bit of the initial runoff before it ever got to points where it could be gauged. Even so, Denver’s water supply reservoirs are hardly brimming and topped up. At its quarterly board meeting on April 20, leaders of the Colorado River District met to discuss current matters regarding the state’s river waters. In an update to the board, Director of Science and Interstate Matters Dave Kanzer and Senior Water Engineer Don Meyer reported that the reservoirs across Colorado are very unlikely to fill before the summer. As of late April, Denver Water reported that Dillon Reservoir, their primary Western Slope storage facility, was only 78% full.
This group’s report also noted that, much along the lines of my reasoning above: “Lower-than-normal snowpack across the West continues to be exacerbated by dry soil conditions and ‘thirsty’ atmospheric conditions. This is expected to continue to cause the larger-than-normal demand for water that has adversely impacted reservoir storage across the Upper Colorado River Basin, and the prospect for refilling them is not optimistic. In other words, the significant cumulative hydrological deficit continues to grow; it will take multiple years of above average conditions to recover.”
In sum, there appear to be multiple factors that are decoupling the relationship between snowpack melt rate and river flows this year. At the moment, the cumulative flow hydrograph for the Colorado River at the Cisco gauge, near the Utah line, is tracking the pattern from the very dry year of 2002, which was the lowest ever recorded. If that continues, it is going to be a bleak runoff year indeed, and my sense is that BOR already realizes this, thus their recent emergency measures to stabilize a system that is in far worse shape than they thought it would be even two months ago. This is all just the cumulative result of a 20-year drought that with each passing year is trending more toward a pattern of long-term regional aridification, and until this climate pattern changes, the entire region is going to have major water issues that will impact all users up and down the Colorado River system. The question is not going to be whether we can fill Lake Powell, but whether we can fill any of the major reservoirs, and if so, which ones and why.
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