From Dr. Roy Spencer’s World Warming Weblog
by Roy W. Spencer, Ph. D.
Key Factors
- Opposite to claims that drought is inflicting Lake Mead water ranges to fall, the Colorado River pure flows into Lake Mead present no long-term development since 1930.
- Decadal time scale variations in river circulation do happen, although, associated to the Pacific Decadal Oscillation (PDO).
- Since about 2000, use of Lake Mead water has exceeded river influx, inflicting water ranges to drop. The destructive part of the PDO since that point has exacerbated the issue.
Pure Water Flows into Lake Mead: No Lengthy-Time period Pattern
Report low water ranges in Lake Mead are extensively blamed on drought, though what “drought” means is seldom specified. The general public notion is that decrease precipitation quantities have lowered water provide to Lake Mead (which comes from the Colorado River), often attributed to human-caused local weather change, and that this is the reason water ranges are falling.
However information from the U.S. Bureau of Reclamation (USBR) present that there was no long-term development in pure Colorado River circulation into Lake Mead:
The flows in Fig. 1 have been barely adjusted for native human-caused adjustments to the flows upstream, and supply our greatest reply to the query of whether or not long-term international local weather change is accountable for a lower of river water circulation into Lake Mead.
The reply is “no”.
Does Local weather Change Concept Even Predict Lowered Precipitation? No
The following query is, does local weather change even predict future reductions of precipitation over the Colorado River watershed? The next plot reveals a median of 183 local weather mannequin simulations of common yearly precipitation in an space approximating the Colorado River watershed. The fashions counsel a slight improve in whole precipitation with warming.
Many of the water coming into Lake Mead is from snowmelt within the mountains; little of the water falling on decrease elevations tends for use by native vegetation with little runoff reaching the Colorado River. Fig. 3 reveals there was no long-term development within the snowpack measurements within the higher Colorado River watershed.
So, not solely has there been no noticed long-term discount in water circulation into Lake Mead, or discount within the watershed snowpack, local weather change idea doesn’t even help such a change as much as the present time (and even to 2050).
So, Why are Lake Mead Water Ranges Falling?
What has modified since Hoover Dam was constructed within the Thirties is the quantity of water being faraway from Lake Mead. Since about 2000, that water use has exceeded the water enter into the lake. That is the latest obtainable demonstration of that reality, revealed in 2012:
So long as water use exceeds provide, Lake Mead water ranges will proceed to fall. (That is considerably dependent upon the regulated releases from Lake Powell, upstream. There’s a “Fill Mead First” initiative that may draw down Lake Powell in an try to boost Lake Mead, based mostly upon calculations that web pure water losses from mixed evaporation and financial institution seepage from Mead and Powell could be lowered.)
The Position of the Pacific Decadal Oscillation (PDO) within the Present Drawback
Whereas the main downside with Lake Mead is overuse, there are multi-decadal fluctuations in Colorado River flows which have made issues worse since roughly 2000. If we take the river circulation information in Fig. 1 and compute the gathered departures from the long-term common circulation (as a result of that is how a reservoir like Lake Mead responds), we discover that there have been intervals of lesser and better flows.
Earlier than the Nineteen Eighties, there was considerably lowered river circulation into Lake Mead, nevertheless it made little distinction as a result of water use (Fig. 4) was nonetheless low.
Then from the 1982-83 tremendous El Nino 12 months to roughly 2000 there have been above common flows, so Lake Mead may deal with the growing water utilization. In truth, the lake reached close to full-pool standing.
However as utilization peaked round 2000, river enter to the lake was lowered as soon as once more. This put Lake Mead into an unsustainable state the place extra water was being extracted than the Colorado River may replenish it.
It has been lengthy recognized (e.g. right here) that precipitation on this area is affected by El Nino (extra precip) and La Nina (much less precip). Additionally, the Pacific Decadal Oscillation (PDO), which is principally a low-frequency manifestation of El Nino and La Nina exercise is said to precipitation on this space.
I computed the cumulative common departures from the long-term imply of each the PDO index and the MEI (Multivariate ENSO Index). The PDO is considerably increased correlated (r=0.52) with the cumulative river circulation information in Fig. 5. As Fig. 6 reveals, constructive PDO intervals are typically related to increased stream flows, and destructive PDO with decrease stream flows. Most notably, the interval since 2000 has seen extra destructive PDO exercise, which is worsening the issue with Lake Mead not getting sufficient water. After all, this may finally reverse when the PDO flips again into its constructive part.
Conclusions
The favored narrative that drought on account of local weather change is inflicting Lake Mead to have much less water obtainable to it’s incorrect. Since 1930, there was no long-term change within the Colorado River circulation upstream of what’s now Lake Mead.
The most recent local weather fashions don’t even predict a discount in precipitation within the higher Colorado River watershed.
Multi-decadal adjustments in river circulation do happen, although, and are associated to the Pacific Decadal Oscillation, a pure fluctuation in climate patterns over the northeast Pacific. Current record-low water ranges in Lake Mead are primarily on account of file excessive water demand from the lake, since roughly 2000. The issue is being made considerably worse by the destructive part of the PDO, additionally since roughly 2000.
