Hi, I'm Jeff Lukas with the Western Water Assessment Program at the University of Colorado Boulder. Thanks for joining us. In this lecture, we'll continue our study of the Colorado River Basin. This time looking at the basin through the lens of climate. In how climate drives the variability of water resources and how water varies over both space and time, the Colorado River basin shares key characteristics with most other parts of the West. Complex topography, with high mountain areas that are cool and wet receiving more precipitation up to 60 inches per year than the surrounding basins, plateaus and canyons and planes which mainly receive 10-15 inches per year or less. So even though the mountains cover less than 10 percent of the basin's land area, they provide nearly all of the runoff and thus the water supply. Since most of the precipitation in the mountains falls as snow, there was a snow melt dominated hydrology with a pronounced peak and runoff in the spring and early summer as the deep winter snowpack rapidly melts. There are large fluctuations in precipitation in streamflow from year to year as the changes in the circulation of the atmosphere and oceans far from the West influence the location of the storm tracks across the Western US. These fluctuations include periodic persistent droughts lasting several years. As described in the previous lecture, the Colorado River has been intensively managed and used as a system for over a century. Critical to that management is understanding how much water can be provided by the river, the long-term average and the variability. The long-term average dictates how much water is available to be a portion to the basin states and individual users, and whether those allocations are sustainable over time. The variability dictates how much reservoir storage is needed to store surplus water during wet periods to buffer the impacts of the inevitable droughts. Our understanding of how much water is available has changed over time, and there have been major changes in our thinking about the river as new information has emerged. First, the basic approach for monitoring stream flow is the stream gauge, a technology developed in the late 1800s and not much different today. The water level of the river or stage height is measured and then compared with a rating curve developed from thorough measurements of the river's discharge or flow at different stage heights. By the early 1920s, a few gauges on the Colorado River at or below Lee's Ferry. So reflecting all of the runoff from the upper Colorado River basin had been operating for about 20 years. So the understanding of the river's flow look like this. An average of 16-17 million acre feet with drought years down to about 10 million acre feet and wet years well over 20 million acre feet. It was based on this understanding that the Colorado River Compact was negotiated in 1922 and the river's flow divided between the upper basin states and the lower basin states. But the next several decades of gauged records suggested that this initial impression was too optimistic and the long-term average streamflow progressively shifted downward closer to 15 million acre feet or 10-15 percent less than the prior assumption. Extreme drought years like 1934 delivered only seven million acre feet of runoff. The bigger message was that even 30 years of stream gauge records was inadequate to represent the longer-term picture. Over the next 30 years, while the long-term average still hovered around 15 million acre feet, the climate system produced variability and the river that hadn't been seen before. There were large decadal swings from wet to dry and embedded within those swings were a new record wet year of 25 million acre feet in 1984 and a new record drought year of six million acre feet in 1977. Since 2000, the Colorado River has had below average flow in 12 out of 15 years and stream flow has overall been 20 percent lower than the 20th century average. In fact this has been the driest 15-year period on record in the Colorado River basin. This has led to Lake Powell and Lake Mead, the huge reservoirs, being drawn down to about half of their combined capacity. The same period was also the warmest such period on record. Part of a long-term warming trend in the West that has been largely attributed to human-caused climate change. So are the low flow conditions since 2000 due to climate change? We'll return to that question a little later. While managers have kept a close eye on the gauged record of Colorado River flow as it is accumulated one year at a time, a group of researchers have at work to extend their view of the river hundreds of years into the past using tree rings. In the early 1900s, A E Douglas of the University of Arizona recognized that patterns of wide and narrow rings in ponderosa pines and other native conifer trees closely corresponded to the annual precipitation, wet years and dry years in the Southwest. By the early 1940s, Douglass's colleague, Edmund Schulman, found that these tree rings also corresponded closely to annual streamflow in the region. He developed a tree-ring index for the Colorado River Basin that suggested past dry periods have been worse than those since 1900. In 1976, using computer-aided measurement and statistical techniques, the first formal reconstruction of Colorado River stream flow at Lee's Ferry using tree-rings was published. It clearly showed that the early 1900s was in fact, the wettest such period of the last 400 years. It also showed that a drought in the late 1500s had lasted for about 20 years, and appears to be more severe and sustained than the most recent period since 2000. The current state of the art for tree-ring reconstructions of the Colorado River stream flow at Lee's Ferry now extends back over 1200 years. It shows multiple droughts, more severe and sustained than any scene in the past 100 years of gauged stream flow, including a few with greater potential impacts than the 1500's drought or the 2000's drought. Most notably, there was a dry period in the mid 1100s during the medieval period, where the annual stream flow was below average in eight out of 10 years for six decades, a drought worthy of the term mega-drought. Also, this reconstruction shows that even the 100-year average flow has swung up and down over the past millennium with the 20th century being wetter than most centuries in the past. So the tree-rings tell us that there is no single solid historical baseline that we can use to plan for the future, and that the 20th century is not fully representative of the natural variability of the Colorado River. More recently, another important perspective on the Colorado River has emerged. The potential impacts of human-caused climate change on the river. As early as the late 1970s, studies identified a risk to Colorado River annual runoff from significant future warming. As higher temperatures would increase evapotranspiration, that loss of moisture from soils, vegetation, and snow packs. By the early 1990s, climate projections from several global climate models were being used to drive detailed simulations of the water cycle in the Colorado River basin. In most of the simulations, annual stream flows declined because of the future warming. Only in those cases where the model suggested large future increases in precipitation did stream flows increase. In the past 20 years, global climate models have been much improved in how they represent the key climate processes that affect this region. Many additional studies and reports have used the climate model output to estimate the future of the Colorado River, but the general message hasn't changed since the 1990s. The future warming in which we have very high confidence will tend to push flow downwards. We are less certain of future precipitation changes, but only large increases in precipitation would keep stream flow from declining in the future. So most simulations of future Colorado River flow show less water being available on the average. A declining future trend and annual stream flow is not the only challenge to the Colorado River posed by climate change. As noted in a previous lecture, other important aspects of water availability and water use will be affected by further warming. As the climate in the Colorado River basin warms, the timing of snow melt and spring runoff will tend to shift earlier by another one to three weeks by mid-century, continuing a trend that has occurred over the past 30 years. As the peak runoff shifts earlier, late summer flows decrease. This would impact farmers and others who divert water directly from the river and its tributaries for immediate use, and also affect aquatic species that need to have certain flows in the river. As the temperatures rise, both crops and urban landscaping require more water. So farmers would need to change their irrigation technology, plant different crops, or reduce their acreage to stay within their current water usage. Outdoor water use in urban areas would likewise have to change. So to return to the last 15 years since 2000, we see from the tree-rings that low flow periods of this length and severity have occurred in the Colorado River basin multiple times in the past driven by the natural variability in precipitation. From that observation and other evidence, we believe that the low runoff since 2000 is mostly due to the natural variability in precipitation. However, the warming temperatures in the basin over the past few decades are also acting to reduce runoff. This is a taste of what we expect in the future that any drought, as measured by a given deficit in precipitation, will have worse impacts on the Colorado River because of the warmer temperatures. Compared to their counterparts 100 years ago, water resource managers in the Colorado River basin have vastly more information about past variations in climate and water supply. They can also monitor the snow pack and forecast the coming springs runoff in ways that their predecessors could only dream about. But in planning for the long-term, five, 10, 20, 50 years out, managers now like those in the early 1900s, face a great deal of uncertainty. Will the next decade be overall drier or wetter than average? We don't know. The climate projections strongly suggests a very long-term trend towards less runoff in the Colorado River. But there are many possible pathways that are consistent with the past behavior of the system, as well as those future projections of climate. One thing that has changed in the last 10 years is how water resource managers at the state, local, and federal levels look at the future. Instead of assuming a single future-based on one historical baseline, they're moving to consider multiple plausible future scenarios. One example of this new approach is found in the Colorado River Basin Water Supply and Demand Study, which is the subject of the next lecture. So we don't know exactly what the climate system in the Colorado River basin will dish out in the next year, the next decade, or the next half century. But we can say that managers of the river system armed with more information are less likely to be surprised by future changes in the river. As to whether their planning and preparation will be sufficient to meet those challenges, so that the river continues to meet the needs of its users while still providing ecosystem services, only time will tell.
