Water Managers and Policymakers Need to Heed Clues from the Past

Just like the weather, the amount of water available in a region in any given year has its ups and downs.  Those ups and downs are particularly important in the Southwest, where scarce water forces tough decisions and conservation in everything from watering lawns to farming to industrial uses of water. An unusual number of recent storms and heavy snowfalls that have boosted snowpack across much of the upper Colorado River basin by about 151% of average¹ will make it easier to meet the region’s water demands this year, as melting snow will fill up the rivers, tributaries, and reservoirs in the basin (see map).  “Average” is based on measures of water flow recorded over the past 100 years, which forms the basis of many current water policies.  But here’s the bad news: past climate data show that the region’s average water flow over the past 500 years has been much drier than in the past 100 years. In other words, what is thought of as normal today might not be normal at all.

The Colorado River Basin

Water management in the Colorado River Basin is already very challenging as severe drought conditions have affected much of the region since the early 2000s.  In fact, 2002 and 2004 are among the 10 driest years on record in the upper basin states of Colorado, New Mexico, Utah, and Wyoming. Water storage in the basin’s reservoirs dropped sharply during this period due to very low snowpack and stream flows; for example, 2002 water year flows into Lake Powell were roughly 25 percent of average.  Visitors to Lake Powell noted the “bathtub ring” left by the drop in water levels of the lake.

At around the same time, several studies were produced that “reconstructed” Colorado River flows over the past several centuries. In those studies, data from annual growth rings of trees, which are good indicators of annual moisture availability, indicate that the region has experienced many severe and extended droughts in the past 500 years.  In response to that news, the U.S. Bureau of Reclamation and regional water bureaus in California and Nevada asked the National Research Council to convene a committee of experts to help put those studies and other scientific information into context to inform water policy.  Colorado River Basin Management: Evaluating and Adjusting to Hydroclimatic Variability (NRC, 2007) represents the committee’s consensus conclusions.

A Changing Picture of Colorado River Streamflow

One of the findings in Colorado River Basin Management is that past water management decisions, were signed during a period of unusually wet conditions across the basin, and which provided an overly optimistic assumptions of long-term water availability.  The first and foremost of these is the Colorado River Compact of 1922 which still today governs water allocations between the upper and lower Colorado River basin. The prevailing scientific understanding of Colorado River flows has been based primarily on direct measurements of the river’s flow at stations along the river, the first of which was  established in the late nineteenth century. The record of streamflow measurements taken throughout the twentieth century has led to an implicit assumption that the river has an average annual flow of about 15 million acre-feet/year, around which year-to-year flow variations occur (see Figure 1). Even though the basin experienced wet and dry periods, river flows and weather conditions were expected to return to a “normal” state that was largely defined by the average flow in the 20^th century.

However, tree-ring based reconstructions have provided new evidence that has shifted the scientific view of Colorado River flows.  One important shift is that the long-term annual average flow of the river, based on the 500-year record, is considerably less than 15 million acre-feet.  Another important piece of information is that the basin has experienced several periods of drought that have been even longer and of greater severity than those experienced in the early and mid-2000s  (see Figure 2).

Measured River Flow Over the Past 100 Years

Figure 1. For many years, scientific understanding of Colorado River flows was based primarily on measurements of the river’s flow at stations along the river. The image above shows the 1906-2006 record of the river’s flow at Lees Ferry, Arizona, with figures for annual values (blue bars), a 5-year running total (black line) and the average flow of about 15 million acre-feet/year (red line)—which was thought of as a “normal” average flow value around which year-to-year variations occur.

River Flow Estimates from Tree Ring Data Over the Past 500 Years

Figure 2. Multiple reconstructions of Colorado River flows at Lees Ferry, Arizona, which are based on tree-ring data that date back roughly 500 years, reflect a long-term average annual flow of much less than 15 million acre-feet—the presumed “average” flow that is being used to allocate the river’s water—as well as several pronounced drought periods.

Additional Future Worries: Warmer Temperatures and Rising Population

Adding to the concern about the past climate is the fact that temperature records for much of the Colorado River basin and the western United States document a clear warming trend over the past three decades. These records, along with climate model projections, suggest that temperatures across the region will continue to rise in the foreseeable future. Higher temperatures are predicted to result in less upper basin precipitation falling and being stored as snow and  higher temperatures will increase evaporative losses. There is less consensus regarding future trends in precipitation. However, based on analyses of many climate model simulations, the weight of evidence suggests that warmer future temperatures will reduce future Colorado River streamflow and water supplies.

Meanwhile, rapid population growth across the western United States is driving increases in water demand. For example, from 1990-2000, Arizona’s population increased by about 40 percent, while Colorado’s population increased by about 30 percent. Population projections suggest that this trajectory will continue. Although many innovative urban water conservation programs have reduced water use per person, population growth is driving increases in urban water demands.  Water consumption in Clark County, Nevada (which includes Las Vegas), for example, approximately doubled in the 1985-2000 period. Steadily rising population and increasing urban water demands in the Colorado River region would inevitably result in increasingly costly, controversial, and unavoidable trade-offs.

Limits of Technologies and Conservation Measures

A wide array of technological and conservation measures can be used to help stretch existing water supplies. These measures include underground storage, water reuse, desalination, weather modification, conservation, and creative water pricing structures. These measures may not necessarily be inexpensive or easy to implement, but many of them show promise for augmenting water supplies in future years. However, technological and conservation options for augmenting or extending water supplies—although useful and necessary—in the long run will likely not be enough to remove the fundamental tension between limited water supplies in the Colorado River Basin and inexorably rising population and water demands.

This year’s winter snows are a good thing, but in the long run they don’t change the fact that water managers and policy makers in the Colorado River Basin will have to develop plans and actions for meeting increasing water demands in a potentially warmer and much drier future. Doing so will require more collaboration between the scientific and water management communities and enhanced interstate cooperation.