40. FLOODS IN THE GREENHOUSE: SPINNING THE RIGHT TALE H.F. Lins & T.A. Cohn U.S. Geological Survey, 415 National Center, Reston, VA 20192, USA e-mail:
[email protected];
[email protected] 1
ABSTRACT An element of human-enhanced greenhouse theory is that the hydrological cycle will accelerate. This has led some to hypothesize that extreme events, such as floods and droughts, may increase in frequency and/or severity. Indeed, published studies indicate that precipitation has increased in recent decades, and some have characterized these increases as occurring in “extreme” precipitation. Significantly, however, recent empirical studies from North America and Europe find no evidence of an increase in flood frequency or magnitude during the Twentieth Century, although increases in low to moderate streamflows have been widely reported. What, then, are the likely effects of greenhouse warming on streamflow in general, and on floods in particular? We consider this question using data and the published literature with respect to two issues: What is known about the sensitivity of various return-period floods and annual precipitation? What is the real significance of a given percentage change in precipitation on a flow quantile (e.g., Q100 versus Qmean)? We find that the precipitation sensitivity of mean streamflow is much greater than that of peak streamflow, and that precipitation sensitivity decreases as flood return period increases. This leads us to conclude that human-induced greenhouse warming is more likely to produce noticeable and significant changes in the mean state of hydrological regimes than in hydrological extremes. 1
INTRODUCTION
Some climate change researchers have speculated publicly that, among the many consequences of global warming, one of the most likely will be an increase in severe flooding across much of the globe. Although speculative, the possibility merits serious scientific consideration because floods are already among the costliest natural hazards when measured in economic and human terms (Mileti, 1999). The prospect of increased flooding also raises some notable non-scientific considerations. For example, extreme floods have figured prominently in our cultural myths and the issue of flooding tends to evoke a political response that is often disproportionate to the magnitude of the problem as quantified by science. In such circumstances, considerable caution may be appropriate when discussing the issue with the public, particularly when considering potential human impacts on flooding. Recent empirical studies from North America and Europe find no evidence of an increase in flood frequency or magnitude during the Twentieth Century, although increases in low to moderate streamflows have been widely reported (Lins & Slack, 1999; Zhang et al., 2001; Robson et al., 1998). This reinforces the case for exercising caution when V.R.Thorndycraft, G. Benito, M. Barriendos and M.C. Llasat (2003). Palaeofloods, Historical Floods and Climatic Variability: Applications in Flood Risk Assessment (Proceedings of the PHEFRA Workshop, Barcelona, 16-19th October, 2002).
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communicating the potential for future catastrophic floods to the public. We consider this subject by addressing the following questions: First, based on hydrological theory and published empirical evidence, what can we say with certainty about extreme flooding in a CO2-rich future environment? What seems likely? What seems possible? Second, how will extreme floods in the future compare qualitatively to what we have experienced in the past? Will future floods in most places seem larger than they had been previously? Finally, looking back from 2050, will the changes in flood frequency be nearly as significant as the consequences of changes in the mean annual flow and changes in water availability across the globe? 2 2.1
WHAT DO WE KNOW? Observed Trends in Precipitation and Streamflow
Karl et al. (1995) and Karl & Knight (1998) documented trends in precipitation in the United States during the twentieth century. Their analyses indicated that precipitation had increased about 10 percent across the United States since 1910. They also found that more than one-half of this increase occurred in the upper 10th percentile of the precipitation distribution, which they described as representing “heavy and extreme daily precipitation events.” Analogously, trends in streamflow have been assessed recently by a number of investigators in the United States and Canada. Lettenmaier et al. (1994) examined trends in monthly and annual mean streamflow across the United States and found positive trends for a large percentage of streams. Lins & Slack (1999) provided an expanded assessment by analyzing trends across the entire streamflow distribution, from the annual minimum (daily mean) to the annual maximum (daily mean) discharge. They found that low to moderate streamflow (annual minimum to annual median) had increased across much of the United States, while annual maximum flow had not changed appreciably. These findings were reinforced by Douglas et al. (2000), who also found evidence of an upward trend in low flows, but no trend in flood flows. Most recently, McCabe & Wolock (2002) confirmed the results of the previous studies (i.e., increases in low to moderate streamflows and little change in high flows), and added that the observed trends occurred as a step change around 1970 rather than as a gradual trend. A very similar pattern of trends was reported in Canada by Zhang et al. (2001), particularly for areas bordering the United States. Groisman et al. (2001) reported what appear to be contradictory results of increases in high streamflow in the conterminous U.S., particularly in the eastern U.S. However, the Groisman et al. results are not contradictory but, rather, reflect a significant methodological difference from all the other studies. Rather than reporting how streamflow had changed in each percentile class relative to the average streamflow in that class over time, Groisman et al. reported the change in the mean volume of discharge in each percentile class relative to the mean volume of the entire streamflow distribution. In so doing, very small percentage changes in annual maximum streamflows were magnified. This is because they were large (or small in the case of the minimum and median) volumes relative to the mean of the overall distribution. While the Groisman et al. results are arithmetically correct, they are also easily misinterpreted to suggest that the planet is already experiencing substantially more extreme streamflow.
Floods in the greenhouse: spinning the right tale
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Observed Sensitivity of Streamflow to Precipitation
During the past two decades there have been dozens, if not hundreds, of published studies attempting to document the sensitivity of streamflow to climate in basins around the world. The approaches used by most can be grouped into one of five categories: • Calibrate a watershed model, vary precipitation, and evaluate the effect on discharge; • Analytically derive the sensitivity of streamflow from model parameters; • Fit multivariate regional hydrologic models using climate and streamflow data for many basins within a region; • Empirically estimate changes in discharge from historical changes in climate; and • Use multivariate statistical methods to estimate the relationship between climate and streamflow at a single site. However, sensitivity analyses performed on the same basin using both similar and different approaches have led to very different results. Consider, for example, the case of the Animas River basin in the western United States where four independent investigations arrived at significantly different sensitivities (Table 1). Although Nash & Gleick and Revelle and Waggoner produced similar results, and Schaake and Vogel et al. produced similar results, the discharge increases in the former studies were one-half those in the latter studies. Notice, also, that the results produced by Nash & Gleick (1991) and Schaake (1990) were both based on the National Weather Service River Forecast System (NWSRFS) model.
Study Nash & Gleick (1991) Revelle & Waggoner (1983) Schaake (1990) Vogel et al. (1999)
Percent increase in annual discharge resulting from a 10 percent increase in annual precipitation
Model National Weather Service River Forecast System At-site multivariate regresssion National Weather Service River Forecast System Regional multivariate regression
10.9 10.5 19.7 19.0
Table 1. Comparison of sensitivity estimates for the Animas River basin, Colorado, USA (source: Sankarasubramanian et al. (2001).
In response to this situation, Sankarasubramanian et al. (2001) argue for the use of a robust and approximately unbiased estimator of streamflow sensitivity. Following from the work of Schaake (1990), they make the case for using the concept of elasticity for evaluating the sensitivity of streamflow to changes in precipitation. They define elasticity as
ε P (µ P , µ Q ) =
dQ dP
(1)
where µ P and µ Q denote mean values of precipitation and streamflow, respectively. In practice, a resulting value >1 indicates that a 1 percent change in precipitation can cause a >1 percent change in streamflow. We apply this metric in the following section to selected river basins in the United States.
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WHAT’S THE SIGNIFICANCE?
We used Equation (1) to compute the elasticity (sensitivity) of flood discharges at specific return periods to mean annual precipitation for regions in four States in the western U.S. The discharges were taken from the National Flood Frequency Equations derived by the U.S. Geological Survey. The computed sensitivities appear in Figure 1. The important thing to notice is that the flood flow sensitivities range from about 1.0 to 1.5 for the 2-year flood, 0.8 to 1.3 for the 5-year flood, 0.6 to 1.0 for the 10-year flood, 0.5 to 0.85 for the 25year flood, 0.4 to 0.85 for the 50-year flood, and 0.3 to 0.8 for the 100-year flood. Notably, as the flood return period increases the sensitivity decreases, such that for the 10-year and longer return interval flows, the elasticity is
