The Changing Precipitation Patterns at Devils Lake, ND 

 

1. Introduction

The landlocked Devils Lake in North Dakota is the largest natural water body in the state, having expanded from the size of a large pond in 1940 to an area of almost 300 square miles in June 2011.  The decades-long expansion of the lake accelerated in the early 1990s and has brought widespread flooding to the surrounding lands and communities.  A continued rise of the lake level will cause ongoing hardship and economic damage and may eventually lead to an uncontrolled outflow into the Sheyenne River, with accompanying degradation of downstream water quality (Vecchia 2008).

On a timescale of several years to decades, changes in the elevation of Devils Lake are controlled by variations in the quantity of precipitation that falls over the 3810 square mile drainage basin.  Figure 1 shows that the lowest elevation of the lake occurred in 1940 after more than two decades of unusually dry conditions and that a sudden increase in precipitation beginning in the 1990s was accompanied by a dramatic rise in the lake level.

Figure 1.  Elevation of Devils Lake (black line) and annual (light blue line) and trailing decadal (dark blue line) basin-average precipitation.

 

The striking difference between the precipitation regimes of the early 20th century and of recent years is shown in Table 1:

Table 1.    Range of decadal average precipitation occurring in years for which the Devils Lake elevation was below 1410 feet or above 1440 feet.

Lake Level

Number of Years

Year Range

10-Year Annual Average Precipitation

Below 1410 feet

17

1933-1949

14.3-17.8 inches

Above 1440 feet

14

1998-2011

18.7-20.5 inches

 

All of the years in which the lake level was below 1410 feet had preceding decadal average precipitation of less than 17.8 inches per year, but all of the past 14 years in which the level has exceeded 1440 feet have had preceding decadal averages greater than 18.7 inches per year.

The three primary goals of this study were: first, to examine the characteristics of the precipitation changes that have profoundly affected the Devils Lake region; second, to examine atmospheric processes and mechanisms that may explain the changing precipitation regimes at Devils Lake; and third, to assess the likelihood of future scenarios for Devils Lake using both climate model simulations and statistical prediction methods.  This page presents findings in regard to the first topic; the remaining topics are addressed under the "Science" tab on this page's top menu.

 

2. Analysis of Precipitation Changes

Several high-quality gridded historical precipitation datasets make it possible to examine in detail the changes that have taken place in the characteristics of precipitation over and near Devils Lake.

Temporal Distribution of Precipitation Changes

Annual precipitation totals for the Devils Lake drainage basin are shown in Figure 2.  For this analysis, basin-average precipitation totals were extracted from the high-resolution Parameter-elevation Regressions on Independent Slopes Model (PRISM) precipitation dataset (Daly et al. 2002).  The recent wet regime appears to have begun in 1991, when the annual precipitation was 22.5 inches, greater than in all but two years in the previous 96 years of record.  Since 1991, this amount of precipitation has been exceeded in five additional years, and 2010 was the wettest year in the historical record.  Figure 3 shows that the annual totals since 1991 have been concentrated in the upper half of the 115-year distribution; the difference in median annual totals before and after 1991 is more than 3 inches.

The regime shift in 1991 is very distinct and statistically significant, as shown by a likelihood ratio test for gamma distributions fitted separately to the annual precipitation amounts between 1948 and 1990 and between 1991 and 2011.  The year 1948 was chosen for the beginning of the first period both because the 1948-1990 period was a time of relative stability in the decadally-averaged precipitation (Figure 2) and because high-quality gridded atmospheric data are first available from 1948 (see "Observed Changes in Atmosphere and Ocean" under the "Science" tab).  The likelihood ratio test examines the probability that annual precipitation values in the two periods were drawn from the same gamma distribution.  The outcome of this test shows that the probability that the two distributions are drawn from different populations is approximately 0.995, and thus the change is highly statistically significant.

The recent increase in precipitation at Devils Lake has not occurred evenly throughout the year but has been concentrated in certain months.  Figure 4 shows the median precipitation by month for years between 1948 and 1990, and for years between 1991 and 2011.  Significantly increased precipitation has been observed in May, June, and July, and also in October. Little change in precipitation has occurred in the winter months (November-April) since 1991 (Figure 2).

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Figure 2.  Annual basin-average precipitation totals at Devils Lake, from the PRISM dataset.  The blue columns show the annual totals summed over November through April, and the green columns show the May-October totals.

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Figure 3.  Histogram of annual basin-average precipitation totals at Devils Lake, from the PRISM dataset.  The blue columns show the annual totals observered between 1895 and 1990, and the red columns show the 1991-2011 totals.

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Figure 4.  Median basin-average precipitation by month, from the PRISM dataset.  The blue columns show the median values from the 1948-1990 period, and the red columns show the median values from 1991-2011.

 

Spatial Distribution of Precipitation Changes

The spatial extent of the increase in precipitation that has affected Devils Lake is revealed by maps of the difference in average precipitation between years before and after 1991 (Figure 5 and Figure 6).  The Global Precipitation Climatology Center (GPCC) dataset (Beck et al. 2005) was used in order to examine the trends for southern Canada as well as the United States.  Figure 5 (top left) shows that average precipitation has increased across almost the entire Great Plains region, from Texas to central Canada.  Larger precipitation amounts have also occurred broadly over the northeastern U.S.  In percentage terms, the precipitation increase is most dramatic (greater than 10 percent) in the central part of the continent, especially in North Dakota and South Dakota, and in north-central Canada (Figure 5, top right).

 

Figure 5.  Differences in average precipitation between the 1948-1990 period and the 1991-2011 period, from the GPCC dataset.  The four panels show annual differences (top left), annual percentage differences (top right), May-July differences (bottom left), and May-July percentage differences (bottom right).

 

Precipitation amounts in May through July— the period showing the most significant increase at Devils Lake—have also increased broadly, with the greatest percentage changes occurring in parts of the U.S. Midwest, along the U.S. - Canada border from British Columbia to the Dakotas, and also in California and in north-central Canada (Figure 5, bottom right).

The changes near Devils Lake are examined more closely in Figure 6, which is obtained from the PRISM dataset.  Annual totals have increased by more than 50 mm (10 percent) in the eastern Dakotas and northwestern Minnesota.  For the May-July totals, the most dramatic percentage increases have occurred in the region surrounding Devils Lake in northeastern North Dakota (Figure 6, lower right).

 

Figure 6.    Differences in average precipitation between the 1948-1990 period and the 1991-2011 period, from the PRISM dataset.  The four panels show annual differences (top left), annual percentage differences (top right), May-July differences (bottom left), and May-July percentage differences (bottom right).  The Devils lake drainage basin is outlined in red.

 

It is clear from the spatial analysis of precipitation changes since 1991 that wetter conditions have occurred very broadly in central and eastern North America.  However, a zone of particularly marked summertime increases in precipitation is found in the Great Plains near the U.S.-Canada border.  Thus Devils Lake appears to have experienced some of the most dramatic changes in average precipitation to have occurred across the entire continent.

Frequency Distribution of Precipitation Changes

In view of the widespread, significant increase in precipitation near Devils Lake since 1991, it is of interest to examine whether precipitation events have become more frequent, or more intense, or both.  This analysis was based on the daily unified gridded precipitation analysis of the Climate Prediction Center (Chen et al. 2008).  The daily basin-average precipitation was extracted for the Devils Lake drainage basin; the resulting frequency distributions (Figure 7) show that the frequency has increased in the 1991-2011 period for every category of daily precipitation amount.  The overall frequency of measurable (at least 0.01 inches of) precipitation increased from 31.5 percent of days to 35.7 percent of days.

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Figure 7.  Frequency of occurrence of daily basin-average precipitation amounts at Devils Lake, from the CPC Unified Precipitation Analysis.  The blue columns show the frequencies from 1948 to 1990, and the red columns show the 1991-2011 frequencies.

In the highest category of most intense precipitation events (1 inch or more), the frequency increased from 0.36 percent (1.3 events per year) to 0.59 percent (2.2 events per year), a proportional increase of 66 percent.  This increase is much larger than the proportional increases at smaller precipitation categories.  Figure 8 shows that the changes in the larger precipitation categories have accounted for the majority of the observed increase in average annual precipitation since 1991; specifically, the total precipitation falling in daily precipitation events of 0.5 inches or more has accounted for 2.4 inches (64 percent) of the 3.6 inch increase in average annual precipitation.

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Figure 8.  Average annual precipitation totals occurring in categories of daily amount at Devils Lake, from the CPC Unified Precipitation Analysis.  The blue columns show the annual averages from 1948 to 1990, and the red columns show the 1991-2011 averages.

 

Figure 9 illustrates the spatial distribution of proportional changes in the average frequency of daily precipitation amounts greater than 1 inch, confirming that events of 1 inch or greater have become about 60 percent more frequent near Devils Lake since 1991.  Very large proportional increases have also occurred in other regions of eastern North Dakota and northeastern South Dakota.  In view of the dramatic changes in the precipitation distribution in these areas, it is perhaps not surprising that major water management issues have arisen.

Figure 9.    Percent difference in average frequency of daily precipitation events of more than 1 inch, between 1948-1990 and 1991-2011.  Grid points having less than 1 event per year, on average, are masked.  The Devils Lake drainage basin is outlined in red.  Source: CPC Unified Precipitation Analysis.

 

3. Conclusions

The recent dramatic expansion of Devils Lake, North Dakota, has occurred in response to a significant increase in precipitation that began in 1991 and has been concentrated mainly in the months of May, June, July, and October.  Median annual precipitation has increased by nearly 20 percent, a highly statistically significant change.  Precipitation events in the Devils Lake basin have become both more frequent and more intense, on average, but it is the increased frequency of heavy precipitation events that has contributed the majority of the total increase in precipitation.

The changes at Devils Lake have occurred as part of a widespread shift to increased precipitation across most of central North America from Texas to north-central Canada.  However, in percentage terms the changes in North Dakota have been among the largest on the continent.

 

References

Beck, C., J. Gieser, and B. Rudolf, 2005: A New Monthly Precipitation Climatology for the Global Land Areas for the Period 1951 to 2000.  DWD, Klimastatusbericht 2004, ISSN 1437-7691, ISBN 3-88148-402-7, 181-190.

Chen, M., W. Shi, P. Xie, V. B. S. Silva, V. E. Kousky, R. W. Higgins, and J. E. Janowiak, 2008: Assessing Objective Techniques for Gauge-Based Analyses of Global Daily Precipitation.  J. Geophys. Res., 113, D04110, doi: 10.1029/2007JD009132.

Daly, C., W. P. Gibson, G. H. Taylor, G. L. Johnson, and P. Pasteris, 2002: A Knowledge-Based Approach to the Statistical Mapping of Climate. Climate Research, 22, 99-113.

Vecchia, A. V., 2008: Climate Simulation and Flood Risk Analysis for 2008-40 for Devils Lake, North Dakota.  U.S. Geological Survey Scientific Investigations Report 2008-5011, 28p.