U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005-3020

March 2005

Trends in the Water Budget of the Mississippi 

River Basin, 1949-1997

... from the National Streamflow Information Program

Printed on recycled paper

This Fact Sheet is one in a series that highlights information or recent research findings from the USGS 

National Streamflow Information Program (NSIP). The investigations and scientific results reported in this 

series require a nationally consistent streamgaging network with stable long-term monitoring sites and a 

rigorous program of data quality assurance, management, archiving, and synthesis. NSIP produces multipurpose, unbiased surface water information that is readily accessible to all.

Introduction

The Mississippi River flows southward from Minnesota to the Gulf of 

Mexico (fig. 1), draining areas as far east 

as the Appalachian Mountains, as far 

north as Canada, and as far west as the 

Continental Divide. Runoff from 

31 states drains through the Mississippi 

to the Gulf. Discharge of the Mississippi 

River is affected both by climate and by 

human activities. Understanding these 

influences, which is key to management of the water in the basin, requires 

information about the basin hydrology, 

especially characterization of long-term 

and seasonal streamflow, including both 

flood and drought. Gulf of Mexico

1

Appalachian Mountains

3 4

5

Continental Divide

0 500

Mean water 

discharge, in cubic 

kilometers per year

1. Atchafalaya River

2. Red River

3. Missouri River

4. Mississippi River

5. Ohio River

0

100

200 MILES

0

100

200 KILOMETERS 1

2

95 

90 85 

80 75 

115 

45 

40 

35 

110 105 100 

CANADA

Keokuk

Natchez

Vicksburg

0

200

0

200

400

600

800

1000

1200

1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

(Millimeters per year)

Discharge

(Millimeters per year)

Precipitation

Figure 1. Mississippi River basin with major tributaries and state boundaries. The width of a river 

indicates its mean water discharge (modified from Circular 1133, Figure 5).

Figure 2. Annual precipitation in and discharge from the Mississippi River basin. The red lines 

represent long-term mean values of precipitation and discharge.

Climatic Influences

The flow of the Mississippi River 

is maintained by runoff of rainfall and 

snowmelt. However, not all rainfall or 

snowmelt runs off. Some infiltrates the 

soil and is subsequently transpired into 

the atmosphere by plants or evaporates 

directly into the atmosphere. In total 

over the basin, evapotranspirative losses 

consume more than three quarters of the 

annual precipitation, but these losses vary 

regionally. In the arid western portion of 

the basin, very little water is left to run 

off after evapotranspiration extracts its 

share; most of the river’s flow is runoff 

from the relatively moist eastern part of 

the basin (fig. 1). Flow also varies in 

time corresponding to variability in precipitation and evapotranspiration 
(fig. 2). For example, in 1988, precipitation was 18 percent below the longterm average [835 millimeters per year 

(mm/year)] and discharge was 29 percent 

below the long-term average (187 mm/

year). In 1993, however, precipitation 

was 19 percent above the long-term average and discharge was 52 percent above 

the long-term average.

Human Influences

The balance between evapotranspiration and runoff is modified, both directly 

and indirectly, by human activity. The 

greatest human effect is associated with 

agricultural irrigation, which is widely 

practiced in the arid west. The net effect 

of this activity is to divert water from runoff into evapotranspiration. The second 

greatest human effect is associated with 

evaporation of water from the surface 

of reservoirs. An estimated 6 percent of 

natural runoff is diverted to evapotranspiration as a direct result of human activity 

(table 1, (C/E)•100).

The creation of surface-water reservoirs and the development of groundwater resources have led to changes in 

storage of water inside the Mississippi 

River basin. During the second half of the 

20th century, the discharge of the Mississippi River was about 0.5 percent smaller 

than it would have been in the absence of 

extractions to fill surface reservoirs (table 

1, (F/H)•100). On the other hand, extraction of water from ground-water storage 

averted an approximate 1-percent reduction of discharge that would otherwise 

have been needed to satisfy water users 

(table 1, (G/H)•100).

Trends in the Water Budget

The trends over time in various waterbudget terms are affected by climate and 

human activities. During the latter half 

of the 20th century, the most significant 

control has been a 2.1-percent per decade 

upward trend in annual precipitation 

(fig. 2, table 1). As a result of nonlinearity in natural runoff processes, this 

upward trend has disproportionately 

affected runoff, which increased at a rate 

of 5.5 percent per decade (table 1).

The increase in runoff has been greater 

than the increase of human-induced 

evapotranspiration. If runoff rates would 

have been constant, then the increase of 

human-induced evapotranspiration would 

have caused a decrease in discharge, at a 

rate of about 1 percent per decade.

The rate of filling of surface-water 

reservoirs changed markedly during 

1949-1997, because most reservoirs were 

filled during the first half of this period. 

The decreasing rate of storage helped to 

free up runoff to support the increase of 

human-induced evapotranspiration. On 

the other hand, estimated net extraction 

of ground water also declined, reducing 

somewhat the availability of water to support use and discharge.

The Future

Trends are history; they cannot be 

extrapolated confidently to forecast the 

future. In the Mississippi River basin, 

research (Milly and Dunne, 1998) indicates that much of the observed upward 

trend in precipitation may have been a 

manifestation of natural variability. Thus, 

the upward trend will not necessarily 

be sustained into the future. Cessation 

or reversal of the trend would decrease 

flows through the basin, thus having 

consequences for water supply.

Superimposed upon natural variability may be changes in precipitation 

associated with anthropogenic climate 

change. Such trends during the coming decades are not expected to be as 

large as the recent precipitation trends 

in the basin. However, their direction is 

unknown. On the other hand, in contrast 

to changes driven by natural variability, 

these changes may be cumulative and 

sustained.

Toward the end of the 20th century, the 

rate of increase of water use and associated human-induced change in evapotranspiration slowed or stopped. If water 

use were to stabilize in the future, then 

further increases in runoff would not be 

needed in order to maintain the rate of 

discharge.

For additional information, contact:

P.C.D. “Chris” Milly

U.S. Geological Survey

GFDL/NOAA

P.O. Box 308

Princeton, NJ 08542

(609) 452-6507

cmilly@USGS.gov

References

Milly, P.C.D., and Dunne, K.A., 2001, 

Trends in evaporation and surface 

cooling in the Mississippi River basin: 

Geophys. Res. Lett., v. 28, p. 1219-

1222.

Milly, P.C.D., and Dunne, K.A., 1998, 

Non-detectability of 20th-Century 

trends in river discharge from large 

basins--observational and modelbased results. In Ninth Symposium 

on Global Change Studies and Namias Symposium on the Status and 

Prospects for Climate Predication, 

American Meteorological Society, p. 

162-163.

Table 1. Major terms in the water balance of the Mississippi River basin, 1949-1997. 

All flows are expressed as volumes of water per unit area of the basin per year, or equivalently, depths of water per year. Mean flows, as well as rates of change in millimeters 

per year per decade, obey the following equations: A=B+E; B+C=D; E-C-F+G=H. 

Some of these equations are not obeyed exactly by the numbers in the table because of 

rounding. (Data from Milly and Dunne, 2001.)

Process

Mean Rate of Change

(Millimeters 

per year)

(Millimeters per 

year per decade)

( Percent 

per decade)

A Precipitation 835 17.8 2.1

B Natural evapotranspiration 637 6.9 1.1

C Human-induced evapotranspiration 12 2.6 22

D Total evapotranspiration 649 9.5 1.5

E Natural runoff 197 10.9 5.5

F Runoff lost to fill surface reservoirs 1 -0.5 -50

G Net removal of ground water 2 -0.3 -15

H Mississippi River discharge 187 8.5 4.5