The impact of climate must be included in all evaluations of water availability in Minnesota. Human activity aside, surface and groundwater quantity is driven by the balance between atmospheric input from precipitation, and losses due to evapotranspiration. Very few of the watersheds in Minnesota extend beyond the borders of the State. Therefore, knowledge of Minnesota's climate patterns provides important insight into water availability issues.
Because it is located near the center of the North American continent, Minnesota is subject to a variety of air masses that make up its climate. Cold, dry continental polar air dominates the winter season, occasionally replaced by somewhat milder maritime polar air. During the summer, hot and dry continental tropical air masses from the desert southwest share predominance with warm and moist maritime tropical air that originates over the Gulf of Mexico. The spring and fall seasons are transition periods composed of alternate intrusions of air from various sources. The diverse nature of the air masses impacting Minnesota's climate leads to a high degree of variability across space (spatial variation) and time (temporal variation).
The primary source of moisture for precipitation in Minnesota is the tropical maritime air that moves into the State from the south and southeast. The spatial variation of average (normal) annual precipitation across Minnesota is determined by proximity to these moist air masses coming northward out of the Gulf of Mexico. Therefore, southeastern Minnesota, averaging near 32 inches, receives more precipitation than northwestern Minnesota, less than 19 inches. The normal annual precipitation for Minnesota (1961-1990) is 27.01 inches.
The presence of moist vs. dry air masses also helps to determine the atmosphere's ability to absorb water vapor evaporating from soil and open-water surfaces, or transpiring from leaf surfaces (evaporation plus transpiration is called "evapotranspiration"). Western Minnesota, more frequently under the influence of dry air masses, has higher evapotranspiration rates than the eastern half of the state. Temperature plays an important role in determining the amount of energy available for evapotranspiration. Because spatial temperature patterns are determined mainly by latitude, southern Minnesota experiences more evapotranspiration than in the north.
Due to its position in the continent, Minnesota is located on the boundary between the semi-humid climate regime of the eastern U.S., and the semi-arid regime to the west. Semi-humid climates are areas where average annual precipitation exceeds average annual evapotranspiration, leading to a net surplus of water. In semi-arid areas, evapotranspiration exceeds precipitation on average, creating a water deficit. In Minnesota, the boundary between the climate regimes cuts the State roughly into east-west halves.
Seasonal variability occurs, as different air masses are dominant. Nearly two thirds of Minnesota's annual precipitation falls during the growing season of May through September, a period during which Gulf of Mexico moisture is often available. Dry spells occur when this moisture source is obstructed or when atmospheric patterns divert storm systems around Minnesota. When hot, dry air prevails, increased evapotranspiration combines with deficient rainfall to create drought conditions. Drought can occur in all areas of Minnesota, however it is more likely in western and northwestern areas more distant from Gulf of Mexico moisture. When Gulf moisture is abundant and numerous storms move through Minnesota, unusually heavy precipitation falls. Repeated rain events can overwhelm surface water systems, raising lake levels and forcing streams out of their banks. Singular, intense rain events can lead to flash floods anywhere in the State.
Only eight percent of average annual precipitation falls in the winter (December through February) when the dry polar air masses prevail. Yet, large scale spring flooding can occur as a result of a combination of a deep late winter snow pack, frozen soil which prohibits infiltration, rapid snow melt due to an intrusion of warm air, and heavy early spring precipitation.
Given the multiple weather scenarios affecting Minnesota, wide ranges of climatic outcomes are the norm. "Normal" is merely a mid-point about which we fluctuate. A look at one Minnesota region illustrates the point. The adjacent figure depicts annual precipitation totals for east central Minnesota from pre-settlement times through the present. A striking feature of this time-series is the range of values. The lowest annual precipitation total recorded over the 161-year period was 10.21 in 1910. Only one year later, precipitation totaled 40.44 inches, the highest on record! Multi-year periods of similar weather also stand out in the climate record. For example, annual precipitation totals show a distinct drying pattern from the turn of the century though and including the "Dust Bowl" years of the 1930's. Note the relative lack of "wet" years over that period. Beginning in roughly 1940, the precipitation trend takes a ladder-step up to an era of tremendous variation including episodes such as the 1976 drought and the extraordinarily wet period in the mid-1980's. It is important to note that climate extremes should not be considered as aberrations, but rather treated as an inherent component of a continental climate. The difficulty comes in learning to live within the extremes knowing that they are not only possible, but likely to occur. Such knowledge does not prevent their occurrence, but does help to shape decisions and make plans that lessen the impact of the extremes upon human activity. When seen in this light, long-term efforts in areas such as water conservation, planning and flood damage reduction take on increased importance.