Figure 2. AFDD at the National Weather Service Station at North Platte, Nebraska, during water year 1960

Hydrological data

Summary of historical data collected

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Figure 2. AFDD at the National Weather

800

Service Station at North Platte, Nebraska,

during water year 1960.

Julian Day (1 Oct is day 1)

temperature is simply the average of the mini-

tor of ice thickness and rapidity of snowmelt. They

mum (Tmin) and maximum (Tmax) air temperatures:

are calculated using the equation

where Tb is the base temperature at which snow-

Freezing degree-days (FDD) are calculated from

melt and ice deterioration are considered to be-

the ADAT using the relation

gin. Because snowmelt and ice deterioration can

occur when the average daily air temperature is

FDD = 32 ADAT.

(2)

less than 32F, the base temperature can range

from about 25 to 32F. Thawing degree-days ac-

A positive value of FDD means that the aver-

age daily air temperature is lower than 32F, while

cumulated after the JD start cold are called ATDD.

a negative value means that the average daily air

If the ATDD drops below zero, accumulation ends

temperature is higher than 32F. Accumulated

and ATDD is considered to be equal to zero until

freezing degree-days (AFDD) are the sum of the

the next positive TDD.

freezing degree-days during the winter, begin-

Snowfall data are important because the pres-

ning 1 October (the start of the water year). Since

ence of snow can affect ice thickness. Snow on top

air temperatures are still relatively high in Octo-

of an ice cover will insulate the ice, slowing its

ber in Nebraska, however, AFDDs are summed

growth during cold weather. Conversely, if there

until some minimum value is reached. The date

is no snow on top of the ice, the ice will thicken

of the minimum AFDD is called the JD start cold.

more quickly during cold periods. Snowfall is also

Upon reaching the JD start cold, AFDD is reset

an indicator of the amount of snowpack available

to zero and begins accumulating again until

during the snowmelt. The combination of deeper

some maximum value is reached (AFDDmax).

snowpacks and rapid snowmelt causes larger in-

creases in stage and discharge than shallower

The date of the AFDDmax is called JD AFDDmax.

snowpacks under the same conditions. Also, the

The AFDDmax marks the change from generally

timing of snowfall can be important since late

colder weather (average daily air temperature

snows are more likely to still be on the ground

below freezing) to weather where the average

when the breakup season arrives. Finally, heavy

daily air temperature is above freezing. After the

snowfall on ice can cause a depression of the ice

AFDDmax is reached, FDDs continue to be ac-

cover that allows water to flood the ice cover. The

cumulated until they drop below zero. Figure 2

saturated snow can freeze into snow ice, increas-

illustrates the parameters derived from the mini-

ing the overall ice thickness. Data on precipita-

mum and maximum temperature data.

tion other than snowfall are also collected because

Negative freezing degree-days are termed

thawing degree-days (TDD). Thawing degree-

precipitation combined with snowmelt can result

days are of interest because they can be an indica-

in rapid runoff and sharp increases in stage that

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