Introduction Page 4 - Answer


Streamflow Explanation

Average monthly precipitation and streamflow for the Watershed 3 (1956-1988).

As you can see above, on average about 100 mm of precipitation falls on the HBEF every month of the year. Precipitation enters the ecosystem, and after short periods of storage in the soil or snowpack, it leaves as streamflow or evapotranspiration. Therefore, over the course of an entire year,

Precipitation = Streamflow - Evapotranspiration.


So, why is streamflow high in March, April, and May, and much smaller in July, August, and September?

* * * * *

  1. Water from melting snow causes high streamflow levels in spring.

  2. Streamflow is very low in late summer because of high levels of evapotranspiration.

During the winter at the HBEF, precipitation usually falls as snow and accumulates as a snowpack. Snowpack is deepest in early March, with depths of about 100-130 cm (40-52 in). In late March temperatures begin to rise, and the snowpack begins to melt. The melting snow runs off watersheds

High streamflow in Cascade Brook.
and into streams, causing higher streamflows.

Melting rates are usually highest in April - and this is when streamflows reach their highest levels. In May the snowpack is smaller, but it continues to melt and streamflow is still higher than normal. Usually by June streamflows have returned to more typical levels.

By July all the trees and other vegetation have fully-developed leaves, and evaportranspiration is very high (trees are taking up more water from the ground than they are during other parts of the year).

Because plants are evapotranspiring at high levels, less water reaches the stream. Therefore streamflow decreases to a very low level.

How did you do? Did you explain this graph correctly, or did you have other ideas?


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