Introduction Page 4A


Water, Soil, and More

Precipitation. Annual precipitation averages about 140 cm (55 in), with about 25-30% falling in the winter as snow. Unlike other ecosystems (for example, deserts in the southwestern US), there is neither a "dry" nor a "rainy" season at the HBEF. In fact, on average, about 11 cm (4 in) of precipitation falls each month. According to the graph below, how much does it usually rain in April in Watershed 3? Click on the graph to view a larger image. You will learn more later about these watersheds.

Average Monthly Precipitation in Watershed 3 at the Hubbard Brook Experimental Forest

HBEF scientists have learned that, on average, there are approximately 111 separate precipitation events each year (2 per week). Our scientists have also learned that there is normally a snowpack present from mid-December until mid-April, with a peak depth in March of about 100-130 cm (40-52 in). How does this compare with the area in which you live?

Geology. The granite bedrock of the HBEF is mostly metamorphic rock of igneous and sedimentary origin, and was scraped clean of soil by glaciers (about 13,000 years ago). Bedrock is now visible only along some of the ridgelines and in some streams; the rest is covered in soil and "unsorted glacial till". What kind of rock do you have near your school?

Bedrock outcropping at the HBEF

One of the important features of the HBEF is that the bedrock has few fractures, so only ~ 1% of the annual precipitation leaves the Valley through seepage. As a result, precipitation entering the watershed can leave primarily by only two paths: streamflow and evapotranspiration.

You will learn more later about how this unique and important feature helps HBEF scientists develop ecosystem budgets.

Soil. Soils around the world are classified into different groups. A majority of the soils at the HBEF are called "Spodosols," and were developed in glacial till. These soils have a "sandy loam texture," contain only very small amounts of clay, and have many rocks (from very small pebbles to large boulders).

The "forest floor," or layer of partially decomposed organic matter at the soil surface, is 2 to 20 cm thick and is present except where the soil surface has been disturbed by fallen trees. The forest floor layer permits rapid infiltration of water (from precipitation), and usually protects the soil from freezing before snow accumulates in winter. Virtually no water flows over land at the HBEF.

HBEF soils, like other similar locations in New Hampshire that have not been disturbed by activities such as farm plowing or road construction, have distinct "horizons," or layers lying on top of one another. A variety of biological (for example, decomposition), chemical (for example, rain and soil interactions), and physical (for example, weathering) processes result in the formation of these horizons.

A typical HB spodosol

At the HBEF, soils often consist of an organic horizon (the "O" horizon) containing decaying plant material at the surface. This is the first layer in this photo, under the leaves and above the grey layer.

Below that are several mineral (non-organic) soil horizons. The "E" horizon is the first mineral layer and is light grey.

Other mineral layers - "B" and "C" - are various shades of brown and lie underneath the E horizon.

Click on the photo to the left to see a larger image of typical HBEF soil horizons.

Streams. There are numerous headwater streams in the HBEF, all of which drain into Hubbard Brook. These streams are generally quite small, and range in temperature from near 0° C (32° F) in winter to 18° C (64° F) in summer. Most of the streams are slightly acidic (pH 4.9).

The most conspicuous streamflow characteristic is the large volume of flow in spring and very low flow in late summer and early autumn (see the figure below). What do you think is responsible for the high flow in spring? Why do you think streamflow is much lower in the summer?

Look at the graph below and come up with reasons to explain it; then click here to read the answer. The y-axis is "Monthly Water Amount," and is in millimeters. This axis shows how much water falls on Watershed 3 as precipitation, and how much leaves the watershed as streamflow. As you will learn later, Watershed 3 is a reference watershed.

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

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