Water plays a major role in so many aspects of our lives, and occurs in so many ways in the landscape that it seems to occur - and play a vital role - in virtually every component.  So in this section we look briefly at the whole subject of water - from the viewpoint of the science of HYDROLOGY.  

     (Text: Chapter 9)

 Hydrologic cycle

  Surface waters

  River flow

  Hydrologic measurement

  Water below the surface

  Surface and subsurface waters together

    There is a general circulation of water around the planet, involving water in the air, in the oceans, on the land in rivers and lakes, and under the earth's surface in the soil and in deeper layers of the earth.

    The water can take many pathways, as the diagram illustrates, but we tend to assume that it has a vague cyclic nature, and we talk about the hydrologic cycle.

We have already considered aspects of water on the planet, and of this cycle, notably when considering evaporation, clouds and precipitation in the section on climate.  We also developed some ideas about water on the earth's surface when considering fluvial landscapes and, later, soil and vegetation development.  Some of these aspects will become important as we delve more deeply into the hydrologic cycle and water supply.

    There is a fixed, finite amount of water on our planet.  Much of it is in the various "reservoirs" - of which the oceans are by far the largest.  Nevertheless, there are continuous water flows - transfers between the various reservoirs - which are the components of the cycle we are especially concerned with here.

    On a long-term, global basis there is a general water balance - as much is returned to a particular reservoir as leaves it.  However, on the short-term, or for a local area, there are frequent imbalances, and it is these imbalances which are vital when we consider water availability for our human use.  Thus in this section we will discuss the local "water budgets"  and how they change with time and from place to place.  

Note that the hydrologic cycle is a concept we use to organize our thinking about water and water movement.  It is an example of the systems approach - with lots of simplifications making it a model of the real planetary water flow.  It is also applicable on any time and space scale. 

It is perhaps rather obvious to say, but surface waters commonly occur in four distinct forms:

Lakes: there are not too many natural lakes in North Carolina. Most are on the Coastal Plain.  There are numerous small ones associated with Carolina Bays (although Lake Waccamaw is not exactly small), with a few, Lake Mattamuskeet being the largest, on the peninsula between the Albemarle and Pamlico Sounds.

Swamps:  much surface water occurs in swamps in our state, mainly in the northeast, but - because of our abundant rainfall - they can occur virtually anywhere with impeded drainage. Although we do not think of them as a direct watre source for human consumption these days, a couple of centuries ago swamp water was regarded as a healthy drink (maybe it was in the days before industrial pollution seeped in).

 Streams: we have an abundance of streams - indeed, it seems that you can rarely go more than a couple of miles without crossing a stream.  The dominant direction, except on the west facing slopes of the mountains, is for flow towards the south-east (see map below)

Coastal waters: although clearly these are 'surface waters', and are vital in many ways, we do not use them for for water supply purposes, so we ignore them here.

    The hydrograph is a record of the changes in river flow over time.  In periods without rain the flow of the river is "base flow" - driven primarily by the slow, steady flow of sub-surface water into the surface streams. This base flow declines very slowly with time - so slowly that it is often impossible to see by looking at a hydrograph for only a few days.

  Once a rain event starts and overland flow reaches the stream, the discharge rises.  This is in addition to the base flow, and is known as the "flood flow" (this is really a technical term, since it may not mean that the river is in what we would normally think of as a flood, only that it is above the base flow).

   The "shape" of the hydrograph once flood flow starts depends on the amount and intensity of the rain, its areal extent and location relative to the river's drainage basin, and the nature of the landscape of the basin.  A basin with plenty of vegetation and deep soils is likely, for the same rain rain character, to have a less rapid rise and a lower peak than a basin with bare for - or a lot of urbanization.

    Many atmospheric measurements are needed for hydrologic purposes. This station - officially known as Chapel Hill 2W, because it is located 2 miles west of the Chapel Hill (downtown) post office - measures temperature (in the beehive-like screen), precipitation (the canister in front of the thermometer screen) and evaporation (the pan to the left of the screen - upturned and not in operation in the view).

    This was the Chapel Hill station in 1985.  Now there is much more development around the site - probably giving some urban warming to the temperature record - and possibly influencing the rainfall and evaporation readings.

    In windy areas it is common to place a shield around the gauge to decrease the number of raindrops which are simply blown across the top of the instrument.

Specialized investigations of the soil water require specialized equipment.  A lysimeter of the type shown here is highly specialized, delicate - and expensive.  There are not many like this anywhere in the country.

Gauging stations - such as this one in Edgecombe County, still partially surrounded by debris from the floods of  Floyd - are more common.

We can think of the water below the surface as occurring in a series of layers:

Soil water: the water retained near the surface, held in the relatively small pores of the soil.  This is available for plant use.  The amount available fluctuates directly in response to rainfall input and evaporation output.  We shall say much more about this is later classes.

Unsaturated Zone: below the soil water layer is a region (usually of weathered, broken down rock) with larger pores and more rapid drainage.  Water passes relatively quickly through this to the:

Water Table: strictly this is simply the top of the saturated zone.  It is seen, and can be measured, because it is the level of standing water in a well. This level varies from place to place, depending on the amount of water percolating down, the depth to bedrock and the amount of sideways drainage - the subsurface flow.  It is convenient to visualize this sub-surface flow as moving "over" the water table - flowing downhill, just as a surface stream might flow over the surface.  However, with the subsurface flow the level of the water table falls as the flow occurs.

Saturated zone:  this zone, where all the pores are filled with water, is between the water table and the solid bedrock.  It represents the ground water potentially available for human use. 

Water flowing "down" the water table will eventually intersect the ground surface.  Most commonly this will occur at a stream.  Ground water will flow into the stream from the sides and also from below - there are extra, capillary, forces at work in this near-stream region. 

   Less common, the intersection occurs away from a stream, creating a spring. 

   During dry period the level of the water table will gradually fall as the water drains to provide the base flow of the stream.  If the drought is prolonged, the water table may become flat and there will be no water movement towards the streams.  Further, ground water may continue to be extracted level as the result of surface evaporation (which may be a very slow process) or by wells for human water supply (which may be a very rapid extraction.  This the water table may eventually fall below the stream level, and water will move from the stream into the ground water, depleting the stream flow.

   The final possibility, of course, is for a stream to cease to flow completely during an extremely long dry period.  Given North Carolina's climate this is extremely unlikely for all except the very smallest, shortest headwater tributaries - where a "very long dry period" may only be a matter of a few days.