1. Chapter One Hydrologic Principles
You know that water evaporates from the ocean, falls as
precipitation onto the land, either evaporates again, soaks into the ground,
or runs off and returns to the ocean.
You know that water evaporates from bodies of water, wet soil, and plants, falls as precipitation onto the earth, either evaporates again, or soaks into the ground, or runs off and returns to the ocean.

2. A watershed or catchment basin is a contiguous area that drains to an outlet, the area around a stream that actually sends water into the stream. Perhaps
the most easily recognized watershed in the US is the Continental Divide.
On one side, water eventually ends up in the Pacific, and on the other, the Atlantic.
A watershed or catchment basin is a contiguous area that drains to a common outlet. It is the area around a stream that actually sends water into the stream.

3. The drainage divide is the locus of points that separates adjacent watersheds. So things that are happening on one side of the divide don’t
affect things on the other.
The drainage divide is the locus of points that separates adjacent watersheds.
Perhaps the most easily recognized divide in the US is the Continental Divide.
On one side, water eventually ends up in the Pacific, and on the other, the Atlantic.

4. In large watersheds with multiple tributary basins it is sometimes convenient to define sub-basins, provided you have a gauge at each sub-basin outlet, and rain gauges, so you know the lag time between peak rainfall and the time of high water.
Watersheds can be defined at a number of different scales. The South Branch of the Raritan has a watershed of its own, and one could consider the South Branch Watershed to encompass the entire drainage around the South Branch. South Branch Watershed would end where it flows into the Raritan River.
However, the Raritan also has a watershed. It encompasses the entire South Branch watershed, and also the North Branch Watershed, ending only where Raritan Bay empties into the Atlantic.

5. On the left is the westernmost subwatershed of the Great Swamp watershed.
Millington Gauge

6. Some symbols
Water falls onto the earth’s surface as rain or snow, marked P for precipitation. Some of the surface water Evaporates (E) or is transpired (T) by plants to the gas phase “Water Vapor”, and returns to the atmosphere. Some of it soaks into the ground, a process called infiltration (F), and becomes a part of groundwater (G), our major source of drinking water. The rest becomes runoff (R), and eventually most of that gets to the sea.

7. Storm Water Component Sequence

8. Interception (part of Evap., E)
LOSS: Interception loss is that part of the precipitation that falls on plants and doesn't reach the ground. It evaporates (or sublimates) from leaves, near-ground plants and leaf litter or, to a lesser extent, is absorbed by plants

9. Evapotranspiration (E+T)
Source of moisture in atmosphere.
Globally, percent of precipitation occurs over land as a result of evapo-transpiration from lakes and wetlands and dense vegetation, in particular tall forests, pumping deep groundwater in the soil’s C-horizon into the air.

10. The Karura Forest, Kenya, in 1972
Big forest trees are transpiration factories. They tap groundwater that crops cannot reach, returning it to the atmosphere.
The Karura Forest, Kenya, in 1972

11. Kenya’s Deforestation
Kenya’s Deforestation
The Mau in 2006, which once contained huge trees

12. Kenya’s Drought
Njoro River, 2009

13. Karura Forest replanting efforts, 2010
It will take 200 years to restore the forest

14. Precipitation (P) The primary input to the system
Rain, snow, hail, etc.

15. Depression Storage (S)
Detention storage eventually returns water downstream, as here. Retention storage holds the water, as in a reservoir.
Storage is very important for flood control. Examples range from huge natural systems such as the everglades and the Mississippi River’s floodplain, to small storm sewer systems with artificial storage ponds.
Very gradual slope
Slow runoff

16. Building on Detention Storage
Unfortunately, many cities allow construction on floodplains, which used to provide natural detention storage. Detention storage should hold storm water, and release it slowly, avoiding floods. Instead:
The Mississippi floodplain
at New Orleans after Katrina
Our Florida storm room
Katrina: don’t forget the axe.

17. Infiltration (F) into Groundwater (G)
Infiltration (symbol F) is controlled by
Intensity and duration of rainfall
Soil texture
Slope of the land
Nature of the vegetative cover
Water can spread nearly horizontally in the zone of aeration (interflow) or can move downward into the zone of saturation.

18. Soil Moisture (part of infiltr
Soil Moisture (part of infiltr. F) Interflow portion may return to surface runoff. Remainder descends into the groundwater
Liquid water in pore spaces of upper zone of aeration

19. Interflow and Base Flow
Interflow may reach the surface
prior to the stream channel
Baseflow is saturated zone water that flows into
the channel. The stream runs even when it hasn’t been raining.

20. Overland Flow, Sheetflow, early Runoff (R)
Intense rainfall, and rain after infiltration slows, runs to the streams

21. Streamflow, late Runoff (R)
Factors that determine velocity
Gradient, or slope
Channel characteristics including shape, size, and roughness
Discharge – the volume of water moving past a given point in a certain amount of time, i.e a FLOWRATE Q=VA
units volume/time

22. The Water Balance
Conservation of Mass with Storage. 
Consider a watershed. We’d like to know, over the course of one month, how much water got added to the watershed, i.e. how much water got stored in it.  
We might care because we’re using that stored surface water for a water supply, or we might be worried that all that excess water will cause a flood.

23. One way of describing this would just be to add up all the things we can think of that add water to the system, and then subtract all the things that we can think of that remove water from the system. The difference would be the change in Storage:
Inputs (Gains)
P = precipitation
I = inflow
Outputs (Losses)
E = evaporation
T = transpiration (these are commonly combined to make “ET” = E + T)
R = surface runoff at outlet
Depending on the control volume (imaginary volume where we know or can calculate everything we want to know):
G = groundwater flow (could be an input, could be an output)

24. Units
The water budget equation below can have units of flow rate, volume/time, say m3/sec
Alternately, we can calculate the change in storage, in depth/time, say inches of water/month, by multiplying all terms by the time interval and dividing by the area of the watershed.

25. Examples
As usual, I’ll do an example, and then you will work on a similar homework problem.