• 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.

Chapter OneChapter One Hydrologic Principles Hydrologic Principles

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.

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.

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.

Millington Gauge

On the left is the westernmost subwatershed of the Great Swamp watershed.

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.

Storm Water Component Sequence

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

Evapotranspiration (E+T)• Source of moisture in atmosphere.• Globally, 65-75 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.

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

Kenya’s Deforestation

The Mau in 2006, which once contained huge trees

http://www.ens-newswire.com/ens/jan2006/2006-01-16-02.asp

Kenya’s Drought

Njoro River, 2009

• Karura Forest replanting efforts, 2010

It will take 200 years to restore the forest

Precipitation (P)

• The primary input to the system

Rain, snow, hail, etc.

Depression Storage (S)• http://www.gohydrology.org/2010_09_01_archive.html

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 slopeSlow runoff

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 floodplainat New Orleans after Katrina

Our Florida storm room

Katrina: don’t forget the axe.

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.

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

http://ipy.arcticportal.org/ipy-blogs/item/1632http://www.crh.noaa.gov/mbrfc/?n=msi

http://wwwbrr.cr.usgs.gov/projects/GW_Unsat/Unsat_Zone_Book/

Interflow and Base Flow

Interflow may reach the surface prior to the stream channelBaseflow is saturated zone water that flows intothe channel. The stream runs even when it hasn’t been raining.

Overland Flow, Sheetflow, early Runoff (R)

• Intense rainfall, and rain after infiltration slows, runs to the streams

Streamflow, late

Runoff (R)

• Factors that determine velocityFactors that determine velocity

–GradientGradient, or slope, or slope–Channel characteristics Channel characteristics including including

shape, size, and roughnessshape, size, and roughness–DischargeDischarge – the volume of water – the volume of water

moving past a given point in a moving past a given point in a certain amount of time, i.e a certain amount of time, i.e a FLOWRATE Q=VA FLOWRATE Q=VA –units volume/timeunits volume/time

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.

• 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)

Units• The water budget equation below can have units of flow rate, volume/time, say m 3/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.

Examples

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