Hydrologic cycle

Readings:
Environmental Hydrology - Ward and Elliot Chps 1, 2, 4(91-102, 112-126 (in brief)), 5 (133-155), 7 (211-229, 245-248)
The depth of reading for above depends on your background

Others are hot-linked below

Components
· Atmospheric water (rain)
· Interception
· Depression storage
· Overland flow (runoff) - sheet, rill
· Infiltration
· ET
· E

Movement in the ground
Interflow - lateral flow in vadose zone
Most flow in vadose zone is vertical (gravity drainage)

Baseflow - flow to streams from groundwater

The hydrologic cycle has significant changes in state of water from gas to liquid and back to gas and to solid form. This involves energy - changes in thermal energy.
· Latent heat of vaporization: energy required to vaporize
· Latent heat of condensation (equivalent to vaporization), reverse of above
· Latent heat of fusion
· Condensation of water to ice (freezing)
· Sublimation

Water Balance
In = Out ± Storage

Ins	Outs	Storage
Precipitation	ET	SW in rivers, lakes etc.
SW inflow	E	Soil moisture
GW inflow	Runoff	Interception
Import	GW Outflow	Ice, snow
	Export	Depression storage
		Groundwater

Condensation and Precipitation
Form: Snow, rain, hail

Adiabatic expansion. Air expands and thus cools.
Adiabatic lapse rate

There are three main processes by which air is lifted and precipitation occurs
Frontal precipitation
Orographic precipitation
Convective precipitation

Storm description:
Form, total precip, intensity, duration, raindrop size, area of coverage

Measurement (relatively easy)
Standard US
Weighing rain gage
Transducer
Tipping bucket

Data collection: NOAA, state weather service
Also in pubs is trend data, deviation from norm

Stephenville weather on-line
Date ET in/d, Temp min-max, min rel humidity, solar radiation, rain, avg wind

NOAA southern region
TX state climatologist
http://www.ncdc.noaa.gov/ lots of data, some free - some not

Time Trends
How do you tell if there has been a change in precipitation over an area? What can you plot?

How many gages do you need to adequately characterize a basin?

Point precipitation statistics
total rainfall
duration
peak intensities for given time intervals
average intensity

Estimation of precipitation over a basin given a network of gages
Arithmetic average, Theissen method, isohyetal method

Rainfall probability
What is the probability of a given rainfall event?
Hazen SCS method
What is probability that a given return period will occur? P = 1 - {1-1/t}^n
t - return period, n - length of interest

PMP maps

River Flow Data
Where obtained
http://water.usgs.gov/

How do you measure stage? discharge?
Field Methods
Read the overview stuff which will tell you how things are done.

7,590 gaging stations in early 1990s, now?
Earliest Rio Grande - 1895
Benchmark stations used to represent natural discharge - 57

Hydrographs
What are components?
How is a hydrograph affected by land use
How is a hydrograph affected by placement in watershed

A hyetograph plots rate of rainfall versus time. These are often plotted with hydrographs for single events.

Flow duration curves
P = 100(m/(n+1))
Recurrence interval

What does the shape of a flow duration curve indicate?

Baseflow recession

Estimations of surface runoff can be made in several ways.

Most common method is SCS curve number approach
Q=(p-Ia)2/(P-Ia+S)
P rainfall depth
Ia = initial - surface storage, interception, infiltration prior to runoff -=0.2S
S=(1000/CN)-10

Curve Numbers
Soil descriptions
AMCs

The runoff equation for small basins (200ac), greatly simplified, is Q=CIA. Suppositions are precip rate constant (very rarely) and infiltration rate constant (almost never). C is based on land use type and read from a table. Now a lot of this work is done by models aided by GIS maps in which land uses can be more accurately portrayed.

Flood estimation techniques - Peak Flows
Rational equation q=1.008CiA if q in cfs, 1.008 conversion factor
i=avg rainfall intensity

Volume predictions generally better than peak flow predictions

Evaporation
Evaporation is the term for vaporization of water from free water surfaces or from bare soil.

Terms:
Saturation humidity
Relative humidity
Dewpoint

Fick's law of diffusion
Flux density = diffusion coefficient * concentration gradient

Evaporation measurements

ET
The combined process of transpiration and evaporation. Transpiration is defined as the loss of water from plants in the form of vapor. Transpiration is dependant on plant type, density of vegetation, growth stage.

Consumptive use is total amount of water needed for crop, usually very close to ET since water stored in plant is very small

PE - potential ET
Penman's definition - the amount of water transpire in unit time by a short green crop completely shading the ground, of uniform height, and never short of water.

AE - actual ET is less than or equal to PE

What affects ET? Solar radiation, wind, plant type, soil type, soil moisture, precipitation regime

Direct measurement:
Soil lysimeters:
Commercial ET gaging device - simulate leaf surface
Measure change in soil water

Methods of estimating:
Thornthwaite
Blaney-Criddle
Penman
Penman-Monteith

Runoff
Sheetflow, rill flow, or gully flow.

Some facts:
US totals: US avg precip 30 inches, US avg ET 21 inches, 9 inches for recharge and runoff
40% of runoff carried by Mississippi

Watershed factors which affect landuse:
Size
Topography
Shape
Geology
Soil
Land Use

Review Questions

What are the components of the hydrologic cycle? Write an equation defining a water budget for a lake.
Discuss at least two methods for looking at time trends in climatic data such as precipitation or streamflow.
Discuss the three methods for determining rainfall distribution over an area.
Be able to determine the probability and return period of a given rainfall event.
There are many watershed factors which directly affect runoff. Discuss.
Be able to discuss a hydrograph curve - its components, how it might be affected by different geology, different land use.
Discuss how a hydrograph would change
    a) from upstream to downstream as the result of an upstream event
    b) if an area became urbanized
    Include discussion of curve shape, peaks, baseflow, lag time
Be able to compare flow duration curves. What do they indicate?
What is a baseflow recession curve? What can it be used for?
The SCS curve method and the Rational equation are used for what purposes? How are they different? What variables do you     need to calculate them? To what settings do they apply?
What are the components of the Manning equation? What do they mean?