Introduction:
In this week’s
exercise we will be estimating one of the most important environmental
variables; evapotranspiration.
Evapotranspiration is one of two variables that define the amount of
water available at the earth’s surface, the other being precipitation (which we will
also begin to look at this week).
In its simplest sense, the amount of water available at the earth’s surface for the growth of vegetation (cultivated and natural) and to fill our streams, lakes, etc. is a result of a simple balance between the water input (precipitation) and water output (evapotranspiration). Thus, we need to be able to accurately estimate the precipitation and evapotranspiration if we want to have some idea about the amount of water available to sustain a local environmental system. Please note that there are many other important variables that determine the water availability including the topography of the region, the soil types, the underlying geology, etc. However, we will not be looking at these in detail in relation to water availability.
We will consider precipitation to mean the total water input to an environmental system from the earth’s atmosphere. This water may fall to the earth in the form of rain, hail, freezing rain, sleet, or snow.
The total amount of water evaporated from the earth’s surface (soil, water bodies, etc.) and transpirated from vegetation we will call evapotranspiration.
Measuring Precipitation and Evapotranspiration:
Although it may seem that both precipitation and evapotranspiration would be “easy” to measure or estimate, it turns out that neither is easily measured.
Precipitation
Precipitation amounts are often estimated using relatively simple instruments. Rain gauges are the most common instrumentation used for measuring water falling from the earth’s atmosphere. There are many different types of rain gauges in use around the world today, some of which we will discuss in class and many that we won’t talk about. Other methods to measure precipitation include the use of Doppler Radar (which we will talk about later) and in some cases remotely sensed data from satellites.
Evapotranspiration
Evapotranspiration is a more difficult variable to estimate or measure than most others that we will look at in this class (at least in the atmospheric environment section). This is because there is no reliable “direct” method to measure the evapotranspiration. Instead, several atmospheric variables must be measured simultaneously, and their values used in an empirically derived equation. Probably the most commonly used equation for the estimation of evapotranspiration is the Penman-Montieth Equation. The Penman-Montieth equation is given for hourly time steps as:
ET = 0.408 L(Rn– G) + g (37/(Thr+273)) u2(eo – ea) / (L + g (1 + 0.34 u2))
Where L, Rn, G, g, eo, and ea are “constants” that are dependent upon the meteorological conditions such as wind speed, incoming solar radiation, air temperature and relative humidity. We will take measurements of all of these meteorological variables (wind speed, solar radiation, temperature, relative humidity) and use them to calculate the instantaneous evapotranspiration. Unfortunately, the calculation of the constants is quite laborious. Therefore, to assist you I will calculate the constants and make them available to you on the Monday after the field exercise.
The Field Exercise:
This week’s field exercise will be
divided into two major tasks. Measuring
precipitation and measuring evapotranspiration.
Precipitation
Since we cannot depend upon
precipitation occurring on the day of our scheduled fieldwork, we will be
measuring precipitation for several weeks with rain gauges that will be supplied
to you in class. You will need to site
these rain gauges at or near your residences in the best possible
location. You should take daily precipitation
observations until October 6th , 2000!! On this date you will hand in your observations and you
will be instructed on analysis techniques.
Evapotranspiration
We will complete estimates of
evapotranspiration at two sites within Nottingham County Park; a deciduous
forest and a savannah site. At each
location we will take measurements of wind speed, incoming solar radiation, air
temperature and relative humidity for a twenty-minute period in order to
estimate the amount of evapotranspiration occurring in each environmental
system. The instruments that we will be
using will include:
Wind speed – an
anemometer, wind count data logger
Solar Radiation –
a pyranometer
Air Temperature –
a handheld electronic thermometer/hygrometer
Relative Humidity
– a handheld electronic thermometer/hygrometer
A suggested table
for recording the information is given below.
Additional details will be provided in class.
Data Table (evapotranspiration)
Site__________ Weather
Conditions______________
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Time |
SWR down |
Air Temp |
R.H. |
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