Evapotranspiration & Cloud Types: Measurement Methods
Actual Evapotranspiration (ETR)
Actual evapotranspiration (ETR) refers to evapotranspiration under real environmental conditions, considering variations in soil moisture and incomplete cover.
Methods for Determining Potential Evapotranspiration
Experimental Methods
Experimental methods are conducted in agricultural experiment stations, either in test plots or in lysimeters.
- Test Plots: Test plots involve preparing plots of land with established vegetation cover. The test controls all moisture inputs to the plot, such as precipitation, irrigation, and water table contribution. Crop consumption can be inferred from the water level height.
- Lysimeters or Evapotranspirometers: Lysimeters consist of calibrated tanks filled with undisturbed soil to develop vegetation cover. Evapotranspiration is measured by balancing moisture inputs and outputs of the lysimeter tank, then deducting the portion corresponding to evapotranspiration.
Adjusted Hargreaves Method
The Adjusted Hargreaves Method involves using mathematical relationships involving meteorological data such as precipitation, temperature, humidity, wind, and radiation.
Clouds
Classification by Shape
- Cirrus and Cirrus: Clouds formed by tiny ice crystals. They appear at altitudes above 8.0 km and have a filamentous appearance.
- Stratus or Strata: These are low, horizontal clouds, mainly seen at night, and may indicate the proximity of rain.
- Cumulus or Clusters: Flat-based, massive clouds formed by rising air currents during the day, especially in summer.
- Nimbus or Nimbus: These are dark gray clouds that bring rain and snow, often clouding the sky.
Classification by Altitude
- High Clouds: Clouds found above 6 km in height.
- Middle Clouds: Clouds found between 2 and 6 km in height.
- Low Clouds: Clouds found from the surface up to 2 km in height.
Observation of Clouds
- Clear Sky: Cloud cover is less than 2 octas.
- Cloudy Sky: Cloud cover is equal to or greater than 2 octas and equal to or less than 6 octas.
- Overcast: Cloud cover is over 6 octas.
Cloud Height Determination
- Using a Nephoscopic Balloon: Balloons filled with hydrogen and a known constant upward velocity are used. A stopwatch measures the time it takes for the balloon to penetrate the cloud.
- By the Vertical Light Projector Method: The incidence of light in the clouds is observed with a theodolite placed at a known distance.
Determining Cloud Speed and Movement
A nephoscope is used to determine the speed and movement of clouds.
- Direct Vision Nephoscope: Consists of a grid of metal bars, one meter long, mounted on a vertical shaft that can rotate. The apparent velocity is recorded by measuring the time it takes for a cloud to move from one bar to another.
- Mirror Nephoscope: Consists of a black glass disc fixed on a tripod with leveling screws.
Lines connecting points of land or any part thereof having the same cloud cover are called “isonets.”