SOLAR RADIATION - The amount of radiant energy emitted by the sun that reaches the Earth, including visible light, infrared light (heat), ultraviolet light, and x-rays. Of essential importance to life on Earth, it is the driving force of weather and climate. Solar radiation sensors measure the combination of direct radiation (that passing directly through the atmosphere without being absorbed or scattered) and diffuse radiation (that scattered by airborne particles or clouds). The highest values occur during sunny clear days and lowest during overcast conditions. Direct solar radiation is zero during heavy overcast or when the sun is below the horizon.

Solar radiation sensors need to be mounted in an area free of shadows or reflections of sunlight by nearby objects. The standard mounting height is 6.5 feet (2 meters) above ground in a southern exposure. The sensor should be mounted horizontally with respect to a point on the ground directly below the sensor. The sensor should be cleaned on weekly basis, since dirt and debris on the diffuser will affect sensor accuracy. Dust with a dry soft brush, or use a cotton swap or soft cloth dampened with denatured alcohol if necessary. Do not touch the diffuser with your fingers, as oil from your skin can also affect the accuracy of the sensor.

EVAPOTRANSPIRATION - The amount of water evaporating from the soil and bodies of water in a given area combined with the water that transpires metabolically from nearby plant leaves. Abbreviated as "ET," evapotranspiration is an indicator of how much water a plant requires over a given period of time (day, week, month, year). Measured in the same units as rainfall (millimeters or inches), it is in effect the opposite variable. Evapotranspiration is an important measurement that growers consider in making their determination of when to irrigate crops.

Agricultural weather stations (such as the Davis Vantage Pro2 Plus) used to calculate evapotranspiration will need to be fitted with weather sensors for solar radiation, air temperature (maximum and minimum), relative humidity, and wind speed. The unit must be placed 6.5 feet (2 meters) above ground.

Evapotranspiration Links:
Kansas State University - What is Evapotranspiration (ET)?
University of Nebraska - Evapotranspiration (ET) or Crop Water Use

ULTRAVIOLET LIGHT - An invisible component of solar radiation with a wavelength longer than x-rays but shorter than visible light. UV radiation can cause cataracts and skin cancer and is responsible for skin tan and burns. The UV sensor should be mounted and maintained in the same manner as the solar radiation sensor.

SOIL TEMPERATURE & MOISTURE - Soil temperature is the temperature of the soil measured at a given depth, typically 2, 4, 6, or 8 inches, and sometimes as much as 20 to 40 inches. Soil temperature impacts many natural processes, such as seed germination, plant growth, soil respiration, and others. Soil and air temperature generally do not correlate. Soil moisture is the total amount of moisture in an unsaturated soil (i.e. above the water table), including water vapor. Soil moisture is a major factor affecting the growth and distribution of plants, soil erosion, soil aeration, and many others.

Soil thermometers and soil moisture sensors should be located together at the rooting depth of your crop, taking also into consideration soil depth and texture. Soil thermometer and moisture sensors must be located in the effective root system of the crop and soil moisture sensors must also be installed "wet," per the manufacturers instructions. Note that some crops need measurement at more than one depth. Guidelines on proper depths for specific crops and conditions can be obtained from your county extension agent.

LEAF WETNESS - Or "surface wetness," is the amount of liquid moisture on the surface of plant leaves or other exposed parts of a plant. It is of interest because of the influence leaf wetness has on plant diseases, since many can only infect plants with wet leaves.

Leaf wetness sensor placement is dependant on the crop and disease. For a consistent response, it is suggested that the leaf wetness sensor be mounted on the side of a horizontal post within the planted area, at the level of the crop leaf height, and pointed towards the north. The sensor should be cleaned on a regular basis to remove accumulated dirt and debris, which will affect its accuracy. Use a clean soft bristled toothbrush with water and a mild detergent liquid to scrub the surface of the sensor, making sure to rinse thoroughly.

 

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