Introductory Agrometeorology/Evaporation and Precipitation

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Objectives of the chapter
  • To understand
    • factors influencing evaporation and transpiration,
    • measurement of evaporation
    • estimation of actual evapotranspiration, significances
    • forms of precipitation, measurement of rainfall and significances in crop production


The change of state of water from solid and liquid to the vapour and its diffusion into the atmosphere is referred to as evaporation. In agricultural meteorology, evaporation is defined as the maximum possible loss of moisture form a wet, horizontal, flat surface exposed to weather parameters, which exist in the vicinity of plants.

  • During evaporation water molecules absorbs energy, which gives them motion to escape from the

surface of liquid or evaporating body and becomes gas.

  • Since energy is consumed in the process of evaporation, the remaining liquid is cooled by an

equivalent amount of energy. So, evaporation is a cooling process and provides cooling effect on evaporating objects.

  • The driving force for evaporation is vapor pressure gradient. The resistant created by soil and wind also affect the process of evaporation.

Factors affecting rate of evaporation[edit]

1. Temperature (increase evaporation as temp. increases)

2. Relative humidity (RH increases evaporation decreases)

3. Wind speed (low wind speed low evaporation)

4. Air pressure (low pressure of the surface high rate of evaporation)

5. Soil characteristics (soil texture, soil profile characteristics etc. At FC – soil characteristics doesn’t affect rate of evaporation)

Measurement of Evaporation[edit]

Following methods are generally practiced to measure the rate of the evaporation 1. Pan measurement method 2. Using Empirical formulae 3. Storage equation method 4. Energy budget method

Evapotranspiration (ET)[edit]

  • Combined loss of water to the atmosphere from soil water and plant surface both as Evaporation

and transpiration.

  • Availability of moisture is crucial for both process.
  • It is used as agroclimatic index to assess the effect of water supply on the growth and yield of the


Reference Evapotranspiration (ET)[edit]

This represents the maximum rate of evapotranspiration from an extended surface of 8–10 centimeters tall green grass cover, actually growing and completely shading the ground under limited supply of water.

Potential evapotranspiration (PET)[edit]

Potential evapotranspiration (PET) for any crop is obtained from reference evapotranspiration and crop factors (Kc) when water supply is unlimited. PET = Kc × ET0

Evapotranspiration is also called water use (WU) or consumptive use (CU). The factors influencing ET are climate and management practices.

Importance of ET and PET[edit]

  1. Estimation of the soil moisture thereby planning irrigation schedule of crops.
  2. Understanding the relationship between the crop yield and irrigation water.
  3. Guiding for the production of a crop with a fully developed canopy.
  4. The evapotranspiration can also help to demarcate soil climatic zones including the drought

prone areas.

  1. These will form the base for developing suitable soil and crop management practices, crop

varieties, water conservation techniques, cropping pattern and ways to improve productivity rainfed crops.

Measurement of ET/CU[edit]


  • Component of Hydrological cycle
  • Water falling in the solid or liquid form on the earth (rain, hail, sleet)
  • It is different with dew, frost, and mist.

Mechanism of Rainfall or precipitation[edit]

  1. First step is condensation – cloud droplets are formed which must be overcome normal buoyancy

and updraft in the atmosphere.

  1. Cloud droplets are so small (10 micrometer) – millions of cloud droplets are required to form a

single raindrop of fairly good size (200 – 700 micrometer).

  1. Process of cloud droplets joins to form large raindrop is not fully understood yet. However, two

mechanisms are proposed: a) Ice crystal theory of Bergeron b) Collision coalescence theory

Forms of precipitation[edit]

i. Rain: most common form – ii. Drizzle: fine droplet of water diameter <0.5 mm and very close to the one another. RH is 100% no evaporation occurs iii. Snow: it is the precipitation of white or opaque grain of ice in the form of hexagonal crystal or star. iv. Sleet: In USA it is called frozen rain (< 5 mm) and In Britain v. Hail:precipitation of small ball or piece of ice about diameter 5 to 50 mm.

Measurement of precipitation[edit]

  1. Measured in terms of

Depth – unit of measurement, mm Intensity – unit of measurement, mm/hr

  1. Two types of Rain gauge

i. Non - Recording type rain gauge ii. Recording type rain gauge


Wind and Humidity · Agrometeorological Normals for Crops