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Evapotranspiration | Characteristics and Factors Affecting Evapotranspiration

The loss of water due to evaporation and plant transpiration is called evapotranspiration. It is also called consumptive use (Cu) of water.

Evapotranspiration(ET) is of two types-

  1. Potential Evapotranspiration (PET): It is the maximum water that is taken by the atmosphere when an infinite supply of water is present.

  2. Actual Evapotranspiration (AET): It is the actual quantity of water taken by the atmosphere when a limited supply of water is present.

Always, AET <= PET. These ETs depend upon the sunshine, temperature, humidity and wind direction. According to the crop water requirements and losses, there are various types of irrigation requirements defined. Let's see what they are.


Some of the irrigation requirements of water are discussed below.

  • Consumptive Irrigation Requirement (CIR): It is the water required to meet the evapotranspiration losses.

CIR = Cu - Effective rainfall

  • Net Irrigation Requirement (NIR): It is the sum of CIR and losses like leaching, percolation and seepage, etc.

NIR = CIR + losses = Cu - Effective rainfall + losses

  • Field Irrigation Requirement (FIR): It is the irrigation requirement including the application efficiency losses too.

FIR = NIR/ application efficiency

  • Gross Irrigation Requirement (GIR): It is the irrigation requirement including the conveyance efficiency losses too.

GIR = FIR / conveyance efficiency


There are many methods to measure evapotranspiration like Meyer's method, Pan evaporation methods, etc. Here, we are going to see about Hargreaves class A pan evaporation method.


Hargreaves Class A pan evaporation method


It is one of the easiest methods to measure evapotranspiration.


Consumptive use, Cu = K x Ep, here, K - pan coefficient and Ep - pan evaporation


Try this question out!


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