Page 17 - soil-plant-water relationship and water requirement

SOIL-PLANT-WATER RELATIONSHIP AND WATER REQUIREMENT

ii. Colorado Sunken Pan
o 920mm square pan made of unpainted GI sheet, 460mm deep, and
buried into the ground within 100mm of the top
o Main advantage of this pan – its aerodynamic and radiation
characteristics are similar to that of a lake
o Disadvantages – difficult to detect leaks, expensive to install, extra care
is needed to keep the surrounding area free from tall grass, dust etc

Pan Coefficient

Evaporation pans are not exact models of large reservoirs. Hence evaporation
measured from a pan has to be corrected to get the evaporation from a large lake
under identical climatic and exposure conditions.

Lake Evaporation = Pan Coefficient x Pan Evaporation

Table 2.1.2 - 1: Values of Pan Coefficients

Sl. No. Types of Pan Average Value Range
1 Class A Land Pan 0.70 0.60 – 0.80

2 Colorado Sunken Pan 0.78 0.75 – 0.86

Evaporation pans are normally located at stations where other hydro-
meteorological data are collected

Using Empirical Equation

Empirical Dalton’s Law Equation

Formula to estimate the evaporation rate prevailing at the water surface.

E  C e  e d 

s

Where,
E = rate of evaporation (mm/day),
C = a constant (mm day-1 kPa-1),
e s = saturation vapor pressure at the temperature of the water
surface (kPa),
e d = actual vapor pressure of the air (es of the air times relative
humidity) (kPa).

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