Question

A deep reservoir had the following average values of climatic parameters during a week:
(i) Saturation vapour pressure = 17.54 mm Hg
(ii) Relative humidity = 50%
(iii) Wind velocity at 1.0 m above ground surface = 16 kmph
An ISI standard pan at the site has pan coefficient 0.80. If this pan indicates an evaporation of 70 mm in a week, estimate the error in the result of Meyer's method relative to pan evaporation measurements.
$[\text{Take}{k}_m=0.36]$

• A. 11%
• B. 4.6%
• C. 6.5%
• D. 9%

Answer 1

The correct option is C 6.5%

$e_s=17.54\text{mm of Hg}$
$\text{Actual vapour pressure}=\text{Relative humidity}\times 17.54=0.5\times 17.54$

$e_a=8.77\text{mm of Hg}$

Let $U_9=$ Wind velocity at a height of 9 m above ground surface
$\therefore U_9=U_1{\left(\frac{9}{1}\right)}^{1/7}=16(9{)}^{1/7}=21.9kmph$

From Meyer's method -
Evaporation loss ,
$E_L=k_m(e_s-e_a)(1+\frac{U_9}{16})$
$=0.36(17.54-8.77)(1+\frac{21.9}{16})=7.48mm/day$

Daily evaporation as per pan evaporimeter
$=\frac{70}{7}\times 0.8=8mm/day$
$\text{So,}\%error=\frac{8-7.48}{8}\times 100=6.5\%$