Nepal Engineering College



Code:nec/DCE/Hydr/HKS/2007/Tuto2

Nepal Engineering College

Changunarayan, Bhaktapur

Program: B.E. Civil Tutorial 2: Evapotranspiration

Year: III Instructor: Dr. Hari Krishna Shrestha

Engineering Hydrology

1. Calculate the evapotranspiration rate from an open water source, if the net radiation is 300 W / m2 and the air temperature is 30°C. Assume value of zero for sensible heat, ground heat flux, heat stored in water body and advected energy. The density of water at 30°C is 996 kg/m3. [Hint: Latent heat of vaporization L = 2501 – 2.37 T (kJ/kg), where T = temperature in °C.]

2. A Class A pan was set up adjacent to a lake. The depth of water in the pan at the beginning of a certain week was 195 mm. In that week there was a rainfall of 45 mm and 15 mm of water was removed from the pan to keep the water level within the specified depth range. If the depth of the water in the pan at the end of the week was 190 mm calculate the pan evaporation. Using a suitable pan coefficient estimate the lake evaporation in that week.

3. A reservoir has an average area of 50 km2 over a year. The normal annual rainfall at the place is 120 cm and the class A pan evaporation is 240 cm. Assuming the land flooded by the reservoir has a runoff coefficient of 0.4, estimate the net annual increase or decrease in the stream flow as a result of the reservoir.

4. At a reservoir in the neighborhood of Delhi the following climatic data were observed. Estimate the mean monthly and annual evaporation from the reservoir using Meyer’s formula.

|Month |Temp (°C) |Relative Humidity (%) |Wind velocity at 2 m above GL (km/h) |

|Jan |12.5 |85 |4.0 |

|Fen |15.8 |82 |5.0 |

|Mar |20.7 |71 |5.0 |

|Apr |27.0 |48 |5.0 |

|May |31.0 |41 |7.8 |

|Jun |33.5 |52 |10.0 |

|Jul |30.6 |78 |8.0 |

|Aug |29.0 |86 |5.5 |

|Sep |28.2 |82 |5.0 |

|Oct |28.8 |75 |4.0 |

|Nov |18.9 |77 |3.6 |

|Dec |13.7 |73 |4.0 |

5. For the lake in problem 4, estimate the evaporation in the month of June by Penman formula by assuming that the lake evaporation is the same as PET, given latitude = 28°N and elevation = 230 m above MSL. Mean observed sunshine = 9 hours.

6. A reservoir had an average surface area of 20 km2 during June 1982. In that month the mean rate of flow = 10 m3/s, outflow = 15 m3/s, monthly rainfall = 10 cm and change in storage = 16 Mm3. Assuming the seepage losses to be 1.8 cm, estimate the evaporation in that month.

Source of the problems 1 to 6: Subramanya, K. 1994. Engineering Hydrology, 2nd ed., Tata McGraw-Hill Publishing Company Ltd. New Delhi, ISBN: 0-07-462449-8, pp. 92-93.

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