Engineering Hydrology (ECIV 4323) Abstraction from Precipitation

Transcription

1 Engineering Hydrology (ECIV 4323) Lecture 07 Abstraction from Precipitation Instructors: Dr. A-Majid Nassar 1

2 LOSSES FROM PRECIPITATION - Evaporation and transpiration are transferred to the atmosphere as water vapour. In engineering hydrology, runoff is the prime subject of study and evaporation and transpiration phases are treated as "losses". - Before the rainfall reaches the outlet of a basin as runoff, certain demands of the catchment such as interception, depression storage and infiltration have to be met. If the precipitation is not available for surface runoff is defined as "loss"

3 EVAPORATION PROCESS Is the process in which a liquid changes to the gaseous state at the free surface below the boiling point through the transfer of heat energy. When some molecules possess sufficient kinetic energy, they may cross over the water surface. The net escape of water molecules from the liquid state to the gaseous state constitutes evaporation

4 Influencing Factors 1. Vapour pressures at the water surface and air above: the rate of evaporation is proportional to the difference between the saturation vapour pressure at the water surface (Dalton s law) Where E L = C (e w e a ) E L : rate of evaporation (mm/day) C : a coefficient depend on wind velocity, atmospheric pressure and other factors e w : the saturation vapour pressure at the water surface (mm of mercury) e a : the actual vapour pressure of air (mm of mercury)

5 Influencing Factors 2. Air and water temperature: the rate of evaporation increases with an increase in the water temperature. 3. Wind : the rate of evaporation increases with the wind speed up to critical speed beyond which any further increase in the wind speed has no influence on the evaporation rate 4. Atmosphere pressure: a decrease in the barometric pressure ( as in high altitudes), increases evaporation. 5. Quality of water: under identical condition evaporation from sea water is about 2-3% less that from fresh water. 6. Size of water body

6 Evaporation Measurement 1. EVAPORIMETER CLASS A Evaporation Pan - The most widely used method of finding or monitoring the water body evaporation. - The standard National Weather Bureau Class A pan ( 1.21 m diameter, 25.5 cm depth, it is placed on a wooden structure of 15 cm height).

7 Pan Coefficient Cp The actual evaporation from a nearby leak is less than that of pan evaporation Why? - The sides of the pan is exposed to the sun - The temperature over the pan is higher that over the lake Lake evaporation = Cp x pan evaporation Cp = pan coefficient and equal 0.7 for Class A land Pan.

8 Evaporation Estimations Empirical Evaporation Equation Most of empirical formulae are based on the Dalton-type equation: E L = Kf(u) (e w - e a ) E L = lake evaporation in mm / day, e w = saturated vapour pressure at the water-surface temperature in mm of mercury, e a = actual vapour pressure of over lying air at a specified height in mm of mercury, f(u) = wind-speed correction function and K = a coefficient. The term e a is measured at the same height at which wind speed in measured.

9 Meyer's Formula (1915): E L = K M (e w - e a ) ( I + u 9 /16 ) Evaporation Estimations u 9 = monthly mean wind velocity about 9 m above ground K M = coefficient of 0.36 for large deep waters and 0.50 for small, shallow waters The limitations of the formulae that at best be expected to give an approximate magnitude of the evaporation.

10 Evaporation Measurement 1. Analytical methods The analytical methods for the determination of lake evaporation can be broadly classified into three categories as : I. Water-budget method, 2. energy-balance method, and 3. mass-transfer method

11 I. Water-budget method, It involves writing the hydrological continuity equation for the lake and determining the evaporation from a knowledge or estimation of other variables. Thus considering the daily average values for a lake, the continuity equation is written as: E L = P + (V is - V os ) + (V ig - V og ) T L Δ S All quantities are in units of volume (m3) or depth (mm) over a reference Area. p,. V is, V os and Δ S can be measured. However, it is not possible to measure V ig, V og and TL and therefore these quantities can only be estimated. If the unit of time is kept large, say weeks or months, better accuracy in the estimate of E L is possible. In view of the various uncertainties in the estimated values and the possibilities of errors in measured variables, the water-budget method cannot be expected to give very accurate results.