Dr. Yaw O-Asante Civil Engineering Dept., CPUT, Bellville Campus. ABC Building Room 1.34 Mr. R M Mambwe Civil Engineering Dept., CPUT, Bellville Campus. ABC Building Room 1.06

General & Definitions Evaporation is the process by which water is transferred from land and water masses to the atmosphere in the vapour state Transpiration is a process by which water absorbed by root systems is discharged to the atmosphere as vapour through the pore spaces of the leaves or leafy parts. Usually it is impractical to separate evaporation and transpiration, and the term evapo-transpiration (ET) is used to describe both. ET generally constitutes the largest component of losses in the rainfall-runoff sequence.

3 3.2 Factors Influencing ET Evaporation is driven by the energy of the sun and depends on : Meteorological factors Physical factors a. Meteorological factors Solar radiation Temperature Humidity Wind speed Atmospheric pressure a. Physical factors Exposure of surface Type of surface Roughness of surface Plant height Quality of evaporating water

4 3.3Measurement of ET The principal methods applicable to measuring or estimating evaporation are : Water Budget Energy Budget and Mass Transfer Only the water budget method is covered in this course. The other two are merely mentioned. The unit of measurement of ET is mm.

Energy budget This is based on the energy conservation principle: “Any body of water must maintain a balance of energy inputs, outputs and storage”. The method accounts for incoming and outgoing energy balanced by the amount of stored energy in the system Mass transfer There are various methods in use based on the estimation of the upward flux of water from an evaporating surface as a function of wind speed and vapour pressure. An example is the Dalton eqn: E = 1.22(e s – e a )u Where E = evaporation rate (mm/day) e s = saturation vapour pressure (kPa) e a = vapour pressure in overlying air (kPa) u = wind speed (m/s)

Water budget Rearranging the water budget equation (Chapter 1) to make evaporation the subject of the formula, we have E =P – R – G – T – ΔS The setback in this method is that, all the errors in measuring P, R, G, T, ΔS are reflected in the final estimate of evaporation, and that is why results from this method is often unreliable. Application of the water budget (balance) equation (i) Lakes and Reservoirs  Inflows = rainfall and stream inflows  Outflows = groundwater losses and stream outflows Ground water losses are difficult to measure. For large reservoirs, ground water losses are small in comparison to reservoir capacity and can be ignored.  S measurements require knowledge of topography

7 (ii) Evaporation pan Method Widely used method; gives a direct measurement Why use pans? Discuss in class Water contained in a standard size container (circular & square) Water depths are recorded daily, at same times  S = ? Change in water depth Measurement of inflows? Have a rain gauge nearby! Measurement of Outflows? No outflows are expected!!!!

8 Hence E from a pan (E p ) is: E p =  I +  S Pans evaporate water faster than open lakes. It is therefore necessary to calibrate pans to find a correction factor - pan coefficient (K): E lake = K pan * E pan or K pan = E 0(actual) /E pan K depends on climate, type of pan, animals, birds, etc. Common Values: 0.67 to 0.81; typical value = 0.7

9 Choosing a suitable site: Site must be described as typical representative of the surrounding area Sun and wind must not be obstructed Never be under a shadow Only direct P must be able to enter into tank, at same time, no rain must be cut off Clean tap water must be readily available for refills Reliable observer must reside or work near site (why?) Tanks must not be erected near dusty roads

10 Symons Tank (S-Tank), Montagu, SA Inclined scale fitted with sliding steel basin to dampen wave action

11 Evaporation Measurement in SA Directorate of Hydrology maintains a large number of evaporation gauging stations Two types of pans are used: Symons tank (S-Tank): American Class A tank (A-tank) 1. Symons tank m square x m deep manufactured from 3 mm thick galvanized steel Painted black and installed at below ground on a 2.0 x 2.0 x 0.1 m concrete slab Disadvantage is the difficult in tracing leakage since it is installed below ground.

12 2. American Class A tank : Round unpainted galvanised iron pan – modified to m diameter x m deep (in SA) Installed on a wooden platform ~ 150 mm above the ground It is used mainly for agricultural purposes in the determination of irrigation water demands of various crops. The South African Weather Bureau also equips all its first class weather stations with A-type pan evaporimeters.

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14 Modified American Class A pan, Montagu, SA Inclined scale fitted with sliding steel basin to dampen wave action

15 Why inclined scales? Discuss in class Scale in S-Tank is 500 units, the zero mark being at the top of the scale (near the rim of the tank) and the 500 mark lower down). Operating range = ; usually filled to 100 units Scale for A-Tank is 150 units; operating range In both A and S tanks, 2 mm on sloping scale represents 1 mm of evaporation

16 Measuring Evaporation The evaporation values from the two types of pans differ significantly. Which of the tanks measures more E and why? A-pan evaporation values on average are 1.1 to 1.3 times higher than those of the S-pan values, depending on the locality of the pan. Bosman (1990) derived equations correlating monthly A- pan with S-pan evaporation values for different environments in South Africa. In a grass environment, his equations are: (S-pan) = (A-pan) (1) or (A-pan) = (S-pan)(2) (Units: millimetre)

17 Gauging frequency Record daily between 07:00 – 08:00 hours for both E and rainfall into record books against the correct date! Calculating E from Observers Records (“RETURN”) E day ‘n’ = 0.5 x { 1 st Reading on day (n+1) – last reading on day ‘n’ } + rainfall measured &recorded on day (n+1) Calculated E values are GROSS and can never be zero! Total Gross E for a period of an uninterrupted reading period, say one month, is given by: E = 0.5 x (  End Readings of gauging cycles -  start readings of gauging cycles) +  rainfall

18 Measuring free surface lake evaporation Many factors govern the relationship between evaporation from large and small water bodies, e.g. depth of the water body extent of the surface area. Small pans gain and lose heat much quicker than larger lakes. Evaporation from free surface lakes (reservoirs) and catchment potential evapotranspiration can be estimated by using the monthly S-pan evaporation data and applying pan factors.

19 Evaporation from free surface lakes can be estimated by using the monthly S-pan evaporation data and applying pan factors/coefficients: Table A: Pan Factors for converting Symons pan readings to dam/lake evaporation in South Africa(Chapman, 1990).

20 Pan factors used to convert monthly Symons pan evaporation values to potential evapotranspiration. Table B: Pan Factors for converting Symons pan readings to potential catchment evapotranspiration in South Africa.

21 (iii) Lysimeter A device called lysimeter directly measures ET by employing the water budget principles. It consists of a large container of soil overlying a permeable bed of gravel Container is supported on a weighing scale which records any changes in moisture storage (  S). The change in weight represents the loss of water due to ET. Water is allowed to drain freely from the large container into a collecting device which can be used to measure the infiltration losses A rain gauge is located nearby to measure all incoming water (P). Computation of ET is thus based on the following water budget equation: ET = P - I -  S

22 Lysimeter and rain gauge

23 The main drawbacks of a lysimeter are: Only calculates ET for one particular crop at a time or for a bare soil. Variation in ET over a whole catchment area can be quite large due to variations in topography and vegetation cover. They are unreliable as it is difficult to have a complete representative vegetation of an area in the unit It can also be very expensive to install and maintain such a device.