Watershed Hydrology: Lesson 12 Evaporation

Lesson 12 Evaporation

12.1 Evaporation Process

Evaporation includes all processes by which water returns to the atmosphere as water vapour: evaporation of intercepted rain and snow; evaporation from bare soil and water bodies, such as ponds, lakes, and streams; and transpiration from plant leaves. Evaporation (and Transpiration) are small for a runoff event and can be neglected.The bulk of these abstractions take place during the time between runoff events, which is usually long.Hence, these are more important during this time interval.

Evaporation requires the following four conditions:

(1) Available water

(2) Higher humidity at the evaporative surface (i.e., vapour pressure) than in the surrounding air

(3) Energy to evaporate the water and

(4) Movement, or transfer, of water vapour away from the evaporative surface.

Energy required to evaporate water depends on incoming solar radiation, reflectivity of the evaporative surface, and air and surface temperature. Diffusion and convection move the vapour away from the surface. Increasing solar radiation, air temperature, and wind speed and decreasing atmospheric humidity all create an increase in evaporation rate. Evaporation is enhanced by warm air flowing over a cooler surface (e.g., air moving from dry rangeland over an irrigated crop or a small lake), but decreases rapidly with distance from the boundary between dry and wet surfaces.Intercepted rain or snow, and open water are in direct contact with the air. Both boundary-layer and aerodynamic resistance affect water loss from these surfaces. The boundary layer is a thin layer adjacent to a surface through which vapour moves by diffusion. Aerodynamic resistance describes vapour movement in the rest of the atmosphere. Both resistances depend on the size and shape of the evaporative surface, and both decrease as wind speed increases. Tree needles have a lower boundarylayer resistance than large leaves and a much lower resistance than that of a lake. Trees generate more turbulence to airflow than smooth surfaces, such as a lake; consequently, trees have lower aerodynamic resistances at the same wind speed. The combined resistances for a wet surface are relatively low compared to the resistance to movement of water from inside leaves or from below a dry soil surface.

12.2 Factors Affecting Evaporation

The factors that affect evaporation are:

1. Wind: When wind speed is high it assists evaporation.

2. Heat:Evaporation is more in summer as compared to winter.

3. Exposed surface area:For instance, a wet cloth spread out dries faster than when folded.

4. Humidity: Dryness assists evaporation; for instance, clothes dry faster in summer than during the monsoon when the air is humid.

5. Nature of the liquid: Rate of evaporation depends upon the type of liquid; for example, petrol evaporates faster than water.

6.Vapour pressure: If pressure is applied on the surface of a liquid, evaporation is hindered; consider, for example, the case of a pressure cooker.

12.3Measurement of Evaporation

12.3.1 Lysimeter

A lysimeter is a measuring device which can be used to measure the amount of actual evapotranspiration which is released by plants, usually crops or trees. By recording the amount of precipitation that an area receives and the amount lost through the soil, the amount of water lost to evapotranspiration can be calculated.In general, a lysimeter consists of the soil-filled inner container and retaining walls or an outer container, as well as special devices for measuring percolation and changes in the soil-moisture content. There is no universal international standard lysimeter for measuring evapotranspiration. The surface area of lysimeters in use varies from 0.05 to some 100 m² and their depth varies from 0.1 to 5 m.

According to their method of operation, lysimeters can be classified into non-weighable and weighable instruments. Each of these devices has its special merits and drawbacks, and the choice of any type of lysimeter depends on the problem to be studied.

Monolithic weighable lysimeters are a tool for water balance studies and solute transport determination.

Lysimeters are of two types:

Weighing
Non-weighing

Non-weighable (percolation-type) lysimeters can be used only for long-term measurements, unless the soil-moisture content can be measured by some independent and reliable technique. Large-area percolation-type lysimeters are used for water budget and evapotranspiration studies of tall, deep rooting vegetation cover, such as mature trees. Small, simple types of lysimeters in areas with bare soil or grass and crop cover could provide useful results for practical purposes under humid conditions. This type of lysimeter can easily be installed and maintained at a low cost and is, therefore, suitable for network operations.

Weighable lysimeters, unless of a simple microlysimeter-type for soil evaporation, are much more expensive, but their advantage is that they secure reliable and precise estimates of short-term values of evapotranspiration, provided that the necessary design, operation and siting precautions have been taken.

Several weighing techniques using mechanical or hydraulic principles have been developed. The simpler, small lysimeters are usually lifted out of their sockets and transferred to mechanical scales by means of mobile cranes. The container of a lysimeter can be mounted on a permanently installed mechanical scale for continuous recording. The design of the weighing and recording system can be considerably simplified by using load cells with strain gauges of variable electrical resistance. The hydraulic weighing systems use theprinciple of fluid displacement resulting from the changing buoyancy of a floating container (socalled floating lysimeter), or the principle of fluid pressure changes in hydraulic load cells.The large weighable and recording lysimeters are recommended for precision measurements in research centres and for standardization and parameterization of other methods of evapotranspiration measurement and the modelling of evapotranspiration. Small weighable types of lysimeters are quite useful and suitable for network operation. Microlysimeters for soil evaporation are a relatively new phenomenon.

12.3.2 Pan Evaporation

Pan evaporation is a measurement that combines or integrates the effects of several climate elements: temperature, humidity, rain fall, drought dispersion, solar radiation, and wind. Evaporation is greatest on hot, windy, dry, sunny days; and is greatly reduced when clouds block the sun and when air is cool, calm, and humid.Pan evaporation measurements enable farmers and ranchers to understand how much water their crops will need. There are many types of evaporation pans used by farmers. However, the universal pan is the United States Weather Bureau (USWB) Class A pan evaporimeter (Fig.12.1). It is important to use the same dimensions as this universal pan, mainly because the effect of wind and temperature on evaporation will vary with the surface area and the depth of water in the pan. Evaporation and irrigation replacements cannot be compared between sites if non standard pans are used.

Fig. 12.1. USWB Class A Pan Evaporimeter.

(Source:http://www.depi.vic.gov.au/agriculture-and-food/farm-management/soil-and- water/irrigation/construction-of-an-evaporation-pan-for-irrigation-scheduling)

Construction:

There are three parts to an evaporimeter (Fig. 12.1). All parts can be made very cheaply with common materials. Alternatively a complete unit can be purchased at considerably greater cost. The following is a description of how to construct the three components of the evaporimeter.

Evaporation Pan:The evaporation pan must be made to the standard specifications of an internal diameter of 1207 mm and height of 254 mm using 20 gauge galvanised iron. The standard material is galvanised iron as alternatives will have different thermal and reflectance properties, therefore altering the evaporation rate. It is best to have the pan made by either a galvanised tank manufacturer or an engineering firm. Before the pan is sited in the field it should be checked for leaks.

Fixed Pointer:

The fixed pointer that sits inside the pan can be made from standard irrigation fittings and a piece of stainless steel rod (Fig. 12.2). There are three parts to the fixed pointer:

Fig. 12.2.Components of fixed pointer.

(Source:http://www.depi.vic.gov.au/agriculture-and-food/farm-management/soil-and- water/irrigation/construction-of-an-evaporation-pan-for-irrigation-scheduling)

the base, a 100 mm PVC flange
The pointer support, a 230 mm long piece of 100 mm PVC pipe. Four equally spaced 9 mm holes 70 mm from the base are drilled to allow the water height around the fixed pointer to quickly adjust to the water height in the pan. A single 15 mm long 5 mm wide elongated hole is also drilled 70 mm from the base of the PVC pipe.
The pointer, a 170 mm long piece of 5 mm stainless steel rod bent at a right angle 60 mm from one end. From the shorter end a thread is tapped for about 15 mm and a point is ground on the other end of the rod.

After fitting the PVC pipe into the flange, the stainless steel rod is inserted into the elongated hole with nuts located on the inside and outside of the PVC pipe. To initially set the stainless steel rod in the correct position, the fixed pointer is placed in the pan and the pan is filled with water to a depth of 190 mm. The rod is then slid up or down in the 5 mm elongated hole so that the point of the rod just breaks the surface of the water.

Measuring Cylinder:

To measure evaporation the pan must be refilled with a known volume of water. The surface area of the pan is 1.14 square metres, so for every mm of evaporation 1.14 litres of water must be added to the pan. A transparent plastic 2 litre measuring jug with vertical sides is an excellent measuring cylinder if it is scaled properly. It is important that the jug actually holds more water than 2 litres so the sides of the jug must extend past the 2 litre mark. The jug is filled with 2.28 litres of water and the water level marked. This can conveniently be done by weighing the jug and adding 2.28 kilograms of water. For most jugs this will just about overflow, which is perfect.

A jug of water filled to the marker will be equivalent to 2 mm of evaporation. To scale the jug when less than 2 mm of water is required to fill the pan, the distance from the top marker to the bottom of the jug is measured and divided by 20. The numbers 0 to 2.0 in increments of 0.1 are then written with a permanent marking pen from the top marker to the bottom of the jug. These numbers are equivalent to the same number of mm of evaporation from the pan.

Measurement:

With evaporation the water level in the pan will fall. To measure the amount of evaporation, water is added to the pan with the measuring jug filled to the top mark. Water is added until the pointer just breaks the surface of the water. The PVC pipe supporting the pointer will help by reducing wave motion. It is important to keep track of the number of jugs used to refill the pan and the reading on the last jug when the pan water level is just broken by the pointer. The total amount of water added equals the amount of evaporation.

It is also essential to measure rainfall in conjunction with evaporation. Both measurements enable evaporation to be calculated on rainy days. After heavy rain the pan may have to be emptied to bring the water level down to the pointer. After rainfall on a hot summer day, less water may have to be removed than actually fell as rain. For example, after a 25 mm rainfall there might only be 12 mm of water removed from the pan with the measuring jug to bring the water level back to the pointer. The difference between the rainfall (25 mm) and the water removed from the pan (12 mm) is the evaporation. In this example it is 13 mm.

If the rain does not fill the pan above the pointer, the rainfall must still be added onto the measured evaporation to give the actual evaporation. For example, if there was 7 mm of rainfall and 6 mm of water was added to the pan with the measuring jug then the evaporation would be 13 mm.

Evaporation measurements should be routinely done every day at 9.00 am and clearly recorded. If measurements are not done routinely then the volume of water in the pan will decrease and take less time to heat up during the day and cool at night. This will induce an error which will become greater as the volume of water in the pan decreases. Evaporation measurements are very simple and take less than 5 minutes.

12.4 Determination of Evaporation from Water Surfaces

Evaporation from water surfaces can be determined by:

(1) Water budget

(2) Energy budget

(3) Mass transfer methods

(4) Combination methods

(5) Evaporation formulas

Water Budget

The water-budget equation for estimating evaporation (Horton, 1943) can be written as:

(12.1)

Where,

E =Evaporation

I = Inflow

P = Precipitation

O = Outflow

O_s = Seepage and

ΔS = Change in storage

Here, Inflow, outflow, precipitation, and change in storage can be measured reasonably accurately .Seepage, O_s, cannot be measured or evaluated directly and accurately, and the extent to which this quantity is accurate will affect the true value of evaporation.

The water-budget method of determining long-term evaporation can be used as a standard for comparing other methods. This method is not perfect, but it is satisfactory for practical purposes.

References

http://www.depi.vic.gov.au/agriculture-and-food/farm-management/soil-and-water/irrigation/construction-of-an-evaporation-pan-for-irrigation-scheduling accessed on 04/01/2014.