# Determining The Evaporation Rate

The rate of evaporation from water or land surfaces can be measured in several ways. The following are a few examples.

1 The change of water level is a direct index of evaporation and routinely measured at many weather stations, generally with an American

'Class-A' evaporation pan (or evaporimeter) shown in Figure 4.6 and Plate 4.1. The rate of evaporation from this standard device is known as the pan evaporation rate, Ep. For a lake or reservoir, one may construct a water balance (Chapter 10). This involves adding all the inputs of water, such as rain (P) or stream flow (I), and subtracting all the losses, such as evaporation (E) and outflow (0), during a month. A rise in water level (R) during that period implies that the inputs exceed the losses, so

The evaporation rate E can be estimated if the other terms are known. A negative rise (R) implies a fall of water level.

Figure 4.6 A Class-Apan evaporimeter for measuring daily evaporation, showing the stilling cylinder for preventing waves affecting the level of the water within. The pan has a diameter of 1.22 m, and a water depth of 180 mm, and stands about 40 mm above the ground on a wooden platform. Sometimes the pan is covered by wire-mesh to prevent birds and animals from drinking the water, though this reduces evaporation by 10 per cent or so. Each day, water is added to the pan until the surface is just level with the tip of the spike in the stilling cylinder. The added water (plus any rain collected since the last refilling) equals the amount evaporated.

Figure 4.6 A Class-Apan evaporimeter for measuring daily evaporation, showing the stilling cylinder for preventing waves affecting the level of the water within. The pan has a diameter of 1.22 m, and a water depth of 180 mm, and stands about 40 mm above the ground on a wooden platform. Sometimes the pan is covered by wire-mesh to prevent birds and animals from drinking the water, though this reduces evaporation by 10 per cent or so. Each day, water is added to the pan until the surface is just level with the tip of the spike in the stilling cylinder. The added water (plus any rain collected since the last refilling) equals the amount evaporated.

Figure 4.7 A drainage lysimeter made from an oil-drum, for measuring the evaporation from a well-watered crop. The central tube is made of plastic, for instance, and reaches down to the sump at the base. The soil rests on a perforated plate.

The evaporation from a well-watered crop can be measured by means of a drainage lysimeter (Figure 4.7), set within the crop. A measured amount of water is applied each day to the soil in the oil drum, and then the water which has drained to the sump since the day before is pumped out and measured. The difference between applied water (and rain) and pumped water is the daily evaporation. For crops in a field, the evaporation rate can be measured by the gravimetric method. This involves taking a sample of soil from the top 300 mm, or more if the crop's roots go deep. The sample is weighed, dried and reweighed, and the loss of mass gives its water content. The process is repeated with numerous other samples to obtain an average water content for the field. Further samples are taken after a week, for instance, and examined similarly. The decrease of average water content (plus any rainfall meanwhile, and minus any runoff) is a measure of the weekly evaporation. But the procedure is obviously tedious, damages the field and ignores the possibility of seepage of moisture down into the ground. In addition, there are several high-tech methods, more suited to research than routine measurements.

Figure 4.8 The relationship between annual mean evaporation rates from Class-A pans at various places in Australia and New Zealand, and the annual mean temperatures there.

Estimation

An alternative to measuring an evaporation rate is to use some relationship, such as that in

Figure 4.8, for estimating the rate instead. In this case, the curve is based on previous measurements of pan evaporation and temperature and allows one to estimate the evaporation corresponding to current thermometer readings, though the scatter of values about the curve shows that such an estimate could only be very rough. The relationship is affected by proximity to the coast, and no account is taken of the local wind speed which Dalton's equation

Plate 4.1 A US Class-A evaporation pan at a climate station in Melbourne. It is covered by mesh to exclude birds, whose drinking or splashing of the water would affect the daily lowering of water level due to evaporation. The congestion of the equipment and the undue proximity to a busy street are unfortunate but typical consequences of the growth of a city.

shows can be important (Note 4.E). A more accurate method of estimating evaporation is mentioned in Chapter 5.

Continue reading here: Various Evaporation Rates