The Hydrologic Cycle: Water Evaporation and Transpiration

The evaporation and transpiration, or evapotranspiration, of water are a small but vital part of the hydrologic cycle. These processes contribute greatly to such things as precipitation, plant growth, and the amount and locations of stormwater runoff. What exactly are these processes, and how do scientists measure them? Hopefully after reading through this article you will know a little bit more about how important water is in our world.

Evaporation occurs when liquid water changes its physical state into a gas. Water can only evaporate when the surrounding air has a relative humidity of less than 100%. Surprisingly, evaporation can happen even if it is cool outside or the sun has gone down. Evaporation also occurs more readily in areas of lower air pressure. Other factors that affect evaporation include: water surface area, purity of the water, ambient temperature, exposure to the sun, wind speed. Over the long term, evaporation can have a large impact on the hydrologic cycle. Evaporation is usually neglected when analyzing storm events as the amount of water removed from the area is insignificant over a short period of time.

Measuring Evaporation Rates

The National Weather Service has a standard set of procedures for how to measure the rate of evaporation for a given area. The pan is circular and has a diameter of 4 feet. Technicians measure the drop in water in one 24 hour period. Each day the pan gets refilled to its original level before it is measured again. The data is collected and compiled to create Annual Evaporation maps.

While this information is great, it isn’t much use to the lay person. You could however, use the maps and data to predict such things as the volume of water lost in a pool, lake, or other body of water over a period of time. Keep in mind that evaporation losses in natural bodies of water are typically 70% of the actual measured rates in the pan method.

Measures of Atmospheric Moisture

There are a number of measurements that can be taken to quantify the amount of water vapor in the air.

Absolute humidity: this is the amount of water (mass) within a specific volume of the air.

Specific humidity: This is the amount of water (mass) per a specific mass of humid air. This would be given as a percentage.

Vapor Ratio: This is the ratio water (mass) to that of the mass of dry air.

Relative Humidity: This is the ratio of the vapor pressure of the water to the saturated vapor pressure at the same temperature. Relative humidity is what is typically reported in a weather report. Values for relative humidity range from 0 (dry) to 100 (saturated).

Dew Point: This is the temperature that air would become saturated at if it was cooled. This would be the point at which precipitation would form (assuming that the ambient pressure remains the same).

Transpiration occurs as moisture in the soil is transferred back into the air by the action of plants. Plants will transpire water vapor through their leaves as part of photosynthesis. About 90% of the water that a plant soaks up will leave it through transpiration. Did you know that an acre of corn will transpire 3,000 to 4,000 gallons of water per day? Transpiration rates are affected by soil moisture availability, soil conductivity, and plant type as well as many of the same factors that affect evaporation.

Measuring Transpiration

It is often difficult to measure transpiration directly as it usually has to be performed in a controlled setting on individual plants. Measuring transpiration can be accomplish by using a lysimeter or potometer. The potometer is constructed with rubber tubes connecting from a water tank and gauge to a plants root structure. A lysimeters is basically a large tank that holds soil and plants. Water that enters and exits the tank system is carefully measure. This is usually accomplished by weighing the tank several times during the water cycle.

Measures of Soil Moisture

Degree of Saturation: This is the ratio of the volume of water in the soil compared to the volume of air (voids).

Water Content (volumetric): This is the ratio of the volume of water in the soil compared to the total volume of the wet soil.

Water Content (gravimetric): This is the ratio of the mass of the water in the soil compared to the total mass of the wet soil.

Hydraulic Conductivity: This is the rate at which water can travel naturally though a soil.