Course: BFM 101 Biofuels Fundamentals

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The Water Cycle

The Earth is called the blue planet because water covers 70 percent of Earth’s surface. The oceans contain 97.5 percent of Earth’s water; land holds 2.4 percent and the atmosphere holds less than .001 percent. The amount of water available on the earth has remained constant over time, with the same water used over and over. Water can be lost from an area, but it is not lost from the Earth; it has simply moved. The water cycle, also known as the hydrologic cycle, describes the movement of water on, in and above the surface of the Earth. Figure 1.1 shows components of the water cycle. Water is an essential resource. The abundance or scarcity of water in any given location affects the quantity and quality of life since water is critical for its existence.

While the Earth’s overall water budget doesn’t change, the form water takes (water vapor, liquid or solid) and the location of available water may change. The annual precipitation rate for Earth is over 30 times greater than the atmosphere’s capacity to hold it. For example, huge amounts of water evaporated from oceans may fall onto land as rain or snow, which can re-enter the ocean system through runoff carried in streams and rivers. While water in its various forms may be in flux, the overall amount of water on Earth (or water budget) remains the same.

The balance of the water budget is determined by the distribution of the water [1]. Water income is a gain of water from precipitation to surface and ground water sources. Water losses occur through evaporation and transpiration. Water incomes and losses are influenced by soil characteristics, topography of land, lake basins and river systems (how the watershed functions), seasons and climate.

Water is stored in the form of ice in glaciers or as liquid in groundwater, lakes and oceans. Current discussions in the scientific community are concerned about the losses of water in glaciers and gains of liquid water causing flooding rivers and increased ocean levels that may affect landforms and coastal communities worldwide. There is also concern about record-breaking precipitation rates causing flooding, while other areas may experience increased drought. Although high volume rain events may cause flooding, the water table (top of the unconfined aquifer) may still remain low because of storm water runoff to surface waters (rivers, lakes and oceans) without significant infiltration or groundwater recharge. The water cycle as we know it is changing and difficult to predict.

Public water uses can impact an aquifer. Though the Great Lakes are the largest freshwater system on Earth, a new basin-wide water availability assessment by the U.S. Geological Survey states that the basin has the potential for local shortages. For example, groundwater pumping has had relatively little effect on water in the basin as a whole; however, pumping in the Chicago and Milwaukee areas has caused local groundwater drawdown of as much as 1,000 feet. Moreover, if pumping were to increase as anticipated in the region, water levels in these areas are estimated to decline an additional 100 feet by 2040 [3].

Irrigation use also may lower the water table. One example of water table decline is in large portions of the Ogallala aquifer, which extends from Texas to South Dakota. Some areas of this aquifer have experienced water table decline by over 40 feet [2].

Water Cycle Processes

Movement of water in the water cycle consists of several processes: precipitation, condensation, infiltration, runoff, evaporation, transpiration and groundwater discharge to the surface.

Condensation and Precipitation - Condensation changes water vapor to liquid water. Water vapor in the air will condense on nuclei, such as smoke or salt particles, when the temperature decreases or pressure changes. Cooling in the atmosphere occurs because atmospheric pressure lessens as you go higher. Lower pressure results in the water molecules moving farther apart and thus cooling.

Precipitation moves condensed water toward the Earth’s surface. Water may fall as rain, snow, sleet or hail. Water vapor spends an average of 8.9 days in the atmosphere before it  condenses on atmospheric particulates (such as salt nuclei or dust particles) and falls as precipitation, or it may cool and condense on surfaces and form dew or frost [4].

About 40 billion gallons of water move in the lower 5 miles of atmosphere over the contiguous United States every day [5]. Around 10% of this water comes down as precipitation [6,7].  Most of this precipitation (67%) returns to the atmosphere as evapotranspiration.  Twenty-eight percent flows into surface water and then into the Atlantic or Pacific Oceans or into Canada or Mexico. Two percent infiltrates into groundwater, and then outflows to surface water where it flows into the oceans. Two percent is consumed by animals, plants, people or industrial use [7].

Evaporation and Transpiration – Evaporation occurs as water changes from a liquid to the gaseous state and goes into the atmosphere from the land, lakes, rivers and oceans. Water on surfaces such as roads, parking lots and rooftops can often be seen evaporating as steam coming off of hot surfaces.

Various factors affect evaporation. High air pressure reduces evaporation from the Earth’s surface, but as the air temperature increases, so does evaporation. In addition, evaporation will increase with the surface temperature of water or soil at the interface with the air. The amount of air humidity also affects evaporation; greater humidity decreases evaporation. Yet another factor is the size of the air-water surface interface. Greater evaporation occurs within a larger area since there will be greater airflow across the surface, which will increase evaporation by stirring up heated surface water molecules. Water currents will also affect evaporation. If the surface temperature is not cooled by currents, it can remain at the level for continued evaporation. Water depth also affects evaporation since shallow water will warm quicker and have less cold water being brought to the surface by convection currents than will a deeper body of water. Since shallow water warms faster, it will experience more evaporation. Likewise, bare soil will warm faster and lose more moisture through evaporation than will covered or planted soil.

The amount of water in the soil will influence soil temperature. The closer the water table is to the surface, the more upward movement of moisture and the cooler the soil. Evaporative cooling as liquid water transforms to gas, will cause further soil cooling. Thus, moister soils will be cooler and have less evaporation.

Soil texture affects upward moisture movement and air movement at the surface. Evaporation from soil is divided into stages. The first stage shows highest evaporation in coarser soil, but the second stage shows evaporation through soil conduction or diffusion of moisture. Studies using different soil types show evaporation in the first stage as being the quickest from sandy soil and the slowest from clay soils. The first stage is controlled by atmospheric conditions, such as relative humidity, temperature and wind velocity. Clay soil conducts more, and thus has greater second stage evaporation; however, the overall loss is still greatest from the sandy soil [8].

Resources by eXtension, Bioenergy & Sustainability, BIOEN3 CC BY-NC-SA 2.5
www.extension.org/pages/63365/bioenergy-curriculum

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