Notes: Mon. 21 Oct

Water Balancing

Introduction and definitions
Most water is in the world's oceans (70%). A small part of that evaporates from the oceans. Some of this water moves over land and is stored in lakes and other bodies of water. The cycle is completed when water from glaciers, rivers, and ground water flow back into the oceans.

The driving force in water flow is topography (in other words, water flows from high points to low points, driven by gravity).

Most water is salt water (in oceans). A small portion is freshwater. Of the freshwater on the Earth, most occurs in glaciers. A small portion of freshwater is present in the atmosphere and streams. Some is in the subsurface groundwater. Of the freshwater in groundwater, approximately equal portions are tied up in shallow and deep depths. There is little water in the soil itself ("soil moisture"). During a water deficit, plants quickly use this soil moisture first.

On land, water evaporates from lakes, rivers, damp soil, etc. Plants also need water for photosynthesis (transpiration). The term evapotranspiration is defined as the passage of moisture from the land surface to the atmosphere through the combined processes of evaporation and transpiration (definitions from de Blij and Muller). The maximum amount of evapotranspiration that could occur in a certain place given its moisture content, vegetation, climate, etc., and ignoring human usage, is called potential evapotranspiration (POTET). The amount of evapotranspiration that actually takes place is called actual evapotranspiration (ACTET). POTET AND ACTET ONLY DIFFER WHEN THE SOIL IS LESS THAN 100% SATURATED. IF THE SOIL MOISTURE IS AT 100% CAPACITY, THEN POTET=ACTET. ACTET CAN NEVER BE BIGGER THAN POTET. (You will never use more water than you need.)

Examine the figure below, taken from de Blij and Muller. The horizontal axis goes from the North Pole to the South Pole. Three curves are shown:

figure 12-9 in de Blij and Muller

We can relate the concepts of water balancing to our model for ocean basins (see end of lecture of 9 Oct.) The western sides of ocean basins get warm, tropical air; the eastern sides get cold, arctic air. You'd expect humidity to be greater on the western side of ocean basins than on eastern sides. Therefore, you'd expect high precip and low evaporation on the western side; low precip and high evaporation on the eastern side of ocean basins. We mentioned these concepts before on Oct. 11. So you'd expect more runoff on the western part of ocean basins than the eastern sides.

Water Resource Balancing

The process of water resource balancing is based on comparing the water input (precipitation) with the water output, or need (evapotranspiration). There are 3 possibilities:

  1. PRECIP>POTET: if the soil started out 100% saturated, then there is runoff because there is a surplus of soil moisture. If the soil was not saturated at first, then the extra PRECIP first goes to recharging the soil moisture. Once it is recharged, then it can have a surplus.
  2. PRECIP<POTET: if the system needs more water than is being supplied, then the actual evapotranspiration is less than the potential evapotranspiration (ACTET<POTET). At first, any stored soil moisture is utilized. After all the reserves are utilized, then there is a state of crisis, a deficit, where plants do not get water, wilt, and die.
  3. PRECIP=POTET: no net change in the system.

Globally, the Earth is a closed system. By that we mean that the water budget is 100% balanced, and PRECIP=POTET. Locally, however, you can have variations. For a given location, the water balance may change between the 3 options above at any time.

Generally, POTET (the need) is highest in the hottest months.

Figure 12-8 in de Blij and Muller shows examples of different PRECIP and POTET curves. The panel on the left is an example of PRECIP<POTET (dry climate) all year-round; the middle panel shows PRECIP>POTET (wet) all year-round; the right panel shows PRECIP>POTET (wet) in winter and PRECIP<POTET (dry)in summer.

figure 12-8 of de Blij and Muller

(More detailed balancing was covered in lab.)

Some facts and figures
Water resources and usage is a very real, very human issue. One textbook gave the following amazing figures for how domestic water is used in the U.S.:

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