Systems Thinking and the Hydrologic Cycle

Throughout this course, we will be dealing with complex systems and “Systems Thinking”. What is Systems Thinking, you may ask? According to Peter Senge, author of The Fifth Discipline Fieldbook, “Systems thinking is a way of thinking about, and a language for describing and understanding, the forces and interrelationships that shape the behavior of systems”. Some systems are very complex, but all systems can be simplified to help understand the relationships between systems components. Systems can be "modeled" to help investigate their dynamics. We do not expect you to become system modelers, per se, but simple models can begin to help you understand how changes in one parameter might influence changes in another.  Let's consider a simple system in which we have a bathtub, fed by a faucet, and drained at its lower level. We could diagram this simple system as follows…

Simple System Diagram

Source: Mike Arthur and Demian Saffer, The Pennsylvania State University

In this system there is a reservoir (the bathtub), an input (the faucet), and an output (the drain). The relationships in this system are simple and, hopefully, intuitive. If you want to run water into the tub for a long time to keep it quite warm, but not have it run over, what are your choices? You could keep the drain closed and run a very slow trickle of warm water into the tub from the faucet, letting it fill gradually, or, you could fill the tub quickly to some level, then open the drain to allow water to leave the tub at the same rate as it is being added to prevent further rise in the water level. Cold water is more dense than warm, so perhaps cooler water would drain preferentially and this would keep the tub water warmer overall. You could also evaluate the time it would take to fill the tub, or drain it, knowing the tub volume (gallons), the maximum input rate through the faucet (gallons/minute), and the maximum drain rate (gallons/minute).

Hydrologic Cycle

The movement of water between these reservoirs, primarily driven by solar energy influx at the Earth’s surface, is known as the hydrologic cycle.

Figure 6. Diagram showing the main components of the hydrologic cycle, including evaporation, transpiration, precipitation, runoff, infiltration, and groundwater runout.

Click Here for Text Alternative for Figure 6

Component	Fluxes in 103 km3/ year
Evaporation	436.5
Precipitation	391
Groundwater Runout	45.5
Evapo-transpiration	65.5

Source: Michael Arthur and Demian Saffer

The hydrologic cycle is a conceptual model that describes the fluxes of water between the oceans, surface water bodies (lakes, rivers, and streams), groundwater in subsurface aquifers, the atmosphere, and the biosphere. One important aspect of the cycle is that no water is gained or lost: water moves between reservoirs but the total mass remains the same. Another way to say this is that the water that currently exists on Earth is the same water that has been here since the time the Earth formed. (Technically, there are small fluxes of water from the Earth’s interior to the surface and atmosphere through volcanism and venting, and small influxes of water from comets and debris, but these are negligible in comparison to the mass of water in the primary reservoirs shown above.)

The movement of water between reservoirs, or the “limbs” of the hydrologic cycle includes five primary processes:

• Evapo-transpiration: the movement of water from oceans or land to the atmosphere, through the combined processes of evaporation and transpiration. Evaporation and transpiration both involve a change in state, from liquid to vapor, which requires an input of energy. Evaporation is simply the change from liquid to vapor as a result of molecular motion, and is affected by temperature and ambient humidity. Transpiration is the movement of water to the atmosphere by plant respiration. In most terrestrial basins, transpiration is the dominant process by which water moves from the Earth’s surface to the atmosphere, whereas over lakes and the oceans, evaporation dominates.
• Precipitation: the movement of water from the atmosphere to the land surface or oceans, in the form of rain, snow, sleet, ice pellets, etc... Precipitation involves a change in state from vapor to liquid, known as condensation. This change in state releases heat energy. After precipitation falls on the land surface, it may flow into surface water bodies (lakes or streams), or percolate through soils and rock into the groundwater system.
• Runoff: the movement of water from the land surface to the oceans in streams or rivers.
• Infiltration: the percolation of water from the land surface or from surface water bodies through soils and into the subsurface. Water that infiltrates becomes part of the groundwater system, and is also known as groundwater recharge.
• Groundwater outflow, also known as subsea outflow: the seepage of water from the groundwater system directly into the oceans. The flux of groundwater outflow is the least constrained component of the hydrologic cycle, and is often estimated by balancing the other fluxes in the cycle.

Because the changes in state that accompany evaporation and precipitation also take in and release energy, the movement of water through the hydrologic cycle is paralleled by redistribution of heat and energy.

Uneven Distribution

Why is water distributed unevenly across the Earth’s surface?

As you probably know, things are far more interesting than a hypothetical case of evenly distributed precipitation! Both precipitation and evaporation vary widely over the Earth’s surface. This unequal distribution of water on the planet drives a diversity in climate and ecosystems (or biomes); water availability for human life, industry, and agriculture; and is fundamentally and intimately tied to the history of politics, economics, food production, population dynamics, and conflict – both in the U.S. and globally.

The abundance of water in some areas and scarcity in others follows systematic and predictable patterns. As part of this module, we’ll explore the physical processes that shape the overall distribution of precipitation - and thus water resources.

Figure 7 Map of average annual precipitations for the U.S. for 1981-2010

Source: PRISM Climate Group, Oregon State University [1], map created 2013.

Figure 8. MODIS satellite image renderings of vegetation coverage for July 2013 (top) and January 2013 (bottom). Gradations of green indicating leafy vegetation with the darkest green reflecting highest coverage of plants. Brown colors are ice or desert and black is "no data" (largely the oceans). Note the seasonal differences most pronounced in the northern hemisphere.

Source: NASA Earth Observations (NEO) [2]

Note the contrasting patterns in the two images in Figure 8 above, based on global satellite coverage. Vegetation in the southern hemisphere, which has relatively more ocean area (and less land area) than the northern hemisphere, changes little seasonally, whereas vegetation distribution in the northern hemisphere undergoes large changes. Why is that? There are probably two impacts on vegetation distribution—precipitation and temperature. Examine the figure below that illustrates the available moisture seasonally (summer vs. winter) and compare to the distribution of vegetation for the same seasons. Think about the role of temperature, precipitation, and soil moisture (water availability to plants), as well as the availability of sunlight for photosynthesis. Yes, there is a more complex relationship between plant growth and other factors, but the hydrologic cycle plays a major role.

Figure 9. Contours of average atmospheric water vapor calculated as equivalent rainfall in millimeters on the basis of satellite observations for July 2003 (top) and January 2003 (bottom). Again note the strong seasonal differences in the northern hemisphere in particular. Can this explain the vegetation differences entirely?

Source: NASA AIRS The Encyclopedia of Earth [3]