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The Hydrological Cycle - Water Budgets

Diagram of conceptual water budget

What is a Hydrologic Budget?

The hydrologic budget or water budget for a watershed consists of inflows, outflows, and storage as shown in the following equation:

Inflow = Outflow +/- Changes in Storage

This is very much like a checking or bank account.  There is income (precipitation), spending (runoff and evaporation/transpiration), savings (recharge/storage/aquifers), and "stealing" from other watersheds (bank transfers). (evaporation and transpiration can be combined as evapotranspiration)

Inflows add water to the different parts of the hydrologic system of the watershed, while outflows remove water. Storage is the retention of water by parts of the system that can include surface impoundments (like lakes), soil moisture, and groundwater aquifers. Because water movement is connected, an inflow for one part of the system is an outflow for another. 

Note: That part of the ground that is saturated with water can be consolidated (various kinds of rock) or unconsolidated (various kinds of sediment like sand or clay).  An aquifer is any of that saturated material through which water can move easily, material like sand (unconsolidated sediment) or sandstone (consolidated rock); if you drill a well into this kind of material, you can quickly pump out reasonable quantities of water. Other materials may have considerable quantities of water in it but if the water cannot easily move through it, you could not pump reasonable quantities of water out of it; it would not be an aquifer.  Clay (unconsolidated sediment) and shale (consolidated rock) are examples of materials that usually do not make good aquifers. The top of the aquifer, or the top of the saturated zone in the ground, is known as the water table.

Precipitation = Evapotranspiration + Total Runoff, whereTotal Runoff = Direct Runoff + Base flow (groundwater component of stream flow)

Looking at an aquifer as an example, the percolation of water into the ground is an inflow to an aquifer. Discharge of groundwater from the aquifer to a stream is an outflow (it is also an inflow from the perspective of the stream). Over time, if inflows to the aquifer are greater than its outflows, the amount of water stored in the aquifer will increase (the water table will rise if it can or, if it cannot because the aquifer is confined, the artesian pressure will increase). Conversely, if the inflows to the aquifer are less than the outflows, the amount of water stored decreases (the water table falls or the artesian pressure becomes less). Inflows and outflows can occur naturally or result from human activity, such as installing a groundwater well or artificially draining a site.

The earth's water supply remains relatively constant, but man is capable of altering the cycle of that fixed supply both locally and globally. Population increases, rising living standards, and industrial and economic growth have placed greater demands on our natural environment. Our activities can create an imbalance in the hydrologic equation and can affect the quantity and quality of natural water resources available to current and future generations.  About 71 percent of the Earth's surface is water-covered, and the oceans hold about 96.5 percent of all Earth's water.   That means at least 96.5 % of the water is not readily drinkable - Great for the Ocean Creatures, but not good for humans.

Water source Water volume, in cubic miles Water volume, in cubic kilometers Percent of freshwater Percent of total water
Oceans, Seas, & Bays 321,000,000 1,338,000,000 -- 96.54
Ice caps, Glaciers, & Permanent Snow 5,773,000 24,064,000 68.7 1.74
Groundwater 5,614,000 23,400,000 -- 1.69
    Fresh 2,526,000 10,530,000 30.1   0.76
    Saline 3,088,000 12,870,000 --   0.93
Soil Moisture 3,959 16,500 0.05 0.001
Ground Ice & Permafrost 71,970 300,000 0.86 0.022
Lakes 42,320 176,400 -- 0.013
    Fresh 21,830 91,000 0.26 0.007
    Saline 20,490 85,400 -- 0.006
Atmosphere 3,095 12,900 0.04 0.001
Swamp Water 2,752 11,470 0.03 0.0008
Rivers 509 2,120 0.006 0.0002
Biological Water 269 1,120 0.003 0.0001

Source: Igor Shiklomanov's chapter "World fresh water resources" in Peter H. Gleick (editor), 1993, Water in Crisis: A Guide to the World's Fresh Water Resources (Oxford University Press, New York).

Distribution of water on planet earth

In addition to knowing where the water is, it is also important to note how that water is moving in the hydrologic cycle. Although there is only a relatively small amount of water in the atmosphere at any given moment, most of the water that is moving from one part of the cycle to another is either moving into the atmosphere (evaporation/transpiration), moving laterally in the atmosphere (advection), or leaving the atmosphere (precipitation).

Water use by households, industries, and farms has increased. People demand clean water at reasonable costs, yet the amount of fresh-water is limited and the easily accessible sources have been developed. As the population increases, so will our need to withdraw more water from rivers, lakes, and aquifers, threatening local resources and future water supplies. A larger population will not only use more water, but will discharge more wastewater. Domestic, agricultural, and industrial wastes, including the intensive use of Pesticides, Herbicides and fertilizers, often overload water supplies with hazardous chemicals and Bacteria. Also, poor irrigation practices raise soil salinity and evaporation rates. These factors contribute to a reduction in the availability of potable water, putting even greater pressure on existing water resources.

Large cities and urban sprawl particularly affect local climate and hydrology. Urbanization is accompanied by accelerated drainage of water through road drains and city sewer systems, which even increases the magnitude of urban flood events. This alters the rates of infiltration, evaporation, and transpiration that would otherwise occur in a natural setting. The replenishing (recharge) of ground-water aquifers does not occur or occurs at a slower rate.

Together, these various effects determine the amount of water in each part of the system and can result in extremely negative consequences for river watersheds, lake levels, aquifers, and the environment as a whole. Therefore, it is vital to learn about and protect our water resources.

Data Sources

Climate - Watershed - Streamflow - Precipitation and Temperature Data

Climate Data Online

Climate Data and Watershed Boundaries

US Geological Survey

USGS - StreamStats

Additional Resources

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