11.2: Water Basin and Budgets

Drainage Basins

The basic unit of division of the landscape is the drainage basin. A drainage basin, also known as a catchment or watershed, is the area of land that captures precipitation and contributes runoff to a stream or stream segment. Drainage divides are local topographic high points that separate one drainage basin from another [5]. Water that falls on one side of the divide goes to one stream, and water that falls on the other side of the divide goes to a different stream. Each stream, tributary and streamlet has its own drainage basin. In areas with flatter topography, drainage divides are not as easily identified but they still exist [6].

Figure \(\PageIndex{1}\): Map view of a drainage basin with main trunk streams and many tributaries with drainage divide in the dashed red line.

Figure \(\PageIndex{2}\): Oblique view of the drainage basin and divide of the Latorita River, Romania.

The headwater is where the stream begins. Streams only flow downhill and smaller tributary streams combine downhill to make the larger trunk of the stream. The mouth is where the stream finally reaches its end. The mouth of most streams is at the ocean. However, a rare number of streams do not flow to the ocean, but rather end in a closed basin (or endorheic basin) where the water evaporates from a stream or lake before it can reach the ocean. Most streams in the Great Basin of western North America are in closed basins. For example, in Utah, the Little Cottonwood Creek and the Jordan River flow into the Great Salt Lake where the water evaporates.

Figure \(\PageIndex{3}\): Major drainage basins color-coded to match the related ocean. Closed basins (or endorheic basins) are shown in gray.

Perennial streams flow all year round. Perennial streams occur in humid or temperate climates where there is sufficient rainfall and low evaporation rates. Water levels rise and fall with the seasons, depending on the discharge. Ephemeral streams flow only during rain events or the wet season. In arid climates, like Utah, many streams are ephemeral. These streams occur in dry climates with low amounts of rainfall and high evaporation rates. Their channels are often dry washes or arroyos for much of the year and their sudden flow causes flash floods.

Along Utah’s Wasatch Front, the urban area extending north to south from Brigham City to Provo, there are several watersheds that are designated as “watershed protection areas” that limit the type of use allowed in those drainages in order to protect culinary water. Dogs and swimming are limited in those watersheds because of the possibility of contamination by harmful bacteria and substances to the drinking supply of Salt Lake City and surrounding municipalities.

Water Budgets

Water in the water cycle is very much like money in a personal budget. Income includes precipitation and stream and groundwater inflow. Expenses include groundwater withdrawal, evaporation, and stream and groundwater outflow. If the expenses outweigh the income, the water budget is not balanced. In this case, water is removed from savings, i.e. water storage, if available. Reservoirs, snow, ice, soil moisture, and aquifers all serve as storage in a water budget. In dry regions, water is critical for sustaining human activities. Understanding and managing the water budget is an ongoing political and social challenge.

Hydrologists can make groundwater budgets within any designated boundary, but they are generally made for watershed (basin) boundaries because groundwater and surface water are easier to account for within these boundaries. However, water budgets can be created for state, county, or aquifer extent boundaries as well. The groundwater budget is an essential component of the hydrologic model; hydrologists use measured data with a conceptual workflow of the model to better understand the water system.

References

5. Seaber PR, Kapinos FP, Knapp GL (1987) Hydrologic unit maps

6. Charlton R (2007) Fundamentals of fluvial geomorphology. Taylor & Francis