AOM 4643

Principles and Issues in Environmental Hydrology

Definitions

• Hydrologic Science– Focuses on the global hydrologic cycle and the processes involved in

the land phase of that cycle.

– Predicts spatial and temporal distribution of water in the terrestrial, oceanic and atmospheric compartments of the global water system

– Predicts movement of water on and under earth’s land surfaces and the physical/ chemical/ biological processes that accompany, conduct or affect movement

• Applied or Engineering hydrology – uses this understanding to design and operate flood control, water

supply, irrigation and drainage, pollution abatement, wildlife protection systems. i.e., for planning and management of water resources.

The Hydrologic Cycle

• Describes the continuous circulation of water from land and sea to the atmosphere and back again.

• Concept is based on mass balance and is simply that water changes state and is transported in a closed system which extends approximately 1 km down into the earth’s crust and about 15 km up into the atmosphere.

• The cycle is only closed earth-wide, not on a watershed or continental scale. Thus practicing hydrologists are typically faced with an open system.

• The energy required to keep the hydrologic cycle going is provided by the sun.

Definitions• precipitation - movement of water from the atmosphere to

the earth as rain or snow

• evapotranspiration - combined consumptive evaporative process by which water is released to the atmosphere through vegetation and soil

• throughfall - water not intercepted by vegetation

• interflow - water at shallow depths within soil structure

• overland flow - precipitated water which moves over the land surface ultimately infiltrating into the ground or discharging into streams as surface runoff.

Definitions

• infiltration - precipitated water “absorbed” by soil surface

• percolation - water movement into deep aquifers (recharge)

• exfiltration - rising of soil moisture due to tension and capillary forces

• sublimation - release of water from snowpack and icecaps directly to the atmosphere as vapor

Global Hydrology

• Ocean activity dominates the global hydrologic cycle -- receiving 79% of the earth’s rainfall and contributing 88% of the evapotranspiration.

• More rainfall falls on the oceans than land (because of larger surface area).

• Land receives more water than it evaporates while oceans evaporate more water than receive as precipitation.

• Excess water on land returns to ocean as surface and groundwater outflow to balance the system.

Distribution of water throughout the earth

• oceans and saline groundwater: 97.5%

• fresh water: 2.5% – ice caps 1.7%

– groundwater 0.7%

– lakes, rivers, streams, etc. 0.02%

– atmosphere 0.001%

– soil moisture 0.001%

– biological water 0.0001%

Global Stores and Fluxes of Water

Reservoir Volume(km3)

Source(km3/yr)

Sink(km3/yr)

Ocean 1338x106 Rain: 458x103

Runoff: 47x103Evap: 505x103

Atmosphere 0.013x106 Land Evap: 73x103

Ocean Evap: 505x103Rain: 577x103

Land(surface andgroundwater)

48x106 Rain: 119x103 km3 Evap: 72x103

Runoff: 47x103

Residence Times• The residence time of a water molecule in a component of the

system equals average volume of water stored a component of the system divided by the volumetric flow rate through the system

• Tr = Storage = 12,900 km3 = 8.2 days

Flow Rate 577,000 km3/yr

• Thus very short residence time indicates that atmospheric moisture is replaced approximately 40 times per year. part of what makes weather prediction so difficult.

• Atmospheric portion of the hydrologic cycle is very active and is driving force for surface hydrology

Residence Times

0.001%8.2 days

97%2650 yr

GW0.6%

4790 yr

History of Hydrology

Challenges in Hydrology

• Understanding the basic physics of individual hydrologic processes which occur instantaneously at a point does not always extrapolate easily to an understanding of hydrologic processes over larger space and time scales, due to– non-linearity of many hydrologic processes

– high degree of spatiotemporal variability in natural systems

– difficulty and expense in obtaining data to characterize variability.

Goals for this Class• We will cover basic physical principals which govern the major

hydrologic processes: Precipitation, Evapotranspiration, Infiltration, Groundwater Flow, Overland Flow, Streamflow.

• We will focus on relatively simple quantitative representations of these processes in order to develop a sound intuitive sense of the way water moves through the land based portion of the water cycle.

• These physical principals are powerful tools which constitute the foundation of hydrologic science. However, the degree of knowledge that can be obtained with the tools we will discuss is limited primarily by the availability and quality of field data and sometimes because they are conceptually inappropriate.