Principles and Issues in Environmental Hydrology AOM 4643 Principles and Issues in Environmental Hydrology

Definitions Hydrologic Science Applied or Engineering hydrology 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.

The Hydrologic Cycle

Definitions precipitation movement of water from the atmosphere to the earth as rain or snow throughfall precipitation that reaches the soil surface (i.e. not intercepted by vegetation) infiltration precipitated water “absorbed” by soil surface

Definitions evapotranspiration combined consumptive evaporative process by which water is released to the atmosphere through vegetation, soil, and water bodies exfiltration rising of soil moisture due to tension and capillary forces percolation water movement into deep aquifers (recharge)

Definitions overland flow precipitated water which moves over the land surface ultimately infiltrating into the ground or discharging into streams as surface runoff. interflow water flowing horizontally at shallow depths within soil structure 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

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, i.e Tr = Storage /Flow Rate e.g. for atmosphere: Tr = 12,900 km3/ 577,000 km3/yr = 0.22 yr= 8.2 days This 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 P ET I Atmosphere 8.2 days 0.001% Surface Water Groundwater 4790 yr 0.7% Surface Water 17.3 days 0.02% Ice cap 1.7% Ocean 2650 yr 97.5% Atmosphere 8.2 days 0.001% I

History of Hydrology Greeks - philosophy 1000 BC -Science of hydrology began with concept of the hydrologic cycle (Plato, Aristotle, Hippocrates) 1BC Vitruvius postulated that mountains received precipitation which gave rise to streamflow Romans 500BC-500AD - practical application Romans had extensive practical knowledge of hydrology and hydraulics (aqueducts). Theory sketchy, based on Greek philosophy

History of Hydrology 16th-18th Century - observations, empiricism, qualitative description 1500 AD - Da Vinci (Itlay) and Palissy (France) asserted on the basis of field measurements that water in rivers came from precipitation 1600 AD - Perrault proved by measurement that rain could account for streamflow 1700 AD - Halley quantified hydrologic cycled by estimating amounts of water in ocean-atmosphere-river-ocean cycle of Mediterranean 18th Century- advances in hydraulics and fluid mechanics by Bernoulli, Chezy, Pitot, and Euler

History of Hydrology 19th-20th Century- beginning of theoretical quantitative approaches Dalton quantifies the nature of evaporation and established the present concept of the global hydrologic cycle Experimental work on groundwater flow by Darcy and Dupuit Manning, Reynolds, Stokes, Poiseuille quantify surface hydraulics Green-Ampt, Richards quantify flow in the unsaturated zone Thornthwaite, Penman quantify evapotranspiration Horton quantifies infiltration Sherman quantifies surface runoff

Current 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.

Goals for this Class 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.