Basic Hydrological Concepts AOM 4643 Principles and Issues in Environmental Hydrology

Structure and Properties of Water Water is a held together by a covalent bond– one side has a negative charge and the other a positive charge. The positive end of one H 2 O molecule attracts the negative end of another => called hydrogen bond hydrogen bond is weaker than a covalent bond but very important. Hydrogen bond determines most of water’s unique properties O 2- H+H+ H+H o Polar covalent bond (strong)

Thermal Properties of Water  boiling point and freezing point are higher than expected for its molecular weight (because of intermolecular attraction i.e. hydrogen bonds)  water exists in solid, liquid & gas phases on earth.  maximum 4 o C  ice floats, caused by hydrogen bonds forming tetrahedra at low temp;  important in determining earth’s climate  high specific heat capacity  a large input of energy raises the temperature a relatively small amount; energy goes into breaking hydrogen bonds rather than raising the temperature

Structural Properties of Water  cohesive, sticks to itself  high surface tension  drops of water are spherical.  capillarity – results of combination of adhesion to solid surfaces i.e. glass (water molecules are attracted to oxygen atoms in glass) by hydrogen bonds and cohesion to itself through surface tension; important for circulation of blood in body and water in soil  capillary forces are what allow moist sand to maintain vertical trench walls, whereas dry sand can only maintain a slope of 30 o  tiny menisci hold sand grains together through the hydrogen bonds

Water as a Solvent universal solvent  given enough time only a few natural substances will not dissolve in water. water dissolves substances by: –forming hydrogen bonds with its molecules (polar molecules) –surrounding individual ions of the substance (electrolytes) water alone cannot carry an electrical current due to the hydrogen bonds which do not allow hydrogen and oxygen atoms to move around freely of one another. Electrolytes in water can cause the solution to carry a charge. The higher the salt content the higher the electrical conductivity.

Basic Hydrologic Concepts 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 Hydrologic cycle is closed only globally, not on a watershed or continental scale. Hydrologic phenomena (precipitation, ET, infiltration, groundwater, overland, streamflow) are extremely complex and although quantifiable at lab scale, may never be fully predictable at the watershed scale. Thus we represent them in a simplified way by means of the systems concept.

Definition A hydrologic system is defined as a structure (surface or subsurface) or volume (atmospheric) in space, surrounded by a boundary, that accepts water and other inputs (such as air or heat energy), operates (physical, chemical, biological) on them internally and produces them as outputs. We treat the hydrologic cycle as a system whose components are precipitation, evapotranspiration, interception, runoff, infiltration, etc.. We give up the quest to know the precise spatiotemporal water flow patterns within the system and settle instead for knowing total water storage, and spatially averaged water fluxes in and out of the control volume.

Example ET(t) Q(t) overland flow groundwater discharge surface runoff The basic relations of physical hydrology for this system are derived from fundamental laws of classical physics. Particularly: Conservation of mass (m = mass of water) Conservation of energy (internal energy, kinetic energy and potential energy of the fluid)

Conservation of Mass The most useful principal in hydrologic analysis and is required in almost all problems. Stated mathematically: For our watershed problem: If have a steady flow problem, inflows=outflows:

Conservation of Energy Second fundamental physical law utilized in physical hydrology is the conservation of energy. Total energy =internal energy + kinetic energy +potential energy Total Energy E = E u + 1/2 mV 2 + mgz Energy per e = e u + 1/2 V 2 + gz unit mass

Internal Energy Internal energy is the sum of sensible heat and latent heat. Sensible heat is that part of the internal energy that is proportional to the substance’s temperature, i.e. de u = C p dT Latent heat - Amount of heat exchange required for inducing a phase change per gram of substance without a change in temperature. Usually a function of temperature.

Latent Heat Values for Water liquid water to vapor L e = latent heat of evaporation = T cal/g (2.5x T J/kg) This is heat absorbed (by vaporized water from surroundings) to break H bonds so evaporation can take place  evaporation always accompanied by transfer of heat out of water body or surroundings to vapor  latent heat transfer vapor to liquid water L c = latent heat of condensation = T cal/g (-2.5x T J/kg) This is heat released to surroundings when H bonds formed during condensation

Latent Heat Values for Water ice to liquid L m = latent heat of melting = 79.7 cal/g( 0.33 * 10 6 J/kg) This is energy required to disrupt tetrahedral molecular structure. liquid to ice L f = latent heat of fusion = cal/g ( 0.33 * 10 6 J/kg) This is energy released as tetrahedral molecular structure is formed. ice to vapor L s = latent heat of sublimation = T cal/g This is energy needed to a) disrupt molecular structure then b) break H bonds

Latent Heat Values for Water At typical atmospheric temp. and pressure on earth, energy required to sublimate ice to vapor generally greater than that required to melt ice through evaporation. Therefore, usually water goes through liquid phase first.

Latent Heat Transfer Jumps in curve  latent heat transfer to water Slope in curve  sensible heat transfer to water

Examples of Use of Latent Heat Properties In the SW use the latent heat of evaporation for air- conditioning houses  water and air is run into evaporative cooler on roofs of houses -- as water evaporates absorbs heat from air. Cooled air is returned to house. Irrigation of plants to protect from freezing.  when irrigation water freezes it releases heat to the environment which increases air temperature slightly and protects plant. Latent heat transfer is the dominant cause of internal energy change for water in most hydrologic applications  temperatures usually only change a few degrees C so sensible heat transfer is small.