Hydrological processes within a river basin

definition of a drainage basin (also often: watershed, river basin, or catchment): area that topographically appears to contribute all the water that passes through a given cross section of a stream
widely recognized as the natural unit of water management and for the scientific study of hydrological processes
example: major basins in NY state (Fig) State lines are shown in white, county lines are shown in light gray, streams are shown in light blue, and river basin boundries in orange (http://h2o-nwisw.er.usgs.gov/nwis-w/NY/)
water balance of a drainage basin: Runoff = Precipitation - Evaporation +/- delta Storage (terms are always positive)
runoff: total amount of water leaving the system as surface water or groundwater
there may be groundwater flowing in or out of the system; an alternative approach to formulate the balance is: P+ET+Q+G=dS; P: precipitation, ET: Evapotranspiration, Q: discharge of river, G: Groundwater flow, dS: change in storage; fluxes in are positive, fluxes out are negative (Fig)
we'll discuss the processes in a drainage basin in more detail below

on land Dalton's law only describes the potential evaporation (PE), i.e. the evaporation rate given an unrestricted water supply, more sophisticated approaches:

based on temperature, mass transfer, energy balance, or combinations of the latter (see Jones (1997), table 3.2)
Penman equation or combination equation (Fig) has developed to a standard method in hydrology to estimate free water evaporation
actual evaporation (AE) over land is lower, it depends on:

this model includes the integrated canopy conductance and the atmospheric conductance of water vapor (see resources below for more details), it describes reality reasonably well (Fig D 7-19)
potential evapotranspiration (PET): "evapotranspiration from an extended surface of short green crop, actively growing, completely shading the ground, of uniform height and not short of water"

process by which precipitation falls on vegetative surfaces (the canopy), where it is subject to evaporation
intercepted water that is lost (Interception loss) contributes significantly to total ET, it has a high surface/volume ratio and is held higher in the air, where windspeeds are higher
interception losses range from 10%-40% of the precipitation (Fig J 3.4)
afforestation could increase evaporation losses by up to 50 to 100%, clearcutting increases runoff
stemflow (flow along the stem), throughfall (the precip that has avoided interception or dripped off the canopy)
interception has also an influence on water quality

excercise: plot the saturated water vapor pressure (svp) as a function of the temperature (T) in the range between 0 and 30^oC

excercise: go to the LDEO climate data base and plot temperature and precipitation as a combined line/bar diagram for the year 1996

find the NOAA NCDC daily temperature and precipitation data for the NYC, LGA station
select all of 1996
download the data as table with two columns (Table)
convert the data into EXCEL and make a combination plot, T as line, P as bars

infiltration: flow of water into the subsurface, process much slower than surface processes (evaporation, surface runoff)
infiltration rate, given for example in mm/yr
unsaturated zone, saturated zone, groundwater, capillary fringe, soil moisture, water table (Fig)
water balance for a block of soil: dSMC = f + c - d - ET + v + TFLOin - FLOout (Fig)
dSMC: change in soil moisture content, f: infiltration rate, c: rate of water raised by capillary action from the water table, d: rate of drainage (recharge) to the water table, ET: evapotranspiration, v: changes in water vapor content (small) and TFLO lateral seepage (throughflow, sometimes also known as: interflow) (Fig)
saturated infiltration capacity: limiting infiltration capacity after the soil reaches a maximum degree of saturation, varies between different soils (cracks, clay swelling), as a function of vegetation (Fig)
processes during a rain event (Fig), precipitation, infiltration rates, and surface runoff as a function of time, note: precipitation stands for the fraction of total precipitation that actually reaches the ground

experiment: wetting soil in a beaker, what makes the grains stick to each other?
grain sizes in soils vary over several orders of magnitude (Fig)
look at sand gauges in class
definitions of porosity, field capacity, specific retention, specific yield (Fig)
porosity, field capacity, and wilting points for typical soils (Fig)
water pressure in soils given as "cm water column" (pressure head ), pressure = force per area
this pressure is negative in unsaturated soils and is often called pressure, suction or tension head (psi), it can be measured with a tensiometer (Fig)
tension as a function of water content (Fig)
soil water status as a function of pressure (tension) (Fig)
driving forces for flow of water in the unsaturated zone are: gravity and adhesion/cohesion
Darcy's law describes the flow in porous media:

pressure head and elevation as a function of depth in saturated and unsaturated zone
wetting fronts

Dingman, S.L. (1994) Physical Hydrology. Prentice Hall, Englewood Cliffs, 575pp.