GLY 15/518 - Groundwater Geology
Chapter 2: Sections 2.1-1.3, 2.5, 2.7-2.8
Lecture on 9/9/12
Elements of the Hydrologic Cycle
evaporation
saturation humidity: the maximum amount of water vapor that air can hold increases
(greatly) with increasing temperature
it varies through the day and also through the year
land pans: used to measure evaporation rate by regular measurements of
water level in the pan and any precipitation that occurs
also measure: wind speed, solar radiation, and temperature
can make corrections for excess heating of land pan to estimate true evaporation from a
lake or large stock pond
evapotranspiration
natural landscapes loose water via evaporation from free water surfaces, evaporation
from the shallow soil, and transpiration via vegetation
it is difficult to determine the exact contribution of each to the total evapotranspiration
plant transpiration is the major factor in water returned to the atmosphere in many
landscapes
potential evapotranspiration (POTET) is the maximum water loss if there is always
sufficient soil moisture for plants to fully utilize
POTET is generally greatest closer to the equator (higher mean temperatures) and
lower farther from the equator (lower mean temperatures)
actual evapotranspiration (ACTET) may be much less than POTET depending on
climate conditions, vegetation cover, and time of year
ACTET = POTET only for parts of the year that precipitation is high and temperatures,
evaporation and plant growth are low,
e.g., winter and spring months in temperate climates
precipitation
lifting mechanisms: condensation, cloud growth, and precipitation are the result of
rising air which results in expansion, cooling, and condensation
warm fronts: warm air pushes gradually over cold air resulting in broad region of
clouds and rain
cold fronts: cold air forces warm air rapidly upward resulting in a narrow front of
intense thunderstorms
convection: local ground heating causes rising air to build up localized
thunderstorms
orographic lifting: prevailing winds rising over mountains results in clouds and
precipitation
calculating rainfall over a drainage basin - effective uniform depth (EUD)
arithmetic mean: simplest method; only valid for uniform distribution of rain gauges
isohyetal method: most accurate method; but must remap and re-measure areas for
every precipitation event
Thiessen polygon method: good estimate for non-uniform rain gauge distribution
only need to calculate areas once
Thiessen Polygons For Estimating EUD of Precipitation Over a Drainage Basin
1. draw connector lines between adjacent stations
2. mark bisectors to each line
3. extend each bisector, perpendicular to the connector lines, until the bisector lines
meet to form the vertices of a polygon
4. determine the area of the drainage basin and of each polygon to determine each
polygon's proportion of the basin area
- to estimate the areas, place a sheet of graph paper under a printout of the basin &
polygons or place thin, translucent graph paper over the printout of basin &
polygons
- count the number of grid boxes and partial grid boxes inside each polygon and the
whole drainage basin
5. calculate the weighted mean rainfall
Isohyetal Method For Estimating EUD of Precipitation Over a Drainage Basin
1. draw isohyetal (contour) lines of rainfall at regular increments based on rain gauge
data
2. use a grid to determine the proportion of the basin within each contoured space; use
the average of the 2 bounding contour lines as the rainfall in that space
3. calculate the weighted mean precipitation
characterization of precipitation
intensity
duration
antecedent precipitation
processes during precipitation
interception of precipitation by vegetation before water reaches the ground
throughfall: some water passes through to ground
stemflow: water flowing down stems and trunks to the ground
evapotranspiration
overland flow
infiltration
of that intercepted by vegetation
much either evaporates
or is absorbed by plants and transpired
the proportion of precipitation intercepted depends on
the intensity and duration of the precipitation
type of vegetation (e.g., pines catch more than oaks)
season (leaves on or leaves off)
some intercepted precipitation does reach the ground via
" throughfall and stemflow
infiltration
rate of infiltration depends on
" vegetation
" soil permeability and porosity
" slope
" soil saturation from antecedent precipitation
rate of infiltration
higher in coarse textured soils
lower in fine textured soils
initial rate of infiltration for all soils is high
but decreases as soil absorbs water
equilibrium infiltration capacity is the infiltration rate for saturated soil
the rate that gravity can pull soil water downward toward the water table
infiltration rate vs. precipitation rate
rainfall rate ≤ equilibrium infiltration capacity
infiltration proceeds at the rate of precipitation (all precipitation may infiltrate)
rainfall rate > equilibrium infiltration capacity < initial infiltration capacity
all rainfall initially absorbed, but then only a portion infiltrates and a portion runs
off as pore spaces are filled and infiltration capacity decreases
rainfall rate ≥ initial infiltration capacity
some of the rain becomes surface water runoff from the onset
the proportion of runoff increases as the infiltration rate decreases
depression storage
before surface water (non-infiltrated precipitation) can become overland flow
the small depressions in uneven ground must be filled to overflowing