Chapter 6
The Global Hydrologic Cycle and Surface Water Balance
2. Explain the difference between percolation and infiltration.
Answer: Percolation and infiltration are both processes that move water from the surface into the
subsurface. The difference between percolation and infiltration is that infiltration occurs closer to the
surface. Infiltration delivers water from the surface into the soil and plant rooting zone while percolation
slowly moves water through the lithosphere to replenish groundwater supplies.
4. How does the Penman-Monteith method of calculating PE differ from Thornthwaite’s method?
Answer: The Thornthwaite method of calculating PE is based only on the temperature and latitude of a
location. This simple method using only two variables was created so that PE could be estimated for any
desired location. The Penman-Monteith method of calculating PE is more accurate than Thornthwaite’s
method because the equation is derived from physical laws instead of statistical relationships. The
Penman-Monteith method is not as widely applicable as Thornthwaite’s due to the large number of input
variables that are required to derive the equation, many that are not consistently measured at weather
stations.
6. What is undercatch? How and why does it occur?
Answer: Undercatch is a phenomenon that results in a reduced amount of precipitation being collected in
precipitation gauges. There are three factors that result in an undercatch of precipitation of approximately
eleven percent. The Bernoulli effect creates a decrease in air pressure above the gauge, resulting in
precipitation to fall away from the gauge. The presence of water “stuck” on the side of a rain gauge and
evaporation of collected water inside the gauge also result in the undercatch of precipitation. Despite these
errors, precipitation data in the United States are of relatively high quality.
8. What advantages does the isohyetal method offer over the Thiessen polygon method and the gridpoint
technique?
Answer: The isohyetal method is considered to be more accurate than the Thiessen polygon method or
gridpoint technique to estimate rainfall totals because it includes the effects of local features. Local
features such as topography, proximity to a body of water, and land cover type significantly influence the
distribution of precipitation; the Thiessen polygon method and the gridpoint technique weighs a station
precipitation total only on distance from the station.
10. How does porosity relate to soil moisture storage capacity?
Answer: Porosity is related to the texture, shape and uniformity of the soil. The porosity of a soil
influences its soil moisture storage capacity because the pore spaces determine the volume of water that
the soil can hold. The porosity of sandy soils is usually greater than soils composed of silt or clay because
the grains are larger, resulting in more pore space. On average, roughly 25 percent of the volume of a soil
may be pore space.
12. Which of the surface water balance variables is most indicative of drought? Why?
Answer: The deficit variable in the surface water balance is the most indicative of drought because it can
be considered to represent the part of PE that is not available to be transmitted to the atmosphere. A
deficit is a condition when the atmosphere has the energy to evaporate more water than it can acquire
from the surface. A drought occurs whenever serious deficit in the surface water balance occurs.
14. What is the difference between an equal availability and a decreasing availability water balance
model?
Answer: The equal availability water balance model assumes that soil water is equally available to be
evapotranspirated regardless of the amount of soil moisture storage. Equal availability water balance
models assume that regardless of soil moisture content, water will be withdrawn equally during wet or dry
periods. The decreasing availability water balance model assumes that as soil moisture withdrawal
increases, the water becomes increasingly more difficult to remove from the soil because of resistance
from vegetation. The decreasing availability water balance model replicates “real-world” processes more
accurately because plants have an increasing resistance to moisture loss as the soil begins to dry out.