CHAPTER 6 THE GLOBAL HYDROLOGIC CYCLE AND SURFACE WATER BALANCE
The Global Hydrologic Cycle
 Most of the earth’s water is in saltwater oceans and seas
 Successively less water is stored in ice caps/glaciers, groundwater, freshwater lakes, saline
lakes, soil moisture, the atmosphere, and freshwater rivers and streams
 Water is continually cycled through these various reservoirs, with the possibility for the cycle
to be modified at climatological time scales
The Surface Water Balance
 Potential Evapotranspiration (PE)
o Represents the climatic demand for water from the landscape driven by the
amount of insolation at the surface, so it peaks in the high-sun season
o Can be considered to represent the maximum amount of evapotranspiration that
could occur from a waterlogged grass field
o Can be estimated using latitude and temperature, calculated more accurately if
many input variables are known, or measured via lysimeter
 Evapotranspiration (ET)
o Represents the actual amount of water transpired by plants or evaporated that
does occur rather than the atmospheric demand for water
o Always less than or equal to PE
 Precipitation
o The “input” water to the surface in the surface water balance
o Peaks at different times of year in different locations, depending on the timing of
the circulation-forcing regimes
o Usually measured with low-tech gauges, and subject to errors
o Spatial analysis methods such as the Isohyetal method, Thiessen Polygon
method, and Gridpoint technique have been used to estimate totals at locations
where it isn’t measured correctly or at all
 Soil Moisture Storage
o The amount of water (in cm or inches of precipitation equivalent) that saturated
soil retains against gravity
o The theoretical maximum soil moisture storage is known as field capacity
o The threshold of water stored in the soil below which plants will not be nourished is
known as the wilting point
 Deficit
o Results when the atmospheric demand for surface water (PE) is not fulfilled by
some combination of precipitation and by water withdrawn from storage in the soil
o An indicator of environmental stress/drought
 Surplus and Runoff
o Surplus is water that falls as precipitation beyond that needed for ET or soilmoisture recharge (i.e., bringing the soil moisture storage to its field capacity)
o Some surplus goes into interception, depression storage, or percolation to
deep groundwater supplies; the rest becomes runoff to streams
o Runoff is modeled using a water balance approach for purposes of forecasting
floods and navigation hazards
Putting It All Together: A Worked Example of the Surface Water Balance
Types of Surface Water Balance Models
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Equal availability models assume that soil water is equally available to be evapotranspired
regardless of the amount of storage in the soil
Decreasing availability models assume that as soil-moisture withdrawal increases, the water
becomes increasingly more difficult to be moved upward via ET
The surface zone can be divided into two or more layers, such that water in the model is
allowed to be lost more freely from the top layer but more stringently from successively lower
layers
Water Balance Diagrams
 Provide a graphical representation of the amount of water in precipitation, PE, ET, soilmoisture storage, soil-moisture recharge, deficit, and surplus over a given period of time
Drought Indices
 The water balance approach can be used to derive various indices of moisture
conditions on a near-real-time basis
o The Palmer Drought Severity Index (PDSI) provides an assessment of long-term
moisture conditions on a weekly basis by climate division across the United States
o The Palmer Hydrological Drought Index (PHDI) is used for even longer-term
analysis than the PDSI, and reflects conditions relating to groundwater availability and
reservoir supplies
o The Crop Moisture Index (CMI) is more useful for short-term agricultural interests
because it is weighted heavily in the uppermost soil layers
o The Keetch-Byram Drought Index (KBDI) was designed by the U.S. Forest Service
to assess the risk of fire potential by examining the relationship between ET and
precipitation in the organic matter on a forest floor and in the uppermost soil layers