Global Drought Monitoring Service through the GEOSS Architecture ...

Architectural Implementation Pilot, Phase 3 Version: 2.0
Global Drought Monitoring and European Drought
Observatory-Water SBA Engineering Report
Date: 11/Feb/2011
The American Meteorological Society Glossary defines “drought” as “a period of
abnormally dry weather sufficiently long enough to cause a serious hydrological imbalance.”
Agricultural drought is defined as “conditions that result in adverse crop responses, usually
because plants cannot meet potential transpiration as a result of high atmospheric demand and/or
limited soil moisture.” Hydrologic drought is defined “prolonged period of below-normal
precipitation, causing deficiencies in water supply, as measured by below-normal streamflow,
lake and reservoir levels, groundwater levels, and depleted soil moisture.” The definition of
agricultural drought stipulates that soil moisture monitoring is the methodology of choice for
monitoring drought afflicting agriculture. The definition of hydrologic drought stipulates that
monitoring of streamflow (including baseflow), groundwater levels, and soil moisture may be
necessary in order to monitor hydrologic droughts. Indeed, the complexities of water cycle
processes found in semiarid terrain, particularly processes in the vadose zone, may be critical in
identifying drought’s early stages. Global drought monitoring capability includes the capability
to monitor drought in many diverse semiarid conditions. The definition of the different types of
droughts, particularly hydrological droughts stipulate that monitoring capability of groundwater,
stream flow, soil moisture, snow storage at the start of spring meltwater season, and river water
level may be prerequisites or user requirements for an effective global drought monitoring
program. These, in turn, establish user requirements for an information system that support
global and regional drought monitoring.
1.3.1 Description of the Water Cycle
The water cycle begins—after evaporation of water over the oceans—as rain out over
land through which precipitation—if temperatures are low enough—which falls as frozen water
which accumulates on top of the surface of land as layers of snow or glacial layers.
Alternatively, precipitation falls—if temperatures are high enough—in its liquid form and
infiltrates into soil (unless the soil has a precondition of already being water saturated. This
infiltration and percolation occurs both as flow through the pores of the soil and flow through
macropores or fractured rock. Drainage may occur from topsoil through thick vadose zones in
semi-arid areas, until the water reaches layers of saturation of pores with water, called
groundwater. The proximity of groundwater to the surface determines whether water is
exchanged between groundwater, with groundwater discharge occurring into streams or rivers or
groundwater recharge occurring through river or streamflow. Furthermore, semiarid areas may
be characterized by ephemeral flashfloods, making the occurrence of such sources of water
difficult to typify statistically. One key difference is that flow of water through pores in soil is a
very slow, diffusive process which occurs over much longer time scales—decades or longer—
than the more rapid, prompt runoff processes occurring at the surface. The point to be made here
is that drought originates as a deficiency of frozen precipitation stored on the surface or liquid
precipitation that slowly works its way through the processes of the hydrologic cycle. Some of
these processes and events, such as decline of soil water, occur rather rapidly and impact the
growth stage of a crop in agriculture (agricultural drought indicator), while other processes,
drawdown of groundwater level and lowered discharge of groundwater to river baseflow can
occur over seasonal time scales or longer (hydrologic drought) (Van Lanen et al., 2004).
Drought can also exhaust municipal water supplies, both as drops in reservoir levels of
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