Global Drought Monitoring Service through the GEOSS Architecture ...

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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 Page 10
Architectural Implementation Pilot, Phase 3 Version: 2.0 Global Drought Monitoring and European Drought Observatory-Water SBA Engineering Report Date: 11/Feb/2011 stored water or declines of water surface elevations within stream and river networks. Environmental flow requirements are not met, causing environmental impacts as well. Cooling water for thermal power plants is not available. All of these water cycle processes are often lumped under the generic term hydrologic drought, but the actual nature of the drought may be caused by a multiplicity of factors. Hydrologic droughts can occur through groundwater flow or streamflow. Groundwater droughts can be the result of long periods with below average precipitation. Van Lanen & Tallaksen (2007) have compared different terrains having a slow and a fast responding groundwater system to conclude the effect of the groundwater system on the frequency and duration of droughts was larger than the effect of different soil types. The groundwater system has large influence on the propagation of droughts through the hydrological cycle and hence on drought characterization (Van Lanen & Tallaksen (2008); Wanders, van Lanen, and van Loon 2010). The Total Storage Deficit Index, developed by Yirdaw et al (2008) used NASA Gravity Recovery and climate Experiment observations to attempt to quantify the groundwater role in hydrologic drought in the Canadian prairies. Terrestrial water storage changes can also be adopted for drought monitoring strategies (Rodell). Drought indicators have also been developed for evapotranspiration (Anderson) 1.3.1.1 Difficulties in Identifying Drought Conditions Drought lacks a precise and universally accepted definition. The detection of the threshold beyond which a drought episode begins is difficult to determine out of the statistical noise that creates random fluctuations (V. Castillo 2009; Moreira et al 2008). Requirements for drought detection include methodology that can select drought events from the remainder of the meteorological or hydrological time series, a truncation level or threshold which divides the time series into “above normal” and “below normal” sections (Dracup et al 1980). The truncation level can be set to cut the series at several places, and “run length” is the distance between successive crossings across the threshold; the run intensity is the average deviation from the threshold (van Lanen et al 2008). Probabilistic prediction tools have also been developed. 1.4 What are the User Requirements for an effective Drought Monitoring and Forecasting Information System? The integration of drought information (indices and impact indicators) in a comprehensive framework (composite index and maps) is the starting point for developing a drought monitoring system. Several integrating methodologies have been explored in AIP-3. Drought Monitoring may be summarized as a back-end information system, linked to an application that, in turn, is at the back-end of a user accessible portal. Page 11
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