92
R. Kimura
• Fine weather, high evapotranspiration demand, and nearly zero annual rainfall;
• A short season for hot weather human activity, from May to September;
• Sand storms and/or dust events can occur throughout the season, but are less
frequent compared with Kharga Oasis 120 km to the east;
• Humidity is high relative to typical arid regions;
• Temperature, wind speed, and potential evapotranspiration are lower compared
with Kharga Oasis.
Herein, these climatic features are explained using synoptic meteorological data
from 2007 to 2016. These data were downloaded from the National Climatic Data
Center (https://www.ncdc.noaa.gov/). However, because solar radiation data cannot
be observed at the Dakhla meteorological station, sunshine data from 2008 to 2010
collected from Kharga Oasis (120 km east of Dakhla Oasis) were used in the sunshine
duration analysis (note that there was a high proportion of missing data and no
observations since 2011).
4.1 Sunshine Duration, Precipitation, and Aridity
Data for these analyses were collected from two meteorological stations: the Dakhla
meteorological station, located at 25.5°N latitude and 28.967°E longitude at an elevation of 117 m, and the Kharga station, located at 25.45 °N latitude and 30.533 °E
longitude at an elevation of 73 m. From 1990 to 2005, the relative sunshine duration (defined as the percent of actual sunshine duration out of the possible sunshine
duration) averaged 87% (from a low of 82% in January to a high of 90% during June
to August) at the Kharga station (Robaa 2008). The highest amount of solar radiation in the world occurs around the southwestern desert of Egypt (Budyco 1956).
More recent relative sunshine duration data from the Kharga meteorological station
(2008–2010) reveal similarly high rates of sunshine duration as well as high rates of
solar radiation (Table 1).
Annual precipitation was 0 mm from 2007 to 2016, except for 72 mm of rainfall on
1 December 2016. The Dakhla Oasis climate is controlled by high-pressure weather
patterns; therefore, apart from the ITCZ, few factors contribute to rain formation.
The aridity index (AI) defined by the United Nations Environment Program
(UNEP 1997) shows that this region is in a hyper-arid. Herein, AI is defined as
the ratio of annual rainfall to potential evapotranspiration (ET p ), where ET p is the
amount of water that would be removed from the land surface by evaporation and
transpiration if the amount of water already present in the land surface were not a
limiting factor. Thus, ET p can be determined only based on climatic demand. Annual
and monthly values of ET p were derived from meteorological data from the Dakhla
and Kharga stations by using the FAO Penman–Monteith method (Allen et al. 1998).
The average annual ET p was calculated to be 1781 mm, with monthly totals from
May to August exceeding 200 mm (Fig. 1). As a point of comparison, in Tottori,
Japan, where the annual rainfall is 2000 mm/year, ET p is 700 mm/year, or less than
R. Kimura
• Fine weather, high evapotranspiration demand, and nearly zero annual rainfall;
• A short season for hot weather human activity, from May to September;
• Sand storms and/or dust events can occur throughout the season, but are less
frequent compared with Kharga Oasis 120 km to the east;
• Humidity is high relative to typical arid regions;
• Temperature, wind speed, and potential evapotranspiration are lower compared
with Kharga Oasis.
Herein, these climatic features are explained using synoptic meteorological data
from 2007 to 2016. These data were downloaded from the National Climatic Data
Center (https://www.ncdc.noaa.gov/). However, because solar radiation data cannot
be observed at the Dakhla meteorological station, sunshine data from 2008 to 2010
collected from Kharga Oasis (120 km east of Dakhla Oasis) were used in the sunshine
duration analysis (note that there was a high proportion of missing data and no
observations since 2011).
4.1 Sunshine Duration, Precipitation, and Aridity
Data for these analyses were collected from two meteorological stations: the Dakhla
meteorological station, located at 25.5°N latitude and 28.967°E longitude at an elevation of 117 m, and the Kharga station, located at 25.45 °N latitude and 30.533 °E
longitude at an elevation of 73 m. From 1990 to 2005, the relative sunshine duration (defined as the percent of actual sunshine duration out of the possible sunshine
duration) averaged 87% (from a low of 82% in January to a high of 90% during June
to August) at the Kharga station (Robaa 2008). The highest amount of solar radiation in the world occurs around the southwestern desert of Egypt (Budyco 1956).
More recent relative sunshine duration data from the Kharga meteorological station
(2008–2010) reveal similarly high rates of sunshine duration as well as high rates of
solar radiation (Table 1).
Annual precipitation was 0 mm from 2007 to 2016, except for 72 mm of rainfall on
1 December 2016. The Dakhla Oasis climate is controlled by high-pressure weather
patterns; therefore, apart from the ITCZ, few factors contribute to rain formation.
The aridity index (AI) defined by the United Nations Environment Program
(UNEP 1997) shows that this region is in a hyper-arid. Herein, AI is defined as
the ratio of annual rainfall to potential evapotranspiration (ET p ), where ET p is the
amount of water that would be removed from the land surface by evaporation and
transpiration if the amount of water already present in the land surface were not a
limiting factor. Thus, ET p can be determined only based on climatic demand. Annual
and monthly values of ET p were derived from meteorological data from the Dakhla
and Kharga stations by using the FAO Penman–Monteith method (Allen et al. 1998).
The average annual ET p was calculated to be 1781 mm, with monthly totals from
May to August exceeding 200 mm (Fig. 1). As a point of comparison, in Tottori,
Japan, where the annual rainfall is 2000 mm/year, ET p is 700 mm/year, or less than