Drought and Vegetataion Monitoring in the Arid and Semi-Arid Regions of the Mongolia using Remote Sensing and Ground Data
Yu. Bayarjargal1, Ts. Adyasuren2 and Sh. Munkhtuya3
1Remote Sensing Lab., J.Blaustein Institute for Desert Research,
Ben Gurion University, Sede Boqer-84990, ISRAEL
Tel: +972-7-6596856, Fax: +972-7-6596805;
E-mail: yub@bgumail.bgu.ac.il
2National Committee of the Combat Desertification,
Ministry for Nature and the Environment,
3National Remote Sensing Center,
Ministry for Nature and the Environment, MONGOLIA
Keywords
Drought, land surface temperature, vegetation index
Abstract
The Gobi Desert and the Desert Steppe regions of the Central Asia within an area of approximately 60% of the country's territory of Mongolia are naturally the main part of the grazing livestock husbandry. The natural disasters such as drought and heavy snowfall states are often observed there. Over this regions drought occurs on an average of once every two or three years. Short-term operational detecting and monitoring of the natural hazards as well as droughts over large territory will contribute to meeting requirements of the economical development of the country. Coarse spatial resolution, high temporal frequency satellite data from the NOAA/AVHRR based technology and methods are widely used to monitor vegetation cover and drought episode throughout the world. This paper presents some results of detection of drought-affected regions by calculating the Normalized Difference Vegetation Index (NDVI) and the Land Surface Temperature (LST) values of the drought and wet years. The NDVI-LST space based drought indicator was tested and by use of this indicator can be detect and operationally monitor of drought distribution over the territory of the country.
1.Introduction
Mongolia has expansive areas (almost 90%) of natural grasslands and current dominant industry in the agricultural sector of the country is the nomadic livestock husbandry which is highly dependent upon the conditions and changes in nature and the environment during the four seasons of a year. Almost 99% of the Gobi Desert and Desert Steppe - arid and semi-arid zone are used as natural pasture. In the above-mentioned vast Gobi Desert and Desert Steppe area are often occur the natural disasters such as drought and heavy snowfall states. Over this regions drought occurs on an average of once every two or three years and the heavy snowfalls occurs every 5 to 6 years and once every 2 to 3 years covering half and quarter of country's territory, respectively (Shiirevdamba, 1998). The summer frequent droughts and severe winter's forces are intensifying desertification in arid, semi-arid and sub-humid areas of Mongolia. Coarse spatial resolution, high temporal frequency satellite data from the Advanced Very High Resolution Radiometer (AVHRR) operated by the National Oceanic Atmospheric Administration (NOAA) based technology and methods are widely used to monitor vegetation cover and drought events throughout the world (Goward et al., 1985; Walsh, 1987; Reed, 1993; Kogan, 1997; Karnieli 1999). In Mongolia, several studies involving the use of the NOAA/AVHRR data have been published. Most of them had concerned with determining vegetation cover study (Oyun et al., 1994; Adyasuren and Bayarjargal, 1995; Bayarjargal, 1995). Recently, Adyasuren and Bayarjargal (1995) mapped drought conditions of the Central Asian zone based on the multi-temporal Global Vegetation Index data from 1982-1987 and noticed that when a drought events occur in the Mongolian Gobi Desert zone, the Normalized Difference Vegetation Index (NDVI) values became to a low values, 0.0-0.05 units, same as value of the extra-arid land - Taklimakan Desert. As mentioned, previous works have introduced that the AVHRR derived NDVI images are useful for analysing spatial vegetation pattern and for assessing vegetation dynamics. However, from a synoptic point of view is required to improve the vegetation cover and dynamic estimation technology as well as to assess the impact of natural drought and its overall extent in space and time, especially in the Gobi Desert and the Steppe area of the Mongolia.
2. Climate Description of Study Area
Mongolia is a land-locked country which covers an area of 1.5 million square kilometers on the southernmost fringe of the Great Siberian boreal forest and the northernmost Central Asian deserts and vast steppes. The main characteristics of the climate of Mongolia are sunny days, long and cold winters. The average mean air temperature in the warmest month is 15-20oC in the north, and 20-25oC in the south of Mongolia. In the Gobi Desert and Steppe zones, the summer continues over 3 months. The maximum summer air temperature can reach anywhere 35-39oC in the north and 38-41oC in the south. The total annual precipitation in mountainous regions averages to about 400 mm, in the steppe 150-250 mm and in the desert-steppe less than 100 mm. About 85 to 90 per cent of the precipitation falls during the three summer months (Shiirevdamba, 1998). The number of rainy days decreases from north to south. The temporal and spatial distribution of precipitation in Mongolia is variable. There is very little precipitation at the beginning of the growing season but much more in the second half of the season when cool air starts to spread around the country. This variation has considerable effects on the growth of several spring plants. Summer, autumn and winter precipitation is a source of soil moisture but it is insufficient for vegetation to thrive in this country.
3. Methodology
In order to carry out the study, we classified the territory of Mongolia into four dryness zones such as the humid, the sub-humid, the semi-arid and the arid zone, by NDVI data, vegetation types and as well as according with desertification and geobotanic maps (Kharin et. al., 2000; Shiirevdanba, 1998). The fourteen plots shown in figure 1 have been chosen which are corresponds to the arid and semi-arid zones. Monthly total rainfall data and monthly composite of the NDVI and LST data for the period April to September of 1992-1995 were used. Prolonged drought, which has been affecting arid and the semi-arid areas, as well as high land surface temperatures which are reached in summer with June and July. The original images from the NOAA/NASA Pathfinder AVHRR Land Data Set, which have been monthly composite and 8 km spatial data, were derived. Prior to analysis, the images were geometrically corrected to a master image using a ground control points and applying a first order transformation into Geographic Projection. The NDVI data calculated from AVHRR channel-1 and channel-2 reflectances and the NDVI is has been shown to be highly correlated with vegetation parameters such as green-leaf biomass and green-leaf area and, hence, is of considerable value for vegetation discrimination (Justice et al. 1985). The NDVI defined mathematically as NDVI=[NIR-R]/[NIR+R], where R and NIR are the radiance or reflectances in the red and the near-infrared spectral channels, respectively (Rouse et al. 1974). The LST computed from AVHRR channel-4 and channel-5 brightness temperatures through the split window methods for land surfaces (Price, 1984; Kerr et al., 1992; Qin and Karnieli, 1999). The LST is correlative to evapotranspiration since it is highly controlled by the radiation received at the surface and the latent heat flux from the surface to atmosphere. This heat loss is responsible to the LST. The
