Fjölrit RALA - 05.12.1999, Blaðsíða 110

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Desertification in Mongolia
Ubsu-Nuur (Largest lake in Mongolia 3423 km2) where eolian processes are becom-
ing active.
There is evidence of increased moisture resources during the last three centuries.
The annual precipitation shows steady tendency to growth from the first half of the
19th century (Dagvadorj et al. 1994). In this regard not only mere climatic variation,
but also sharp continental climate condition and impact of human activities should be
considered as the most important factors of ecosystem vulnerability and desertification
risk in Mongolia.
Climatic factors
The climate of Mongolia is characterized by a high moisture deficit (Figure 1), low
humidity (Figure 2) and low levels of incident energy. Despite 260 days (more than
3000 hours) of sunshine, total heat units above 10°C rarely exceed 2000 and in some
areas are less than 1000. Snow cover is very light so soils are completely frozen in the
winter. As a consequence the effective vegetation growing period is short, generally
from 80 to 100, days although it can vary from 70 to 130 days depending on altitude
and location.
Precipitation is generally low, ranging from less than 50 mm per year in the ex-
treme south (Gobi desert region) to just over 500 mm per year in limited areas in the
north (Figure 3). The average countrywide precipitation is about 230 mm which
translated into an average of 361.1 km ! of rainwater. Roughly 90% of this amount re-
tums to the atmosphere through evapotranspiration. This is an extremely high evapo-
transpiration rate compared to other regions of the world. Of the remaining 10%, 63%
becomes surface runoff. Most of this surface flow component (95%) flows out of
country, while a small portion flows into lakes and basins within the country. About
36% infiltrate into the soil and contribute to subsurface flow. Thus of the total annual
precipitation, only about 3% infiltrates into the soil to replenish aquifers and becomes
potentially available as a water resource in the form of soil moisture or ground water.
This proportion is very low compared to the water balances of other regions in Asia in
which not less than 30-40% of precipitation remains available (for example, the Amur
and Enisei river basins, Russia).
Because of the continental climate, fluctuations in temperature are extreme, both
annually and diumally. Fluctuations can be as high as 30°C in single day and the dif-
ference between average winter low temperature and summer high temperature in ex-
cess of 50°C as compared to 25°C range in Europe. Moreover, it is possible even
during summer to get sharp falls in temperature and unseasonal frosts can cause har-
vest losses of between 10 and 30% of crops. In addition, the spring-summer droughts,
on the average, occur once in every five years in the Gobi region, and once in every
ten years over most other parts of the country (Figure 4).
The danger of sudden local natural calamities should not be ignored as they can
also cause long-term disturbances in ecological systems. A significant portion of the
land resources in Mongolia is degraded not only due to frequent strong winds but also
due to heavy rainfalls. For instance, in 1991, a hurricane with strong winds of velocity
of 140-160 km an hour destroyed and carried away fertile soil layer in large areas in
the southem part of Mongolia in only a few hours. These soils will probably take