Zhu Zhenda, Liu Shu, Yang Youling
Desertification is a process of environmental degradation on the aspect of declining land productivity in arid and semi-arld (include some areas in semi-humid) zones,and is a phenomenon of disaccords between the human activities and environmental resources,namely,it is the result of the destruction of fragile ecobalance under the interaction of over-utilization of land resources and sand deposits conditions in dry and windy season.From these processes,the desert-like landscape,where winds and sands are active,is oceured on surface,and they are called desertification processes and the lands affected by them is called desertified land. There are 11,750,000 of population,40% of total population in arid and semi-arid areas in Northern China,in 81 counties and banners in the inter-di-stributing areas for agricultural and graziery practices where desertification is occured;and approximately 261,000 km2 of land area here,of which cultivated fields occupy about 4,500,000ha.,and cover 47% of that in arid and semi-arid areas in Northern China,The deserfification processes are developing rapidly at present and 61.5% of land area in the inter-distributing areas for agricultural and graziery practices is occupied by the desertified land.As for the desertified land above-mentioned,26.9% of it was occupied by severe desertified land;25.7% by most severe desertified land;and 47.4% by on-going desertified land. It is estimated that,on the basis of the data combined the analysis of airphoto with ground investigation and surv ey,the desertified land in the inter-distributing areas for agricultural and graziery practices increased for 3.4 million ha,during last 30 years.Of which 42.9% is formed by over-cultivation for farming; 31.1% by transcendence of carrying capacity and over-grazing;22.2% by conti-nuous firewood collection and the rest caused by construction of mills,mines,co-mmunication lines and new towns and by the destruction of vegetation and the misuse of water resources.Because of the difference of formational type,the de-velopment of desertification landscape is also different relatively.A diagramme of formation of desertification processes in the inter-distributing areas for agricultural and graziery practices is given in the paper. Huanghua Tala Commune in Naiman Banner,Inner Mongolia,for example,was a sandy steppe and annual rainfall is about 360mm,but the deserfified land area developed to occupy 81% of the total land area in the commune due to over-recla-matiion and over-cultivation of steppe and firewood collections.Since 1970,the land use rate which centred on dry farming has been re-managed;the specific gravity of forest and forage has been enlarged;the measures of combination of trees,shrubs and grass,of plantation of tree-belts and forests in patches have been adapted;the basic farmland and fodder farm has been protected;the tillage influenced by desertification has been cut down unceasingly and the measures for fencing-Sand-to-cultivating-grass have been integrated with other measures.At pre-sent,the land-use rate of the commune has been readjusted to the structure of 21:52:27 in the midst of agriculture,forestry and graziery practices.The deserti-fied land at first has been controlled initially,the total grain output has been increased 3.36 times that before controlling and the desertification process has been rationally reversed basically. By sythesizing above analysis,the desertified land in the inter-distributing areas for agricultural and graziery practices possesses not only the possibility.to remanage,but also the reverse rotation of desertification is becoming real in a great deal of typical region.
where,D =monthly average total dissolved solid quantity in water (mg/l); Q = monthly average river discharge (m3/s); T=seconds for each month.The amount of main ions transported into the sea annually by rivers are shown in Table 1. The dissolved solid quantity of various regions and monthly dissolved solid amount of main rivers in China are given in Tables 2 and 8, respectively.
