历史时期西北干旱区生态环境演变过程和演变阶段

在历史时期，西北干旱区的生态环境演变过程是绿洲化与沙漠化并存，人进沙退和沙进人退皆有。灌溉绿洲是通过人工水系(渠、库、井)、栽培植物和耕作土壤，逐步替代自然水系、自然植被和土壤形成的。沙漠化则是由于人类活动改变了水资源的地域分配，使河流中下游地表水减少，地下水位下降，植被衰败向发展扩大。在受人类干扰很少的自然水系情况下，本区生态处自然平衡；在较强干扰下的半自然水系时，生态平衡失调；在强烈干扰的人工水系时期，河流上游生态改善，下游急剧恶化。

Evolution Processes and Stages of the Ecological Environment in the Arid Areas in Northwest China during the Historical Periods

Both oasis development and desertification and both human advance but desert retreat and desertadvance but human retreat existed in the evolution processes of the ecological environment in the arid areas innorthwest China during the historical periods. The irrigated oases are developed under the conditions that thenatural stream systems are gradually replaced by artificial irrigation systems (including reservoirs, canals andwells), the natural soils are replaced by cultivated soils, and the natural vegetation is replaced by planted crops.Desertification develops under the conditions that the regional distribution of water resources is changed byhuman activities, the volume of surface water in the middle and lower reaches of rivers is reduced, drawdown ofgroundwater level occurs, and the vegetation is degenerated. The local ecology is in a balanced status under theconditions of natural stream systems when human disturbance is slight; the local ecology is in an unbalancedstatus under the conditions of semi-natural stream systems when human disturbance is relatively violent; and theecology is improved in the upper reaches but sharply degenerated in the lower reaches under artificial irrigationsystems when human disturbance is extremely violent.