黄土高原和六盘山区森林面积增加对产水量的影响

Effects of Forest Area Increase on Water Yield on the Loess Plateau and Liupan Mountains Region of Northwest China

[目的]定量评价黄土高原地区造林对流域产水量的影响,指导林水协调的林业发展和森林管理。[方法]综合分析多年在黄土高原、泾河流域、六盘山区开展的森林产流影响的多尺度研究结果。[结果]在黄土高原区域尺度,林地平均年径流(16 mm)比非林地(39 mm)减少59%;在典型流域尺度(泾河干流上游流域),森林年均产水量(76 mm)比自然草地减少51%,其中,半湿润土石山区的森林年均产水量(88.8 mm)比自然草地减少58%;在典型小流域尺度(半湿润的香水河小流域),完全覆盖森林后的年均产流比无森林覆盖情景减少59%。森林增加导致的年产流减幅在干旱地区和干旱年份会更大,并可能超过100%,如年降水量低于450 mm的黄土流域、半干旱的泾河上游黄土区的林地及半干旱的叠叠沟小流域的坡面乔木林分样地,其年产水量常为零或负值,即需消耗土壤水分和坡上汇入径流等维持生存。低度间伐不能显著减少森林蒸散耗水。[结论]在黄土高原研究的小流域、流域和区域空间尺度内,造林减少年产流量的平均幅度都在50%~60%以上,且随干旱程度的增加可达到100%,并因消耗降水外的其他水源而出现负产流。需依据水分承载力合理选择待恢复植被类型和确定森林覆盖率,这是保障区域供水安全和实现林水协调管理的基本途径。

[Objective]To quantitatively evaluate the effects of afforestation on the water yield of watersheds on the Loess Plateau,and to provide references for integrated forestry development and forest management. [Method]The long-term and multiple-scaled study results about the effects of forest on water yield carried out on the Loess Plateau,Jinghe Watershed and Liupan Mountains region were comprehensively analyzed. [Result] At the regional scale of the Loess Plateau,the mean annual runoff（ 16 mm） from forestland was reduced by 59% compared with that from non-forestland（ 39 mm）. At the typical watershed scale（ the upper stream of Jinghe watershed）,the mean annual water yield from forestland（ 76 mm） was reduced by 51% compared with that from natural grassland,in which the relative reduction in the semi humid mountain area of this watershed was 58%（ 88. 8 mm）. At thetypical small watershed scale（ the semi humid Xiangshuihe watershed）,the mean annual water yield under fully forested scenario was reduced by 59% compared with that of non forested scenario. The water yield reduction caused by forest area increase in heavier arid regions and drier years can be greater than this average range,and even exceed 100%. For example,the annual water yield often appeared as zero（ negative values） for the forestland in the loess watersheds with annual precipitation below 450 mm,and in the semi-arid loess areas of the Jinghe watershed,and for the slope plot of forest stand at the semi-arid Diediegou small watershed. This means that these forests must uptake more soil water and upslope run-in to survive. A low intensity thinning cannot lead to a significant reduction of water consumption by forest evapotranspiration. [Conclusion] Within the several spatial scales（ small watershed,watershed,and region） studied on the Loess Plateau,the mean annual water yield reduction by afforestation will be 50%-60% or more,and this reduction amplitude will increase with rising drought degree as high as to 100%,or even to a negative value of water yield because of the consumption of other water resources besides precipitation. It seems that a rational selection of proper vegetation types and forest cover within the carrying capacity of water resources is the basic approach to guarantee a safe regional water supply and to realize an integrated forest-water management.