无定河水土保持措施减沙效益的临界现象及其意义

Threshold phenomenon of sediment reduction beneficials from soil-water conservation measures in the Wudinghe river

以1956-1969年作为“基准期”,基于该时期的资料建立了流域年产沙量与若干降水特征指标之间的多元回归方程,并运用该方程计算出了“措施期”(1970-1993年)在假定无措施的条件下的历年产沙量。由此得到该年的水土保持减沙效益百分比,并点绘了减沙百分比随时间变化的曲线。结果显示,水土保持效益随时间的变化是非线性的,在总体趋势上表现出3个阶段:①缓慢增大;②急剧增大;③保持不变甚至于减小。点绘了减沙效益与各年末的梯田、造林、种草和坝地累积保存面积的关系,也显示出非线性变化,可以用两条斜率不同的直线来拟合。两条直线之间的转折点反映了水土保持措施面积的临界值,两条直线分别代表水土保持增益高回报区和低回报区。该曲线可以为这一地区的水土保持规划决策提供参考。

Based on the period 1956 - 1969 taking as a ＂baseline＂ period for the Wudinghe river basin, a multiple regression equation is established between the annual sediment yield and some indices of the precipitation characteristics, and used to calculate the sediment yield in the measure period （1970- 1993） , assuming there are no soil-water conservation measures. Then the annual sediment reduction beneficials, defined as the percentage of sediment reduction of the sediment yield without soil-water conservation measures, is calculated, and its temporal variation is plotted. The curve indicates that the soil-water sonservation beneficats is a non-linear variation, which can be divided into three stages： （1） the increasing slowly; （2） increasing rapidly; and （3） remaining unchanged or even decreasing. The relationships are plotted between the sediment reduction beneficials and the areas of the soil-water conservation measures such as land terracing, tree- and grass-planting and the land created through check dams. It also shows the non-linear variations which can be fitted by the two straight lines with different slopes. The turning point between the two straight lines can be regarded as a threshold of the area of the soil-water conservation measures, and the two straight lines represent a high-return area and a low-return area of the sediment reduction beneficials, respectively. The curve can provide some useful information for the soil-water conservation planning in the studied area.