基于数值试验的山坡尺度降雨产流关系

Threshold effect for the rainfall-runoff relationship at the hillslope scale based on numerical simulations

山坡是流域水文响应的基本单元,对山坡降雨产流关系特别是山坡降雨产流的阈值现象的研究,有助于增进山坡水文过程的了解,进而改进流域的降雨径流模拟。本研究利用经过野外实验详细验证的二维山坡水动力学数值模型THRM,借鉴统计物理的思路,将土壤水分运动由微观网格升尺度到宏观山坡。通过数值试验分析山坡尺度降雨径流关系的特征,结果显示:山坡尺度降雨径流响应普遍存在阈值-线性的两阶段规律,即降雨低于某阈值时不产流,大于该阈值后降雨量与产流量呈现线性关系。数值试验还揭示了降雨阈值出现的过程机理,即降水入渗至土壤-基岩界面,填满基岩地形的凹陷后,交界面土壤水分达到近似饱和状态,山坡达到蓄满状态,土壤基岩交界面成为水流的优先通道而开始产流。山坡产流的降雨阈值可以认为是使山坡蓄量空间充满的降雨量。定义山坡蓄满后降雨量与产流量的相关系数为山坡产流系数,研究表明降雨阈值随着坡度的增加而减少,而山坡产流系数随着坡度的增加而增大。

A hillslope is the basic unit for hydrological responses.The rainfall-runoff relationship at the hillslope scale,especially the threshold effect for the rainfall-runoff relationship,controls hillslope and watershed behaviors.A 2Dnumerical hillslope model,THRM,was verified against field data for a real hillslope and used to upscale the movement of soil moisture from the grid scale to the macro-hillslope scale.Statistical analysis of a large number of a numerical experiment was used to develop the rainfall-runoff relationships.A ＂threshold-linear＂two-phase rule was obtained,with the runoff generated only when the rainfall exceeds a certain threshold,with a linear relationship between the rainfall and runoff after the threshold.The analysis also shows that the fill and spill hypothesis is the most likely mechanism for the threshold effect.The rainfall threshold can be considered to be the amount to fill the＂hillslope storage capacity＂.The rainfall-runoff coefficient after the threshold is defined as the hillslope runoff generation coefficient,with this research showing that the rainfall threshold decreases with slope,but the runoff generation coefficient increases with the slope.