Rainfall partitioning and its effects on regional water balances: Evidence from the conversion of traditional cropland to apple orchards in a semi-humid region

Rainfall partitioning by vegetation cover plays an important role in local and regional water balances. Large areas of traditional cropland have been converted to apple orchards on the Loess Plateau, China, so the effect of the conversion of traditional cropland to these orchards on rainfall partitioning cannot be ignored. In this study, we measured precipitation, throughfall (TF), and stemflow (SF) and calculated canopy interception (I) and canopy storage capacity (S) in two neighbouring apple orchards (8 and 18 years old in 2013) on the plateau during the four growing seasons of 2013, 2014, 2015, and 2016. Besides, we also summarized the percentage of rainfall partitioning of various crops and tree species and assessed the effect of land use change on regional water balances. The results showed that the percentage of rainfall partitioning and S differed between the two orchards. Mean annual I, TF, and SF for the young and mature orchards during the 4 years accounted for 7.9, 89.8, and 2.6% and for 10.3, 87.5, and 2.3%, respectively, of the rainfall partitioning. The percentage accounted for by mean annual I and TF differed significantly between the two orchards, but the percentage of mean annual SF did not differ significantly between the two orchards. Mean annual S for 2013–2016 was significantly higher for the mature than the young orchard. Although the conversion of traditional cropland to apple orchards led to a more serious soil desiccation in this region, the I loss percentage was higher in maize (12.5%) than the apple orchards. Therefore, we inferred that the effect of the conversion of traditional cropland to apple orchards on regional water balances was likely not caused by differences in rainfall partitioning. Differences in tree morphology due to tree age accounted for the differences in rainfall partitioning and S between the two orchards. Thus, tree age should be taken into account when assessing the effect of apple orchards on rainfall partitioning in this or similar regions.     

Simple generalization approach for intensity–duration–frequency relationships

Rainfall intensity–duration–frequency (IDF) relationships describe rainfall intensity as a function of duration and return period, and they are significant for water resources planning, as well as for the design of hydraulic constructions. In this study, the two‐parameter lognormal (LN2) and Gumbel distributions are used as parent distribution functions. Derivation of the IDF relationship by this approach is quite simple, because it only requires an appropriate function of the mean of annual maximum rainfall intensity as a function of rainfall duration. It is shown that the monotonic temporal trend in the mean rainfall intensity can successfully be described by this parametric function which comprises a combination of the parameters of the quantile function a(T) and completely the duration function b(d) of the separable IDF relationship. In the case study of Aegean Region (Turkey), the IDF relationships derived through this simple generalization procedure (SGP) may produce IDF relationships as successfully as does the well‐known robust estimation procedure (REP), which is based on minimization of the nonparametric Kruskal–Wallis test statistic with respect to the parameters θ and η of the duration function. Because the approach proposed herein is based on lower‐order sample statistics, risks and uncertainties arising from sampling errors in higher‐order sample statistics were significantly reduced. The authors recommend to establish the separable IDF relationships by the SGP for a statistically favorable two‐parameter parent distribution, because it uses the same assumptions as the REP does, it maintains the observed temporal trend in the mean additionally, it is easy to handle analytically and requires considerably less computational effort. Copyright © 2012 John Wiley & Sons, Ltd.     

Accounting for soil parameter uncertainty in a physically based and distributed approach for rainfall‐triggered landslides

In this study we propose a probabilistic approach for coupled distributed hydrological‐hillslope stability models that accounts for soil parameters uncertainty at basin scale. The geotechnical and soil retention curve parameters are treated as random variables across the basin and theoretical probability distributions of the Factor of Safety (FS) are estimated. The derived distributions are used to obtain the spatio‐temporal dynamics of probability of failure, in terms of parameters uncertainty, conditioned to soil moisture dynamics. The framework has been implemented in the tRIBS‐VEGGIE (Triangulated Irregular Network (TIN)‐based Real‐time Integrated Basin Simulator‐VEGetation Generator for Interactive Evolution)‐Landslide model and applied to a basin in the Luquillo Experimental Forest (Puerto Rico) where shallow landslides are common. In particular, the methodology was used to evaluate how the spatial and temporal patterns of precipitation, whose variability is significant over the basin, affect the distribution of probability of failure, through event scale analyses. Results indicate that hyetographs where heavy precipitation is near the end of the event lead to the most critical conditions in terms of probability of failure. Copyright © 2015 John Wiley & Sons, Ltd.     

Estimation of instantaneous peak flows from maximum mean daily flows using the HBV hydrological model

The record length and quality of instantaneous peak flows (IPFs) have a great influence on flood design, but these high resolution flow data are not always available. The primary aim of this study is to compare different strategies to derive frequency distributions of IPFs using the Hydrologiska Byråns Vattenbalansavdelning (HBV) hydrologic model. The model is operated on a daily and an hourly time step for 18 catchments in the Aller‐Leine basin, Germany. Subsequently, general extreme value (GEV) distributions are fitted to the simulated annual series of daily and hourly extreme flows. The resulting maximum mean daily flow (MDF) quantiles from daily simulations are transferred into IPF quantiles using a multiple regression model, which enables a direct comparison with the simulated hourly quantiles. As long climate records with a high temporal resolution are not available, the hourly simulations require a disaggregation of the daily rainfall. Additionally, two calibrations strategies are applied: (1) a calibration on flow statistics; (2) a calibration on hydrographs. The results show that: (1) the multiple regression model is capable of predicting IPFs with the simulated MDFs; (2) both daily simulations with post‐correction of flows and hourly simulations with pre‐processing of precipitation enable a reasonable estimation of IPFs; (3) the best results are achieved using disaggregated rainfall for hourly modelling with calibration on flow statistics; and (4) if the IPF observations are not sufficient for model calibration on flow statistics, the transfer of MDFs via multiple regressions is a good alternative for estimating IPFs. Copyright © 2015 John Wiley & Sons, Ltd.     

13e SESSION DU COMITÉ CONSULTATIF KARACHI 4–15 NOVEMBRE 1957

Abstract

Event-based methods are used in flood estimation to obtain the entire flood hydrograph. Previously, such methods adopted in the UK have relied on pre-determined values of the input variables (e.g. rainfall and antecedent conditions) to a rainfall–runoff model, which is expected to result in an output flood of a particular return period. In contrast, this paper presents a method that allows all the input variables to take on values across the full range of their individual distributions. These values are then brought together in all possible combinations as input to an event-based rainfall–runoff model in a Monte Carlo simulation approach. Further, this simulation strategy produces a long string of events (on average 10 per year), where dependencies from one event to the next, as well as between different variables within a single event, are accounted for. Frequency analysis is then applied to the annual maximum peak flows and flow volumes.

Citation Svensson, C., Kjeldsen, T.R., and Jones, D.A., 2013. Flood frequency estimation using a joint probability approach within a Monte Carlo framework. Hydrological Sciences Journal, 58 (1), 1–20.     

基于单、双重渗透介质降雨边界处理的改进

在降雨入渗模拟中,现有研究无法准确实现复杂降雨条件下降雨边界在流量与压力两类边界之间的双向动态转换。基于饱和?非饱和渗流理论,将降雨补给流量与实际入渗流量的差值作为两类边界动态转换的判别条件,改进了单重渗透介质降雨边界动态转换控制方程,并进一步提出了双重渗透介质降雨边界处理方法及控制方程,在此基础上进行了数值试验及工程实例验证。结果表明：改进的降雨边界动态转换控制方程克服了传统降雨边界只能实现流量到压力边界单向转换的局限性,可准确实现流量边界与压力边界的双向动态实时转换;案例边坡的降雨入渗模拟结果说明了改进后降雨边界的正确性和工程适用性。     

甘肃省白龙江流域降雨型潜在泥石流危险性预报模型

【研究目的】泥石流灾害是白龙江流域分布广泛并常引起群死群伤的重大地质灾害,准确评价泥石流活动规模及其危险度,是泥石流危险性预警预报的前提,合理构建危险性预报模型是泥石流防灾减灾的关键。【研究方法】本文以研究区历史泥石流案例和对应降雨资料为基础数据,采用统计分析方法,通过分析形成泥石流关键地质环境条件及其相互关系,构建了白龙江流域潜在泥石流危险度定量评价模型,提出了两类泥石流危险级别临界判别模式。【研究结果】结果表明：（1）以泥石流活动规模、沟床平均比降、流域切割密度、不稳定沟床比例为判断因子的泥石流危险度动态定量计算模型,能快速准确预测未来不同工程情景和降雨频率工况下泥石流危险度;（2）影响降雨型泥石流发生的地形条件由流域面积、10°~40°斜坡坡度面积比、沟床平均纵比降等组成,降雨条件主要由泥石流爆发前的24 h累积降雨量、触发泥石流1 h降雨量或10 min降雨量等组成;（3）依据30条典型泥石流沟危险度计算结果,获得泥石流危险性临界判别值,提出了降雨型潜在泥石流危险性1 h预报模型（Ⅰ类）和10 min预报模型（Ⅱ类）,其中Ⅰ类模型高危险度以上泥石流预测精度大于87.5%,Ⅱ类模型中等危险度以上泥石流预测精度大于80%,而两类预报模型验证准确率为83.3%。【结论】研究成果为泥石流精准预警预报提供了技术支撑,对建立中小尺度泥石流实时化预警系统具有一定参考意义。创新点：通过确定与泥石流相对应关键地质环境因子,构建了泥石流危险度动态定量评价模型,依据泥石流危险性1 h和10 min临界判别模式可准确实现潜在泥石流预警预报。     

基于改进Green-Ampt模型的滑坡稳定时变可靠度分析

降雨入渗滑坡稳定可靠度分析往往仅考虑了土体参数的空间变异性,而忽略土体初始含水率非均匀性空间分布的影响。基于此,推导了不同降雨工况条件下初始含水率任一分布的Green-Ampt入渗模型,利用变步长复化Simpson法求解初始含水率不同空间分布条件下湿润锋深度与降雨历时的函数关系。结合多维正态累计分布函数Mvncdf,计算整个降雨评估基准期滑坡稳定的时变可靠度,并利用某一滑坡实例进行对比验证。计算结果表明,初始含水率空间分布的不同对降雨条件下滑坡稳定时变可靠度的影响十分明显,且相较于初始含水率为均匀和梯形分布;相同降雨条件下,指数分布时滑坡稳定时变可靠度的下降幅度最小。该改进模型适用于土体初始含水率的任意分布,有利于Green-Ampt模型在滑坡稳定性评价的普适性推广应用。     

低山丘陵区典型滑坡-泥石流链生灾害特征与成灾机理

2019年6月10—13日,龙川县发生持续强降雨,导致全县境内发生大量滑坡、泥石流灾害,贝岭镇米贝村是三个重灾区之一。本文以贝岭镇米贝村6号沟发生的滑坡-泥石流链生灾害为研究对象,在野外精细化调查测量基础上,结合数值模拟分析与计算,对链生灾害特征与成灾机理展开研究。研究发现:①6号沟内共发育7处浅层土质小型滑坡,仅3号滑坡体与部分6号滑坡体转化为泥石流,构成泥石流主要物源,其余滑坡未构成持续性影响;②持续降雨下渗,坡体由非饱和向饱和状态转变,坡表形成连续饱和区,孔隙水压力的增加与孔隙水的软化促使土体强度降低,加之坡体饱和自重的增大,斜坡发生浅表层失稳破坏;③降雨的持续下渗与支沟沟源“漏斗状”地形下的地表汇水快速增大滑坡松散堆积体内的含水率,促使其物理性质发生变化,在重力势能下呈流态状启动、运动转化为泥石流。降雨结构影响滑坡-泥石流链生过程,前期降雨引发滑坡、后期降雨启动形成泥石流,滑坡与泥石流的发生表现出阶段性特征。研究成果有助于指导当地政府进一步开展滑坡-泥石流链生灾害的防灾减灾工作,也为该地区未来区域预警研究工作提供理论支撑。     

基于FLO-2D的矿山泥石流运动特征及危险性评价——以甘肃省岷县簸箕沟矿山泥石流为例

岷县簸箕沟金矿因人类开采活动引发了矿山泥石流灾害. 采用FLO-2D软件模拟分析了降雨强度重现期50 a及100 a条件下的簸箕沟泥石流运动特征, 进行危险性评价和分区, 并结合实际发生情况做了精度验证. 结果表明: 簸箕沟泥石流的堆积扇范围、堆积深度以及平均流速等运动特征参数随着降雨重现周期的变长而增大, 堆积扇中部的堆积深度及流速明显大于两翼及前端. 泥石流的危险区集中分布于泥石流沟道以及沟口一定范围内. 随着降雨重现周期的变长, 高危险区面积比例由48%升高至54.0%. 通过精度验证得出模拟结果与实际情况基本相符, 可信度较高.