The relationship between rainfall inputs and flood generation in south–east Spain

Rainfall and flood data are relatively sparse in semi‐arid areas; hence there have been relatively few investigations into the relationships between rainfall inputs and flood generation in these environments. Previous work has shown that flood properties are influenced by a combination of precipitation characteristics including amount, intensity, duration and spatial distribution. Therefore floods may be produced by high intensity, short duration storms, or longer duration, low intensity rainfall. Most of this research has been undertaken in small catchments in either hyper‐arid or relatively high rainfall Mediterranean climates. This paper presents results from a 6 year data record in south‐east Spain from research conducted in two basins, the Rambla Nogalte (171 km²) and the Rambla de Torrealvilla (200 km²). Data cover an area of approximately 500 km² and an annual average rainfall of 300 mm. At coarse temporal resolutions gauges spread over large areas record similar patterns of rainfall, although spells of rain show much more complexity; pulses of rain within storms can vary considerably in total rainfall, intensity and duration over the same area. The analysis for south‐east Spain shows that most storms occur over a period of less than 24 h, but that the number of rainfall events declines as the duration exceeds 8 h. This is at odds with data on floods for the study area suggesting that they are produced by storms lasting longer than 18 h. However, one flood event was produced by a very short (15 min) storm with high intensity rainfall. Most floods tended to occur in May/June or September, which coincides with wetter months of the year (September, October, December and May). Floods are also more highly related to the total rainfall occurring in a spell of rain, than to intensity. The complexity of storm rainfall increases with the storm total, which makes it difficult to generalize on the importance of rainfall intensity for flood generation. Copyright © 2007 John Wiley & Sons, Ltd.     

Modified hydrograph method for arid regions

There is a real need for more research on regionalization of flood estimation methods in arid zones. Such estimations are important because reliable long‐term storm rainfall and corresponding runoff measurements are commonly unavailable. This is the situation in many parts of the Kingdom of Saudi Arabia. Hence, it is necessary to develop a new approach, one that suits the conditions found in the Kingdom, which depend on available, but incomplete measurements and catchment morphological features. In this paper, a modified methodology based on the classical Snyder approach is proposed and it is referred to as the Saudi Geological Survey (SGS) approach in order to reflect works of this kind within the Survey. The basis of the methodology has two phases, namely, logical and empirical. The former phase is valid for any part of the world whether humid or arid, but the latter phase is location specific, which in the case of this paper, is the Arabian Peninsula. The application of the methodology is presented for Wadi Baysh, a major wadi in the south‐western part of the Kingdom. Furthermore, a synthetic unit hydrograph (UH) methodology based on a Gamma distribution function is also presented with applications to some of the Wadi Baysh sub‐basins. For this purpose, a dimensionless UH has been obtained, which is special for the area. Copyright © 2007 John Wiley & Sons, Ltd.     

Role of unusual moisture transport across Equatorial Indian Ocean on the extreme rainfall event during Kerala flood 2018

Southwestern Indian state, Kerala experienced extreme devastating statewide flood event of the century during 2018 monsoon season. In this study, an attempt has been made to bring out the salient dynamical factors contributed to the Kerala flood. There were 3 active spells over Kerala during 2018 Monsoon season. All the three spells were accustomed with the intrinsic factors of low frequency components of the active spells such as strength of monsoon Low Level Jet (LLJ), Monsoon depressions in the Bay of Bengal, favorable Madden-Julian oscillation (MJO) phases and Western Pacific systems. Though all the common ingredients remain same, the third spell is distinct with the less evaporation flux over Western and Central Arabian Sea and unusual moisture transport from maritime continent through South Equatorial Indian ocean (SEIO) towards the Kerala coast across Equator. Strong meridional pressure gradient force created by the combined effect of high pressure anomaly oriented Northwest-Southeast direction across tropical Indian ocean and anomalous low pressure along monsoon trough might have contributed to this unusual moisture transport across SEIO originating from west of Australia. The anomalous high pressure in South Indian ocean was greatly influenced by the position of the Mascarene high. Subtropical Indian ocean dipole (SIOD) also exhibits an influential role by altering tropical Southern Indian ocean dynamics in favor of the unusual moisture transport. The position of the monsoon depression and presence of typhoons in Western Pacific might have aided to this moisture transport. However, the normal moisture transport from Central Arabian sea towards Kerala coast by virtue of the strong LLJ along with additional moisture transport directly from South of maritime continent through SEIO across the Equator towards Kerala coast might have played a dominant role in the historical flood event over entire Kerala state.     

An assessment of large-scale flood modelling based on LiDAR data

Large-scale flood modelling approaches designed for regional to continental scales usually rely on relatively simple assumptions to represent the potentially highly complex river bathymetry at the watershed scale based on digital elevation models (DEMs) with a resolution in the range of 25–30 m. Here, high-resolution (1 m) LiDAR DEMs are employed to present a novel large-scale methodology using a more realistic estimation of bathymetry based on hydrogeomorphological GIS tools to extract water surface slope. The large-scale 1D/2D flood model LISFLOOD-FP is applied to validate the simulated flood levels using detailed water level data in four different watersheds in Quebec (Canada), including continuous profiles over extensive distances measured with the HydroBall technology. A GIS-automated procedure allows to obtain the average width required to run LISFLOOD-FP. The GIS-automated procedure to estimate bathymetry from LiDAR water surface data uses a hydraulic inverse problem based on discharge at the time of acquisition of LiDAR data. A tiling approach, allowing several small independent hydraulic simulations to cover an entire watershed, greatly improves processing time to simulate large watersheds with a 10-m resampled LiDAR DEM. Results show significant improvements to large-scale flood modelling at the watershed scale with standard deviation in the range of 0.30 m and an average fit of around 90%. The main advantage of the proposed approach is to avoid the need to collect expensive bathymetry data to efficiently and accurately simulate flood levels over extensive areas.     

Using stable isotopes to identify major flow pathways in a permafrost influenced alpine meadow hillslope during summer rainfall period

Global warming has leaded to permafrost degradation, with potential impacts on the runoff generation processes of permafrost influenced alpine meadow hillslope. Stable isotopes have the potential to trace the complex runoff generation processes. In this study, precipitation, hillslope surface and subsurface runoff, stream water, and mobile soil water (MSW) at different hillslope positions and depths were collected during the summer rainfall period to analyse the major flow pathway based on stable isotopic signatures. The results indicated that (a) compared with precipitation, the δ²H values of MSW showed little temporal variation but strong heterogeneity with enriched isotopic ratios at lower hillslope positions and in deeper soil layers. (b) The δ²H values of middle-slope surface runoff and shallow subsurface flow were similar to those of precipitation and MSW of the same soil layer, respectively. (c) Middle-slope shallow subsurface flow was the major flow pathway of the permafrost influenced alpine meadow hillslope, which turned into surface runoff at the riparian zone before contributing to the streamflow. (d) The slight variation of δ²H values in stream water was shown to be related to mixing processes of new water (precipitation, 2%) and old water (middle-slope shallow subsurface flow, 98%) in the highly transmissive shallow thawed soil layers. It was inferred that supra-permafrost water levels would be lowered to a less conductive, deeper soil layer under further warming and thawing permafrost, which would result in a declined streamflow and delayed runoff peak. This study explained the “rapid mobilization of old water” paradox in permafrost influenced alpine meadow hillslope and improved our understanding of permafrost hillslope hydrology in alpine regions.     

Determining hydrologic pathways of streamflow using geochemical tracers in a claypan watershed

Despite the low permeability of claypan soils, groundwater has been heavily contaminated by nitrate in agricultural watersheds dominated by claypan soils. However, it is unclear how nitrate concentrations in groundwater affect stream water quality. In this study, streamflow pathways were investigated using natural geochemical tracers in the 73-km² Goodwater Creek Experimental Watershed in northeastern Missouri. Samples were collected from 2011 to 2017 from stream water (weekly-biweekly), precipitation (event-based), groundwater in 25 wells with screened depths varying from 2 to 16 m (bimonthly–seasonal) and interflow above the claypan in 7 shallow piezometers (weekly–monthly). The results of endmember mixing analysis using major ions indicate that streamflow was dominated by near-surface runoff (59 ± 20%), followed by interflow (25 ± 16%) and groundwater (16 ± 13%). Analysis of endmember distances using the mixing space defined by stream water chemistry suggests that groundwater contributions to streamflow came primarily from the intermediate to deep glacial till aquifer near and below 8 m. Near-surface runoff was persistent and dominant even after isolated precipitation events during a prolonged dry period. It is hypothesised that the alluvial aquifer near stream banks acts as a mixing zone to receive and store various source waters, resulting in persistent delivery of runoff to the stream. Groundwater, even though its contribution was limited, plays a significant role in regulating streamflow NO₃⁻ concentrations. This study significantly improves our understanding of claypan hydrology and will lead to the development of models and decision support tools for implementation of management practices that improve groundwater and stream water quality in restrictive layer watersheds.     

The changing malaria risk patterns in East-Central Europe and the North Balkans in the last 27 000 years

Southeast Europe has historically been at the crossroads of migration routes between Western Asia and Europe. In the Holocene, this area might have been home to malaria. However, it is questionable when malaria arrived in this area and whether it could persist continuously or not in the Holocene. To begin to answer these questions, the July potential generation number of two malaria parasites were modelled, based on the reconstructed mean July temperatures of 52 times in the last 27 000 years. The results indicate that in the late Pleistocene era (27–12 kya bp ), vivax malaria might have been present in the south-east Black Sea and Aegean Sea's coastal areas. Vivax malaria could also be present in the Pannonian Basin and the inner parts of the North Balkans at least from the mid-Greenlandian period (~10 kya bp ). Although it is questionable whether falciparum malaria could be endemic in the Pannonian Basin during the mid-Holocene climate optimum (~6 kya bp ), this malaria plausibly could be endemic from the Neolithic era (~12–6.5 kya bp ) in the major river valleys of the North Balkan region, millennia ahead of the Graeco-Roman times (8th century bce to 6th century ce ).     

Hydrological alterations from water infrastructure development in the Mekong floodplains

The Mekong floodplains, which encompasses the region from Kratie Township in Central Cambodia to the Vietnamese East Sea, is a region of globally renown agricultural productivity and biodiversity. The construction of 135 dams across the Mekong basin and the development of delta‐based flood prevention systems have caused public concern given possible threats on the stability of agricultural and ecological systems in the floodplains. Mekong dams store water upstream and regulate flow seasonality, while in situ flood prevention systems re‐distribute water retention capacity in the floodplains. The main aim of this paper is to evaluate possible impacts of the recent development of both hydropower dams and flood prevention systems on hydrological regimes in the Mekong floodplains. An analysis of measured daily and hourly water level data for key stations in the Mekong floodplains from Kratie to the river mouth in Vietnam was conducted. Hydropower dam information was obtained from the hydropower database managed by the Mekong River Commission, and the MODIS satellite imagery was used to detect changes in flooding extent related to the operation of flood prevention systems in the Vietnam Mekong Delta. Results indicate that the upper part of the floodplains, the Cambodian floodplains, may buffer upstream dam impacts to the Vietnam Mekong Delta. Flood prevention up to date has had the greatest effect on the natural hydrological regime of the Mekong floodplains, evidenced by a significant increase of water level rise and fall rates in the upper delta and causing water levels in the middle delta to increase. The development of flood prevention systems has also effected spatial distribution of flooding as indicated via a time series analysis of satellite imagery. While this development leads to increase localized agricultural productivity, our historical data analysis indicates that development of one region detrimentally affects other regions within the delta, which could increase the risk of future conflicts among regions, economic sectors and the ecological value of these important floodplains. Copyright © 2016 John Wiley & Sons, Ltd.     

Modelling detention basins measured from high‐resolution light detection and ranging data

The construction of stormwater detention basins is a best management practice to effectively control floods, to provide additional surface storage for excess floodwater and to compensate for the adverse effects of urban development. Traditional field‐based levelling survey methods are very time consuming and subject to human‐induced arbitrariness and error. This article presents an approach to modelling detention basins measured from light detection and ranging remote sensing data. A case study is illustrated by using the White Oak Bayou watershed of Harris County, Texas. The storage–stage curve obtained from the volumetric analysis is used in a modified detention basins routing model, which was developed by adding the weir structure control to the traditional hydrologic reservoir routing equations. The model simulation showed that the peak flow of the synthetic 100‐year reoccurrence event was effectively reduced and delayed by the detention basins. The comparison with the simulation results from the traditional reservoir routing model suggested that previous studies using the reservoir routing model were likely to underestimate the flood reduction effect of detention basins. The sensitivity analysis of the parameters showed that the detention basin design and evaluation should pay more attention on the weir height and river channel's roughness. Copyright © 2011 John Wiley & Sons, Ltd.     

Flood hydrograph reconstruction from the peak flow value in ephemeral streams using a simplified robust single‐parameter model

Flood hydrographs from ephemeral streams in arid areas provide valuable information for assessing run‐off and groundwater recharge. However, such data are often scarce or incomplete, especially in hyper‐arid regions. The hypothesis of this study was that it is possible to reconstruct a hydrograph of a specific point along an ephemeral stream with the knowledge of only the peak flow rate of a flood event at that point and that this can be done at almost every point along the stream. The feasibility of this approach lies in the shape of the recession stage of the flood hydrograph, which is known to be a repeating phenomenon. The recession stage comes immediately after the peak flow rate, when it begins its decline, and lasts until the flood is extinguished. A general shape of the flood recession stage can be provided. Because the recession stage represents ~80% of the duration of a flood event, it can provide a general idea of the flood hydrograph's shape. A simple model based on geometric progression is suggested to describe the repeating recession stage of a flood. The advantage of the proposed model is that it requires only one parameter: the recession characteristic at a fixed point along the ephemeral stream, termed recession coefficient q. By knowing the recession coefficient of a fixed point and the peak flow rate of a flood event at that point, one can plot the flood hydrograph. A good agreement is shown between the observed and computed values of the recession stage. Copyright © 2016 John Wiley & Sons, Ltd.