Small Ungauged Catchments Research Articles

Exploring soil erosion and reservoir sedimentation through the RUSLE model and bathymetric survey

The aim of the paper was to compare the soil erosion in a river catchment with the sediment volume accumulated at the river mouth. Firstly, the sediment yield was estimated in GIS based on the soil loss according to the RUSLE model, and then further integrated into the sediment production equation. Following this, we estimated the sediment volume accumulated at the river mouth based on the diachronic overlap of topographic and bathymetric data. This methodology was validated for the Eselnita catchment exiting into the Iron Gates I Reservoir. The LS factor has an average value of 4, with lower values for the forest cover. The C factor has an average value of 0.057 being statistically correlated with the RUSLE result. The average soil loss was estimated at approximately 1.89 t ha-1 yr -1, a value that is validated by previous studies as a low risk of erosion at national scale. The sediment transfer model indicates a distribution of cells sediment production strongly correlated with the time travel to the discharge channels. Overall, the sediment volume obtained by using the RUSLE model corresponds to about 70% of the sediment volume accumulated at the river mouth during 53 years (1970-2022). The difference in sedimentation may be due to human activities along the river mouth’s banks to extend the built-up area and to enjoy the waterscape. This paper is relevant for the topic of reservoir sedimentation and recommends the use of the RUSLE model to predict the sediment contribution, especially for small ungauged catchments.

A complete methodology to assess hydraulic risk in small ungauged catchments based on HEC-RAS 2D Rain-On-Grid simulations

This paper explores the use of the rain-on-grid (or direct rainfall) method for flood risk assessment at a basin scale. The method is particularly useful for rural catchments with small vertical variations and complex interactions with man-made obstacles and structures, which may be oversimplified by traditional hydrologically based estimations. The use of a hydrodynamic model solving mass and momentum conservation equations allows the simulation of runoff over the watershed at a basin scale. As a drawback, more detailed and spatially distributed data are needed, and the computational time is extended. On the other hand, a smaller number of parameters is needed compared to a hydrological model. Roughness and rainfall loss coefficients need to be calibrated only. The direct rainfall methodology was here implemented within the two-dimensional HEC-RAS model for the low-land rural, and ungauged, watershed of the Terdoppio River, Northern Italy. The resulting hydrographs at the closing section of the watershed were compared to synthetic design hydrographs evaluated through pure hydrological modelling, showing agreement on the peak discharge values for the low-probability scenarios, but not on the total volumes. The results in terms of water depth and flow velocity maps were used to create flood hazard maps using the Australian Institute for Disaster Resilience methodology. The Index of Proportional Risk model was then adopted to generate a basin-scale flood risk map, by combining flood hazard maps, damage functions for different building-use classes, and the value of reconstruction and content per unit area.

From rainfall to runoff: The role of soil moisture in a mountainous catchment

Soil moisture is essential in runoff generation, chemical transport, vegetation growth, and geological hazards like landslides in mountainous regions. However, due to its difficulty of measurement and its significant spatial heterogeneity and temporal variability, the dynamics and influence of soil moisture in the rainfall-runoff process are still not fully understood. In this study, precipitation, runoff, and soil moisture at multiple depths were measured and analyzed in a small, post-seismic mountainous catchment. Our results suggest that the dynamics of soil moisture could be explained by the characteristics of soil. The drastic change and the anisotropy of saturated hydraulic connectivity in the upper soil of the loose deposits in our catchment would lead to soil stratification at depths between 30 cm and 50 cm. The temporal variability of soil moisture was more highly correlated within the soil layers above and beneath the interface. Above the interface, besides vertical infiltration, occurrences of preferential flow were also frequent, transient perched water table may emerge during the wet period, making it a more active runoff contributor than the soil layers below the interface. When the level of rainfall reached the threshold, the correlation between rainfall and runoff became linear. This threshold response was more significant at soil layers above the interface, especially at the site with large topographic gradient. High antecedent soil moisture could skew the partitioning of precipitation into discharge, leading to an earlier flow peak. Our findings provide guidance on observation spot selection and the soil properties needed for the interpretation of future measurements, and may help develop flood early warnings system in small ungauged mountainous catchments with high flood risk.

Impacts of small-scale irrigation water use on environmental flow of ungauged rivers in Africa

Failure in Environmental flow in quantity, timing, and quality leads to failure to support ecosystems, human livelihoods, and well-being. Irrigation water use is one of the main actors in impacting the water flow of rivers in quantity and time but was not well investigated in many ungauged catchments under smallholder irrigation systems. This study examined the impact of irrigation water use on environmental flow in Arata's small ungauged catchment. The study estimated the flow in sub-catchment using the area ratio method, the crop irrigation water requirement using F.A.O. cropwat 8.0, and the water balance in the Water Evaluation and Planning System tool and the environmental flow in Tennants, Q95, asnd local area thumb rule. The result showed that the minimum environmental flow of the Arata catchment is 290, 310, and 60 li/sec in the Tennant, Q95, and the local thumb rule. Irrigation consumes only 9% of the water resources of the catchment while 91% is contributed to downstream lake Ziway via Ketar river. January and February have unmet water demand and zero environmental flow. In December Tennant's 10% and Q95 recommended environmental flow had 19% and 24% deficit while the thumb rule environmental flow is 291% more than the minimum requirement. The rest of the months are by far more than the minimum environmental flow requirement. Given the result, meeting the environmental flow of the system throughout the year needs the installation of a water storage facility from upstream to downstream, the introduction of different water-saving irrigation technologies, farmers' capacity building in irrigation water management, and a standardized environmental flow estimation mechanism.

Hydromorphological Inventory and Evaluation of the Upland Stream: Case Study of a Small Ungauged Catchment in Western Carpathians, Poland

The hydromorphological conditions of watercourses depend on numerous natural and anthropogenic factors such as buffer zones or human infrastructure near their banks. We hypothesised that, even in a small stream, there can be substantial differences in the hydromorphological forms associated with naturalness and human impact. The paper aims at the field inventory and evaluation of the hydromorphological conditions of a small upland stream in the conditions of contemporary human activity, against the background of meteorological and hydrological conditions. The study concerned a left-bank tributary of the Stradomka River located in the Wiśnicz Foothills (Western Carpathians). The analyses were conducted with the use of the Polish method, the Hydromorphological Index for Rivers (HIR), which conforms to the EU Water Framework Directive (WFD). The hydromorphological condition and quality of habitats were evaluated based on the Hydromorphological Diversity Score (HDS) and Habitat Modification Score (HMS). The study shows that the largest changes in stream hydomorphology and habitat conditions took place in the downstream, urbanised stream catchment area with an intensive development of construction and technical infrastructure. The hydromorphological condition of the examined stream sections was evaluated as good or poor. The best hydromorphological conditions were found in the section located in the semi-natural area, and the worst in the urbanised area. As our research shows, the strong influence of human activity, including weather extremes, and the risks and hydrological hazards of the hydromorphological conditions of the small, ungauged catchment, highlight the necessity to search for other research methods to support the decision-making cycle in the transformation of riverbeds and catchments.

Assessment of suspended sediment dynamics in a small ungauged badland catchment in the Northern Apennines (Italy) using an in-situ laser diffraction method

The volumes and dynamics of suspended sediments drained from a watershed are often unknown, especially in small creeks and tributaries where continuous discharge measurements are seldom available. The objective of this study was to assess in a qualitative and quantitative way the sediment dynamics of a small ungauged watershed in the Northern Apennines, Italy. We analysed and correlated the sediment volume concentration and grain-size distribution of the suspended sediments with the precipitation pattern. The study area is a small ungauged watershed (0.15 km2) that is dominated by intensive soil erosion processes and related landforms. The basin is oriented East-West with the south-facing slopes characterized by badland erosion processes. The north-facing slopes are cultivated and dominated by rill-interrill erosion phenomena. An morphometrical characterization of the basin was performed using a high-resolution DTM with a 1 × 1 m resolution. Subsequently, the physical characteristics of the topsoil were investigated based on grain size laboratory analysis. A detailed NDVI analysis of the vegetation was performed using Sentinel-2A images. Finally, we assessed the suspended sediments at the outlet of the basin using a laser diffraction technique. The Suspended Sediment Volume Concentration (SSC) and the Sauter Mean Diameter (SMD) of the eroded sediments, provide insights into the morphogenetic processes and the sediment transport dynamics of the basin. The measurements were conducted in autumn 2018 after an intense precipitation period and in spring 2019 after a dry phase. The results show a direct relationship between precipitation and SSC with a delay of about 2 h after the most intense precipitation events. Moreover, we reveal that the SMD values are inversely related to the precipitation due to turbidity effects. The SSC and SMD measurements allow for a detailed assessment of the dynamics between precipitation and suspended sediment load even though discharge volumes were not measured directly.

Hydrodynamic parameters of floods and related bank erosion events indicated from tree rings and 2D hydrodynamic model for a small ungauged catchment (Sudeten Mts., Poland)

Small mountain catchments usually lack hydrological monitoring and gauges. Therefore, in such areas, data on past flood and bank erosion are often missing, which makes assessing flood and erosion hazards very limited. We attempt to fill in this gap by dating individual flood and erosion events from growth disturbances produced by trees after their stems are tilted, and their roots are exposed and wounded by transported material. We aimed to develop a conceptual approach to integrate dendrochronology and 2D modelling for indicating and assessing past events of floods and bank erosion on a small mountain river Łomniczka, Sudeten mountains, Poland. We dated growth disturbances resulting from tilting of stems of spruce trees which grow on eroded riverbanks, i.e. tree-ring eccentricity and compression wood. We also dated disturbances resulting from the exposure of roots from under the soil cover, i.e. sudden decreases of cell lumen, and root injuries by debris transported by floods, i.e. scars and traumatic resin ducts. Dendrochronology allow to indicate the occurrence of 28 floods since the 1930s, including 11 floods when bank erosion was also recorded at study sites. The approach enables to identify rates of bank erosion during specific floods which ranged at study sites from 20 to 120 cm. The largest discharge was determined for the 1997 flood (106,7 m3s−1), and the highest flow velocities were obtained for the 1930 floods (4.59 m/s). Results show that the highest shear stress occurred during the floods in 1943 and 1977 (510,3N/m2) and in 1997 flood (469.1 N/m2). We conclude that dendrochronology combined with 2D modelling allowed us to indicate past floods and bank erosion, and to prepare reliable inventories for analyses of flood and erosion hazard. The approach proposed in this paper can also be used as a tool for flood management, spatial management and planning.

Application of Xin'anjiang Model in the flow prediction of ungauged small- and medium-sized catchments in the middle and lower reaches of the Yangtze River Basin

水文资料匮乏流域的洪水预报（PUBs）是水文科学与工程中一个尚未解决的重大挑战.中国湿润山区中小流域大多是水文资料匮乏的流域，在此地区进行洪水预报的重要手段之一就是水文模型参数的估计.对基于参数物理意义的估算方法（以下简称物理估算法）及两种区域化方法进行了研究，将其用于新安江模型参数的估算及移植.皖南山区的29个中小流域被选作水文资料丰富的测量流域，鄂西山区的3个中小流域被视为水文资料匮乏的目标流域，目的是研究目标流域与测量流域空间位置较远但物理条件相似时，区域化等方法是否可以有效估计模型参数.结果表明，即使目标流域与测量流域空间距离较远，区域化及物理估算法也能一定程度上减少参数估计导致的模型效率损失，且在研究区的最优参数估计方案为单流域物理相似法结合回归法及物理估算法.为长江中下游资料匮乏的山区中小流域提出了可行的新安江模型参数估计方案，为该地区的洪水预报提供指导.;Flow prediction in ungauged basins (PUB) is as important as it is difficult. PUB is a major unsolved challenge in scientific and engineering hydrology. So far, empirical and stochastic methods have mainly been used for this purpose. Many small catchments in humid mountainous areas of China are with little or no hydrological data. The principal method of flood forecasting in this area is the parameter estimation of the hydrological model. Xin'anjiang Model was used to predict flow in this study, and Xin'anjiang Model parameters were estimated based on the physical meaning of them and regionalization. Regression and physical similarity approach were included in regionalization methods in this study. The 29 small basins in mountainous areas in southern Anhui Province were selected for gauged catchments, and three small basins in mountainous regions in western Hubei Province were treated as ungauged catchments. The main goal of this approach was to research whether regionalization and other methods could effectively estimate model parameters when the spatial location of the gauged catchments and the ungauged catchments were far away, but the physical conditions were similar. It is shown that:(1) Xin'anjiang Model has high accuracy in flow prediction of study areas. (2) Even if the space distance of gauged catchments and ungauged catchments is far, regionalization and physical-based estimation method can also reduce the efficiency loss caused by parameter estimation to some extent. (3) With the increase in the number of donor catchments, the efficiency of the physical similarity approach is reduced. (4) The optimal parameter estimation scheme in the study area is the ingenious combination of physical similarity approach (only one donor catchment), regression approach and physical-based estimation method. A feasible parameter estimation scheme of the Xin'anjiang Model is proposed in this study, which provides guidance for flow forecasting in small ungauged catchments in humid mountainous areas among the middle and lower reaches of the Yangtze River.

Hydrological impacts of climate change on small ungauged catchments – results from a global climate model–regional climate model–hydrologic model chain

Abstract. Climate change is one of the greatest threats currently facing the world's environment. In Norway, a change in climate will strongly affect the pattern, frequency, and magnitudes of stream flows. However, it is challenging to quantify to what extent the change will affect the flow patterns and floods from small rural catchments due to the unavailability or inadequacy of hydro-meteorological data for the calibration of hydrological models and due to the tailoring of methods to a small-scale level. To provide meaningful climate impact studies at the level of small catchments, it is therefore beneficial to use high-spatial- and high-temporal-resolution climate projections as input to a high-resolution hydrological model. In this study, we used such a model chain to assess the impacts of climate change on the flow patterns and frequency of floods in small ungauged rural catchments in western Norway. We used a new high-resolution regional climate projection, with improved performance regarding the precipitation distribution, and a regionalized hydrological model (distance distribution dynamics) between a reference period (1981–2011) and a future period (2070–2100). The flow-duration curves for all study catchments show more wet periods in the future than during the reference period. The results also show that in the future period, the mean annual flow increases by 16 % to 33 %. The mean annual maximum floods increase by 29 % to 38 %, and floods of 2- to 200-year return periods increase by 16 % to 43 %. The results are based on the RCP8.5 scenario from a single climate model simulation tailored to the Bergen region in western Norway, and the results should be interpreted in this context. The results should therefore be seen in consideration of other scenarios for the region to address the uncertainty. Nevertheless, the study increases our knowledge and understanding of the hydrological impacts of climate change on small catchments in the Bergen area in the western part of Norway.

Approximate calculation of flash flood maximum inundation extent in small catchment with large elevation difference

Inundation analysis is one of the core aspects of flash flood prevention and control in mountainous area. Remote sensing technology can conveniently extract submerged region of a large flood, but infeasible to dynamically obtain flood inundating process in a high temporal resolution. Meanwhile, it is difficult to establish a suitable hydrodynamics-based inundation model under conditions of complex terrain. Moreover, such model usually requires huge input data and long simulation time. The Bulk Method (BM) can quickly calculate flood inundation extent, given the total water yield in the catchment and local Digital Elevation Model (DEM). However, the traditional BM is only applicable for relatively flat terrain. In order to apply BM to calculation of the flash flood inundation extent in the catchment of large elevation difference, the differential BM based on differential element theory is proposed in this paper. In the method, the main flood channel is divided into a certain number of differential channel segments, in which the local elevation difference is relatively small, and the BM can be soundly applied. The flood inundation extent of the whole channel can be obtained by combining the maximum inundation extents of all differential channel segments. Furthermore, it is the key to determine the optimal number of channel segments by repeating the prior steps of the proposed method until the area change ratio of the total inundation extents calculated in two consecutive loops is less than the specified threshold value. Taking Zharugou Catchment as the study area, we obtained the approximate maximum inundation extent of a once in 10-year flood occurred there on July 10th, 2018. As the calculation results were generally in good agreement with the field measuring data, it demonstrated the feasibility and effectiveness of the differential BM in approximately calculating the maximum flood inundation extent in a small ungauged catchment with sharp elevation variation.

Hydrological modelling in small ungauged catchments.

The knowledge of the frequency and magnitude of low flow events is necessary to mitigate social, economic and ecological impacts inside the basin. However, the measurement network in Brazil is still restricted to large drainage areas, while basins with less than 300 km2 remain ungauged. Among different flow estimation methods, we used a rainfall-runoff model designed specifically to estimate flow rates during the dry season in small ungauged basins: the Silveira Method (SM). We tested the model performance for the São Bartolomeu river basin (Minas Gerais, Brazil), a small ungauged basin that experienced severe droughts and water supply shortages in 2014-2016. We tested eleven different scenarios based on the time and duration of drought periods used to estimate the model parameters. In the best scenario, the model underestimated low flow rates by 31% for Q95 and was considered suitable to predict local low flow. Finally, the model results suggest that a water volume higher than the river can support has been granted concession during the dry season, which may lead to an unsustainable water supply scenario soon. This result showed the capacity of SM as a complementary tool for the evaluation of water potential in small basins.

Comparison of the Curve Number Method (SCS-CN) Modifications and the Application of Measures for Soil Erosion Reduction and Flood Protection in Small Ungauged Catchments in the White Carpathian Mountains in Slovakia

Knowledge of soil erosion and runoff processes leads to changes in attitudes towards a rational use of soil. Agricultural practices have a wide range of negative environmental impacts, including accelerated soil erosion, degradation, and increases in the volume of surface runoff. It is, therefore, necessary to investigate these negative influences on arable land. The study area is the municipality of the village of Vrbovce. It is situated in the north-western part of Slovakia in the White Carpathian Mountains. Due to its specific geological (heterogeneous rock masses of flysch) and geomorphological conditions (hilly country, uplands), that part of Slovakia is prone to accelerated soil erosion and mud flood formations. Nevertheless, a significant part of the municipality of Vrbovce is classified as agricultural land. The aim of this study is the soil erosion and runoff volume modelling that is focused on changes in the land cover/land use. We calculated soil loss with the Universal Soil Loss Equation (USLE) for agricultural parcels in the cadastral area of the village of Vrbovce. We are offering a set of suitable measures and agricultural procedures for this area as a result of our research. We estimated the direct runoff for the Teplica River basin up to the Teplica water gauge and five ungauged subcatchments in the cadastral area of the village of Vrbovce (Haluznikov creek, Lulovský creek, Zahutník creek, Zápasečník, and an unnamed creek). The Teplica river is poorly gauged with only a 10-year observation period. That length of hydrological measurements is not enough for the required period for calibrating a hydrological model. We used 3 modifications of the curve number (CN) method for estimating the volume of the direct runoff with various return periods, i.e., the standard CN method with values for the current land use, the worst land use an alternative, and the DesQ program method. We also applied the Pearson Type III distribution for the estimation of the peak flow with a higher frequency of occurrence for the Teplica gauging station. The results of the specific runoff were compared with the values of the specific runoff calculated for the Sobotište water gauge, which is situated downstream of the Teplica river. The results show that the most suitable modification of the CN method is the standard CN method with the use of the values for the current land use. The predictions of the long-term average annual rate of erosion on each parcel from the land parcel identification system (LPIS) were calculated by the Universal Soil Loss Equation (USLE).

Flash Flood Simulation for Ungauged Catchments Based on the Distributed Hydrological Model

Flash flood is a significant threat to those who live in China and beyond. Reproducing them with distributed hydrological models is an effective measure for preventing flash floods. This paper introduces the China flash flood hydrological model (CNFF-HM); explores its principles, key steps, and applicability; and validates its simulation effects in two typical, small ungauged catchments (UCs), namely, Tiezhuling and Qianyangxi. First, the applicability of the model and the portability of the parameters were verified in Qianyangxi. Then, in Tiezhuling, the model parameters were comprehensively determined based on the measured values and simulation results, and the influence of the reservoir on the simulation effect was analyzed. Using the model’s different parameters in different catchments, the characteristics of the basin were compared and analyzed from the aspects of evapotranspiration, runoff, water distribution, and confluence. Finally, the early warning effect was evaluated. The results indicated the following: (1) The reservoir has a significant impact on model simulation. (2) The two UCs have similarities in evapotranspiration, runoff, and water source, and the difference in convergence is obvious. (3) With better early warning effect, the warning can achieve full coverage. These positive results suggest that this method should be further developed and tested.

GIS Based Flood Flow Assessment in Small Catchments for Flood Mapping in Bosnia and Herzegovina

Abstract An initial step in flood hazard mapping is hydrological modelling. We present a recent river flood modelling approach in Bosnia and Herzegovina (BiH) for small ungauged catchments of drainage area up to 32 km2. To estimate peak flow of required probability in small catchments, we use the rational method. The paper focus is GIS based procedure for producing the runoff coefficient map for BiH from the DTM and land cover map. For validation of the peak flow modelling results in small ungauged catchments we use diagrams of peak flow per catchment area (specific runoff) versus catchment area in medium and large gauged catchments. The results indicate agreement in specific runoff for 100 and 500 years return period compared to reference runoff in gauged catchments and a mild drop in specific runoff for 20 years.

The use of Snyder synthetic hydrograph for simulation of overland flow in small ungauged and gauged catchments

The paper presents the results of simulated overland flow on the Třebsín experimental area, Czech Republic, using the Snyder synthetic unit hydrograph. In this research an attempt was made to discover a new approach to overland flow simulation that could give precise results like the KINFIL model for a small ungauged catchment. The provided results also include a comparison with the KINFIL model for N = 10, 20, 50 and 100 year recurrence of rainfall-runoff, with the rainfall time duration t<sub>d</sub> = 10, 20, 30, and 60 min. Concerning a small gauged catchment, one of the most accurate and elegant methodologies, Matrix Inversion Model, can be used for the measurement of both the gross rainfall and the runoff. This method belongs to a matrix algebra concept. For the sake of completeness, we designated this model at the end of the present article to show how exact this forward march can be.

Hydrological and hydraulic analysis of a small lowland watercourse flow capacity and its functioning in the region of Silesian Lowlands in the context of rainfall water management

Abstract Hydrological and hydraulic analysis of a small lowland watercourse flow capacity and its functioning in the region of Silesian Lowlands in the context of rainfall water management. The built-up areas in the catchments of small lowland watercourses faced with the risk of particularly devastating natural hazards. This risk is especially prevalent in the mouth reaches. The increasingly intense extreme natural phenomena, such as violent floods and long lasting droughts, increase the importance of irrigation, drainage and hydraulic structures. This paper presents the results of a comprehensive hydrological and hydraulic analysis of the functioning of a small lowland watercourse flow capacity in the context of water management in its catchment. The case study discussed below is a proposal that could be used by both the specialists and the designers working in the field of water management. Methods are proposed for the determination of heavy rainfall in small ungauged agricultural catchments for the purpose of rainfall water management. The area under study consists of the lower reaches of R-4, a watercourse located in the agricultural village of Dobrzeń Wielki in the Opolskie province, the Upper Odra river basin. The authors have assessed the flow capacity of R-4 based on EN 752:2008 and carried out hydrological calculations for the area being analysed as well as hydraulic calculations for R-4. It was concluded, that the small lowland watercourse and the communication structures (culverts and pipelines) on its way indicated low flow capacity. Consequently, a rainfall water management concept was proposed for the water flowing in the watercourse. A small retention reservoir with a total capacity of 3,132 m3 and a surface area of 2,088 m2 was also proposed to be constructed. In order to calculate the water inflow to the reservoir the Bogdanowicz and Stachy formula was used. This model was selected because the maximum rainfall on agricultural land calculated from this formula meets the requirements of EN 752:2008. Moreover, this formula produced the highest runoff values compared to other models. The research carried out is in line with the goals of water management in rural areas, where the management model has changed.

Mapping dominant runoff processes: an evaluation of different approaches using similarity measures and synthetic runoff simulations

Abstract. The identification of landscapes with similar hydrological behaviour is useful for runoff and flood predictions in small ungauged catchments. An established method for landscape classification is based on the concept of dominant runoff process (DRP). The various DRP-mapping approaches differ with respect to the time and data required for mapping. Manual approaches based on expert knowledge are reliable but time-consuming, whereas automatic GIS-based approaches are easier to implement but rely on simplifications which restrict their application range. To what extent these simplifications are applicable in other catchments is unclear. More information is also needed on how the different complexities of automatic DRP-mapping approaches affect hydrological simulations. In this paper, three automatic approaches were used to map two catchments on the Swiss Plateau. The resulting maps were compared to reference maps obtained with manual mapping. Measures of agreement and association, a class comparison, and a deviation map were derived. The automatically derived DRP maps were used in synthetic runoff simulations with an adapted version of the PREVAH hydrological model, and simulation results compared with those from simulations using the reference maps. The DRP maps derived with the automatic approach with highest complexity and data requirement were the most similar to the reference maps, while those derived with simplified approaches without original soil information differed significantly in terms of both extent and distribution of the DRPs. The runoff simulations derived from the simpler DRP maps were more uncertain due to inaccuracies in the input data and their coarse resolution, but problems were also linked with the use of topography as a proxy for the storage capacity of soils. The perception of the intensity of the DRP classes also seems to vary among the different authors, and a standardised definition of DRPs is still lacking. Furthermore, we argue not to use expert knowledge for only model building and constraining, but also in the phase of landscape classification.

The value of streamflow data in improving TSS predictions – Bayesian multi-objective calibration

Summary The concentration of total suspended solids (TSS) in surface waters is a commonly used indicator of water quality impairments. Its accurate prediction remains, however, problematic because: (i) TSS build-up, erosion, and wash-off are not easily identifiable; (ii) calibrating a TSS model requires observations of sediment loads, which are rare, and streamflow observations to calculate concentrations; and (iii) predicted TSS usually deviate systematically from observations, an effect which is commonly neglected. Ignoring systematic errors during calibration can lead to overconfident (i.e. unreliable) uncertainty estimates during predictions. In this paper, we therefore investigate whether a statistical description of systematic model errors makes it possible to generate reliable predictions for TSS. In addition, we explore how the reliability of TSS predictions increases when streamflow data are additionally used in model calibration. A key aspect of our study is that we use a Bayesian multi-output calibration and a novel autoregressive error model, which describes the model predictive error as a sum of independent random noise and autocorrelated bias. Our results show that using a statistical description of model bias provides more reliable uncertainty estimates of TSS than before and including streamflow data into calibration makes TSS predictions more precise. For a case study of a small ungauged catchment, this improvement was as much as 15%. Our approach can be easily implemented for other water quality variables which are dependent on streamflow.

A method of flood severity assessment for predicting local flood hazards in small ungauged catchments

A flash flood caused by heavy storm in a short period of time is now one of the common natural disasters worldwide. The aim of this paper is to develop a new flood hazard index for use in local flood severity predictions in small ungauged catchments through the regression analysis between the flood hazard index and rainfall features. The flood events with the peak flow above a threshold discharge are targeted to estimate the flood hazard index among the annual maximum flood runoff hydrographs generated from a rainfall–runoff model for the long-term-observed rainfall data. To quantify characteristics of flood runoff hydrographs simulated for ungauged catchments, the new flood hazard index is measured by the average of the three normalized relative severity factors, such as the flood magnitude ratio, the rising curve gradient, and the rising average runoff to the peak time. The proposed flood hazard index shows a high correlation with logarithm of the short-duration rainfall for the two selected small ungauged catchments in the Korean Peninsula. The best-fit regression equation between the flood hazard index and rainfall patterns can be used in predicting the relative flash flood severity for small ungauged catchments.

An improved method for estimating the median annual flood for small ungauged catchments in the United Kingdom

AbstractThis paper presents a new regression equation to estimate the median annual maximum instantaneous flow (QMED) for small ungauged catchments in the UK. The regression equation utilises catchment descriptors that have been used extensively for flood and low‐flow hydrology. To derive this equation, a database of 135 gauging stations that drain areas ranging from 0.5 to 25 km2 has been compiled. This is a larger dataset than has been used in previous equivalent studies. Predictions from the new QMED equation were compared to other widely used flood estimation methods, showing a significant improvement in accuracy in some cases. The merits of including a descriptor quantifying urbanisation in the new regression equation are also discussed.