Ungauged Watersheds Research Articles

Assessment of Drought vulnerability through an integrated approach using AHP and Geoinformatics in the Kangsabati River Basin

This study focuses on the application of multi-sensor satellite archived data products and ancillary data for the evaluation of drought vulnerability. The use of a subjective model, namely Analytical Hierarchy Process (AHP) integrated with Geographical Information System (GIS) and various influencing parameters, emerged as a powerful tool for the gauged and ungauged watershed. The Kangsabati river basin, located in the southern part of West Bengal (India), is facing issues of overexploitation of groundwater, climate variation, and unsystematic agricultural practices that are inducing the water crisis and vulnerability of the basin to drought conditions. In this study, 11 parameters, namely elevation, slope, aspect, LULC, population density, normalized difference vegetation index (NDVI), normalized difference drought index (NDDI), land surface temperature (LST), normalized difference water index (NDWI), vegetation condition index (VCI), and soil moisture index (SMI) were used to delineate the drought in the catchment of Kangsabati River. Findings of this study using Analytical Hierarchy Process (AHP) integrated with a GIS platform, show that, nearly 28.5% of the area falls under the medium drought category, followed by the high (21.1%), no drought (20%), low (19.5%), and very high (10.6%) drought categories in the study area. The study also suggests Nature Based Solution (NBS) in Disaster Risk Reduction (DRR) strategies such as ecosystem-based approaches (EBA) and blue infrastructure for Drought Risk Reduction for drought mitigation in the study area.

Hydrological impacts of climate change in selected ungauged sub-watersheds of Lake Tana Sub-Basin, Upper Blue Nile Basin, Ethiopia: A regionalization approach

Climate change is a global issue affecting the world's hydrology and water resources. Quantifying climate change impacts on hydrology is important for integrated water resources management. The ungauged watersheds of Lake Tana sub-basin are vital for different habitats and communities where various socio-economic activities such as irrigation, and fishing are carried out. Therefore, assessment of the impact of climate change on this sub basin is critical to settingup an integrated water resource management plan for susitnablity of livelihoods. Climate effect evaluation on ungauged catchments' hydrology using continuous hydrological modeling is difficult with the absence of adequate measured hydro-meteorological data for modeling. This study, thus, used a regionalization approach for ungauged watershed impact assessment and an HBV hydrological model for simulation of flow for different RCP scenarios and time domains. Impacts of climate change on the hydrology of the ungauged sub watersheds namely (Gilgel Abay, Gumara, Megech, and Ribb) were studied using two ensemble GCM driven by one RCM(RCA4) under RCP4.5 and RCP8.5 for the 2040s (2020-2049), and 2070s (2050-2079) time domains. The following results were obtained: Summer rainfall drops for RCP4.5, and increments for RCP8.5 with a decline from March to June. Rainfall decreased under RCP4.5 and RCP8.5 in the spring. It was conclude that increased flow in the dry seasons, provided water for agriculture and other uses while, a decrease flow during the rainy seasons would reduce Lake Tana's storage during the summer. Summer storage is useful for the Tana Belles Hydropower project as it, potentially influences water demand and lake's Navigation depth. In the future these changes will potentially have a severe influence on agricultural production by reducing water supply, thereby calling for the successful implementation of adaptation measures to reduce vulnerability. As the study results revealed that there would be future dry flow decreases and wet flow increases, it is therefore,important that the government, and decision-makers should plan for integrated water resources management activities where for example, wet season flow could be stored for dry season use by providing storage structures.

Runoff Estimation in Ungauged Watershed and Sensitivity Analysis According to the Soil Characteristics: Case Study of the Saint Blaise Vallon in France

Water Resources Research announced that, as a result of global warming, the amount of extreme torrential rain globally has increased steadily since the middle of the last century. To cope effectively with climate change, it is important to use consistent and scientific water information of water resources. In this study, we use a hydrological analysis of the Saint Blaise Vallon area to indicate how the damage from natural disasters that may come in the future may be minimized. In addition, a hydrological analysis and a numerical simulation model were implemented to estimate runoff and runoff coefficients derived from a heavy rainfall event that caused serious damage to river flooding. A runoff sensitivity analysis was conducted according to the soil parameters. In particular, a method using the hydrological model and the hydrological sensitivity analysis were applied to the target watershed, and the results of the peak outlet discharge were analyzed in time series so that they could be used for flood prediction and disaster management. In addition, the flood discharge and runoff coefficients during the flooding of the target watershed were presented through the study.

A nonparametric statistical approach for the estimation of water quality characteristics in ungauged streams/watersheds

Water quality (WQ) data are essential for water resources as well as WQ management. However, due to limited fiscal resources, in many cases, historical WQ data may not be available for the watershed(s) of interest. Recently, few artificial intelligent and parametric statistical approaches were proposed for the estimation of WQ characteristics at ungauged watersheds. In this study, the main goal was to develop nonparametric statistical approaches for the estimation of the three main quartiles of different WQ indicators in ungauged watersheds using watershed attributes as predictors. Four nonparametric approaches were based on the region of influence (ROI) and Theil-Sen nonparametric multiple regression (TSMR). The ROI was used to identify watersheds similar to the ungauged watershed, and TSMR was calibrated and validated to estimate WQ quartiles in ungauged watersheds. The main proposed approach combined the ROI and TSMR. In the second approach, the Spearman correlation analysis was introduced as a filter, to identify watershed attributes that are highly correlated with the WQ quartile, to improve the ROI performance, (CO)ROI-TSMR. In the third approach, a step forward selection technique was integrated with the TSMR to facilitate the selection of the model predictors (watershed attributes), ROI-(SF)TSMR. In the fourth approach, both the (CO)ROI and (SF)TSMR were used, (CO)ROI-(SF)TSMR. The four nonparametric statistical approaches were calibrated and validated, to estimate the three main quartiles for three different WQ indicators, using data from 50 watersheds in the Nile Delta, Egypt. A Jackknife validation procedure was applied to evaluate the performance of the four approaches. The results indicated that the (CO)ROI-(SF)TSMR approach outperformed the three other approaches.

Efficacy of Synthetic Sediment Graph Developed using Various Modified Time-Area Methods

Suspended sediment (SS) is an essential indicator for assessing watershed health. However, the temporal variation of SS, called sediment graph (SG) using readily available data, is not always considered, particularly in un-gauged watersheds, which are many in developing countries. Since field measurements of SS are time-consuming and costly, the synthetic SG seems to be a promising alternative. Therefore, it is essential to have reliable SS data for watershed management. This study aimed at simulating SGs through conceptual analysis of soil erosion and sediment yield at the watershed scale. To that end, soil erosion, sediment yield, and sediment routing were modeled using 38 storm events collected during 2011 and 2019 at the Galazchai Watershed in West Azerbaijan Province, Iran. Initially, the Time-Area Method (TAM) was applied, and then two strategies were considered to improve the TAM performance, including RUSLE and sediment delivery ratio (SDR) using gradient ratio and WaTEM/SEDEM methods. Comparing simulated SGs with recorded ones showed that the SDR-based method had the lowest relative error in time to peak and base time, but the peak value had the highest relative error. Results also showed that TAM developed using the spatially distributed travel time method had a better performance than the channel longitudinal profile method. Overall, TAM could not simulate the temporal variation of sediment and needs further research.

Historical climate change impacts on the water balance and storage capacity of agricultural reservoirs in small ungauged watersheds

Study regionAgricultural reservoirs are the principal source of irrigation supplies for sustaining rice production and play a critical role in the water resource management of South Korea. Study focusWe comprehensively evaluated the performance of 400 major agricultural reservoirs, spread throughout the country, as a function of climate change during 1973–2017 considering the constituents of reservoir water balance, such as watershed runoff, irrigation water demand, and evaporation loss. New hydrological insightsBased on the trend analysis during the 45-year study period, the reservoir inflows remained higher than the irrigation water demands because of the gradually increasing annual rainfall. Most reservoirs had enough storage capacities; their resilience gradually declined particularly during the last 15 years. The northern and northeastern region reservoirs had acceptable resilience whereas the reservoirs in the western and southwestern inland and coastal regions showed insufficient resilience. Reservoirs received sufficient inflows but the residence time of the excess inflow has to be increased to suffice the peak irrigation demands in the coming cropping season. Study outcomes could help prioritize the reservoirs requiring immediate rehabilitation located in the central and southwest regions with high failure risks. Results suggested that a reservoir with a watershed-irrigated area ratio > 5 would have a better chance of coping with future climate change threats.

Assessment of diminishing discharge of springs in Central Himalayan region, India

AbstractUttarakhand is witnessing a drastic decrease in spring discharge which leads to scarcity of drinking water for Himalayan inhabitants. This study uses a novel approach to assess the surface‐subsurface interaction of water, pivotal for estimating the status of spring discharge. Hydrological modelling has been used to quantify the diminishing discharge of spring and their associated factors, including changes in land practises and precipitation patterns. The coupling of the soil water assessment tool (SWAT) and MIKE hydro NAM model was proposed in the study to assess the decreasing discharge for an ungauged watershed. The model performance evaluated between the SWAT and the MIKE hydro NAM model indicated a very good index of agreement (d = 0.81). Field‐based spring discharge investigations were conducted in almost all watersheds in the study area. The MIKE hydro NAM model showed a good index of agreement (d = 0.79) with the observed spring discharge. Further, the geographically weighted regression (GWR) technique was applied over 17 driving factors based on hydrological parameters including topography, structural geology, land use/land cover and future annual precipitation (till 2030) under the RCP 4.5 scenario to understand the status of diminishing discharge of springs. The accuracy between the observed spring discharge and the GWR indicated a good index of agreement (d = 0.86). The results indicate that annual spring discharges will be significantly reduced up to 50% from 1975 to near future (2030), posing a threat to the drinking water supply. The proposed approach is replicable and scalable for ungauged river systems of the Himalayas. The study demonstrates the methods such as trend analysis of precipitation behaviour, land use change dynamics using remote sensing techniques, structural geological investigation through field measurements and hydrodynamic modelling are important for understanding surface‐subsurface interaction. The findings of the study help to identify the critical recharge zones for implementing the spring revival and rejuvenation programmes.

WEPPcloud: An online watershed-scale hydrologic modeling tool. Part II. Model performance assessment and applications to forest management and wildfires

Suspended sediment and nutrients following forest management activities or wildfires are transported to streams and lakes via surface runoff and are a major threat to water quality. Land and water managers resort to hydrologic models to test hypotheses that can help them make informed decisions to minimize disturbances and protect water resources. We present applications of an online interface, WEPPcloud, for the Water Erosion Prediction Project (WEPP) model as a pre- and post-disturbance management tool to model various gauged and ungauged forested watersheds throughout the western U.S. We compare simulated streamflow, sediment, and phosphorus to observations at USGS gauging stations and assess the accuracy of the online interface with minimal or no calibration. Specifically, we present modeling results from 28 relatively undisturbed forested watersheds in the states of California, Nevada, Oregon, Washington, and Idaho. Across all watersheds, the NSEs based on the daily streamflow values, were in the range of 0.43 to 0.64 indicating satisfactory agreement between modeled and observed values. Similarly, annual average NSE for sediment yield was 0.61, while for phosphorus it was 0.75, 0.71, and 0.66, for total, particulate, and soluble reactive phosphorus, respectively. Additionally, we demonstrate the utility of the WEPPcloud interface as a tool to compare model results for ungauged watersheds from various disturbed conditions including prescribed fire, thinning, and wildfire to undisturbed model results to better understand the effects of forest management and wildfires on water quality and quantity.

Estimation of Threshold Rainfall in Ungauged Areas Using Machine Learning

In recent years, Korea has seen abnormal changes in precipitation and temperature driven by climate change. These changes highlight the increased risks of climate disasters and rainfall damage. Even with weather forecasts providing quantitative rainfall estimates, it is still difficult to estimate the damage caused by rainfall. Damaged by rainfalls differently for inch watershed, but there is a limit to the analysis coherent to the characteristic factors of the inch watershed. It is time-consuming to analyze rainfall and runoff using hydrological models every time it rains. Therefore, in fact, many analyses rely on simple rainfall data, and in coastal basins, hydrological analysis and physical model analysis are often difficult. To address the issue in this study, watershed characteristic factors such as drainage area (A), mean drainage elevation (H), mean drainage slope (S), drainage density (D), runoff curve number (CN), watershed parameter (Lp), and form factor (Rs) etc. and hydrologic factors were collected and calculated as independent variables, and the threshold rainfall calculated by the Ministry of Land, Infrastructure and Transport (MOLIT) was calculated as a dependent variable and used in the machine learning technique. As for machine learning techniques, this study uses the support vector machine method (SVM), the random forest method, and eXtreme Gradient Boosting (XGBoost). As a result, XGBoost showed good results in performance evaluation with RMSE 20, MAE 14, and RMSLE 0.28, and the threshold rainfall of the ungauged watersheds was calculated using the XGBoost technique and verified through past rainfall events and damage cases. As a result of the verification, it was confirmed that there were cases of damage in the basin where the threshold rainfall was low. If the application results of this study are used, it is judged that it is possible to accurately predict flooding-induced rainfall by calculating the threshold rainfall in the ungauged watersheds where rainfall-outflow analysis is difficult, and through this result, it is possible to prepare for areas vulnerable to flooding.

Incorporating GIS Technique and SCS-CN approach for runoff estimation in the ungauged watershed: A case study west desert of Iraq

Estimation of runoff in an ungauged watershed is a significant part in the process of the water resources management. In the Iraqi western desert, the accessibility reliable surface runoff knowledge is scarce, that affects a critical difficulty for the hydrologic engineers. Estimation of surface runoff quantity in valleys of interrupted flow is significant to mobilize the deficiency water resources and manage valleys flow accurately. The incorporation of the Soil Conservation Service Curve Number (SCS-CN) approach with the geographic information system (GIS) was applied for estimating runoff volume of Wadi Hijlan, Fahamy, and Zgadan. The amount of runoff of the maximum storm were 7388700 m3, 12750000 m3 and 9851590 m3 for Hijlan, Fahamy and Zgadan respectively. In addition, the results showed acquired via the SCS-CN technique, revealed that the runoff depth fluctuated from 12.5 mm to 20.3 mm for (48mm) the maximum storm of rainfall through 2018-2019. The present strategy can be used for planning and development other valleys in the western desert of Iraq.

Sensitivity analysis of the flash flood hazard by coupling variable fuzzy method and mutual entropy

AbstractWatershed characteristics are one of the crucial factors affecting watershed hydrologic processes. Watershed hydrologic process regulation has been shown to be an effective tool for flash flood management. This study aims to propose a coupling method to evaluate the flash flood hazard degree of small hilly ungauged watersheds based on their watershed characteristic factors and identify sensitive factors influencing flash flood hazard. In this regard, nine watershed characteristic factors were selected for flash flood hazard sensitivity analysis. The variable fuzzy method (VFM) was proposed to calculate the eigenvalue of flash flood hazard degree for each small hilly watershed, and mutual entropy was proposed to identify sensitive factors influencing hazard degree. As an example, the sensitive factors in Licheng County with 119 small hilly watersheds in China are identified and further verified by stepwise regression analysis, which are mean watershed slope, soil moisture, average width of the mainstream, and watershed shape factor. This study shows that VFM and mutual entropy can be used jointly to identify the sensitive factors associated with the flash flood hazard in small hilly ungauged watersheds. The results can be used for engineering measure management in flash flood hazard control.

Evaluation of lag time and time of concentration estimation methods in small tropical watersheds in Ethiopia

Study regionSix small agricultural watersheds in three tropical climatic regions of Ethiopia. Three regions of contrasting climate: highland, midland, and lowland regions with respectively high, middle, and low elevations and rainfalls. Study focusLag time (TL) and time of concentration (TC) are two measures of how quickly a stream responds to runoff-producing rainfall. These parameters are the main inputs used to estimate peak flow under flood conditions in ungauged watersheds. Many empirical methods have been proposed to estimate TL and TC, but the validity of none of them has been tested. This study compared 10 commonly used methods by using measured TL and TC. New hydrological insightsMeasured median values of TL and TC for 176 rainfall–runoff events were used to evaluate the performance of empirical methods. For individual watersheds, the estimates of TL and TC differed by up to 2.6 h and 4.4 h, respectively. Most of the empirical methods tended to substantially underestimate TL and TC, which would lead to overestimation of runoff volume. TL and TC computed by two methods that consider both overland and channel flow were closest to the measured values of TL and TC, because such mixed flow is typical of tropical climate regions. Our results show the need for caution when empirical methods developed in regions with a particular climatic and geomorphological conditions are applied elsewhere.

Inference of Parameters for a Global Hydrological Model: Identifiability and Predictive Uncertainties of Climate‐Based Parameters

AbstractCalibration of global hydrological models (GHMs) has been attempted for over two decades; however, an effective and generic calibration method has not been explored. We present a novel framework for calibrating GHMs assuming that parameters can be regionalized by climate similarities. We calibrated four sensitive parameters of the H08 global hydrological model by aggregating the results of 5,000 simulations with randomly generated parameters into 11 Köppen climate classes and using an objective function Nash–Sutcliffe Efficiency (NSE) with random sampling from the proposed parameter distribution. From a 100‐fold split‐sampling test, we found that both the representativeness and robustness of the transferred parameter sets were guaranteed when the upper 5% of the samples were accepted and assign the median of each accepted parameter distribution for the climate class. The simulation with the climate‐based parameters yielded satisfactory (NSE > 0.0) and good (NSE > 0.5) performances at 480 and 234 stations (61.7% and 30.1% of 777 stations), respectively. The storage capacity (SD) and the conductive coefficient (CD) were sensitive to the climate classes and exhibited well‐constrained distributions of the accepted samples, whereas the recession parameters for the subsurface storage (γ and τ) showed little or no explanatory power to climate. The identified parameters for climate classes exhibited consistency with the physical interpretation of soil formation and efficiencies in vapor transfer. The consistency of the identified parameter values with physical underpinnings indicates that the appropriate parameters were determined, which ensured the robustness of parameters, especially when they are transferred to ungauged watersheds.

Application of Soil Conservation Service Curve Number model for deriving rainfall runoff relationship in an ungauged watershed of mountain region

Rainfall and runoff relationship is one of the key inputs required for assessing available water for planning watershed interventions. It is tedious to establish the rainfall and runoff relationship for an ungauged watershed, particularly in the hilly terrains. However, Soil Conservation Service (SCS) Curve Number method is well established equation and applied globally for estimating runoff. Hence, an attempt is made to derive the rainfall and runoff relationship using the SCS Curve Number coupled with the Google Earth Engine (GEE) in Nanjannad hill watershed of the Nilgiris district of Tamil Nadu. The monthly and annual rainfall and runoff analysis for the period of 14 years (2008–2021) showed that the October and November had the highest rainfall and runoff. The rainfall is varied between 880 mm and 1860 mm and the runoff ranged from 148 mm to 345 mm. The average annual runoff of 14 years period was observed as 249 mm (17.1%) and the maximum percentage of rainfall to runoff was estimated as 22.66% in the year 2009, the relationship between rainfall and runoff exhibited the positive correlation (R2 = 0.79). Accordingly, the total water available for harvesting annually stood at 1.08 Mcm in Nanjannad watershed. Hence, it is suggested that, watershed interventions can be planned to manage the 17 to 22% annual rainfall and rainfall and runoff relationship for watershed planning can be considered as 0.17 to 0.22.

Sediment-yield prediction using map correlation method

Sediment yield is of utmost importance in estimating soil erosion, reservoir sedimentation, aquatic life protection, change in river cross-section, and many more. Daily sediment time series can be readily obtained from sediment gauging stations; the problem lies at ungauged sites where information on daily sediment data is unavailable. At ungauged catchments, the most common methods to estimate sediment time series involve the use of a reference sediment gage, which shifts the properties of the sediment flow time series at a reference station to the ungauged catchment. The appropriate selection of reference stations is challenging in the estimation of daily sediment flows at ungauged watersheds. At most times in conventional methods, the parameters at the ungauged sites are determined by assuming the nearest gage station as a reference station. In the present paper, it is established that the near neighbour catchment may not always be the most correlated one. One of the techniques termed as map correlation method is undertaken to identify the most correlated sediment gage station as a reference to an ungauged station. To assess the efficacy of the present method daily sediment data for two years at forty sediment gauging stations of the USA watershed are utilized. Sample and model variograms were used to model the spatial structure of Pearson's r correlation coefficient. Observed and estimated correlations resulting from the cross-validation of the variograms are in line with each other for most of the stations. This method selects the most related reference station for an ungauged sediment site.

Extreme Rainfall-Runo Events Modeling Using HEC-HMS Model for Oued El Hachem Watershed, Northern Algeria

Abstract Flood forecasting has become necessary for dam management during extreme hydrological events. The lack of streamflow data in ungauged watersheds of arid and semi-arid regions makes the assessment of water resources di cult. In this paper, the Hydrologic Modeling System developed by the Hydrologic Engineering Center (HEC-HMS) was applied to the Oued El Hachem watershed. Calibration and validation of the model have been performed, taking into account the lag time and the curve number CN that is expressed as a function of soil group, land use and antecedent runo condition. The model was evaluated on the basis of the coe cient of determination, the Nash Sutcli e E ciency (NSE), and the percentage di erences between peak and volume. Performance indices of calibration showed a good agreements between observed and computed flows. The validation of the model has given satisfactory results. The calibrated model can be used to manage the dam of Boukerdane during extreme rainfall events by forecasting the induced hydrographs from which adequate procedures will be operated in order to ensure the safety of the dam against possible overtopping.

The Influence of Two Different Time of Concentration Equations on the GIUH-based Flood Hydrograph Estimates of Keduang and Temon Sub-Watersheds, Indonesia

Understanding the rainfall-runoff response on a watershed requires the study of both the observed and synthetic unit hydrographs. The application of a synthetic unit hydrograph for an ungauged watershed is the prospective alternative. Model development to obtain the unit hydrograph without rainfall and runoff data requires information on the physical characteristics of the watershed and its geomorphological parameters. This paper presents the results of a study on the application of the Geomorphological Instantaneous Unit Hydrograph (GIUH) model in which the time of concentration equations of Kirpich and Ventura were used as a comparison. The study objects are the Keduang and Temon Sub-watersheds of the Upper Bengawan Solo River, Indonesia. The study utilized the sub-watershed characteristic data obtained from the satellite data of ASTER Global DEM V3, which was then processed with the Watershed Modeling System and ArcMap. The results show that in the Keduang Sub-watershed, the Ventura equation gave results closer to the observed unit hydrograph than the Kirpich equation. In the Keduang Sub-watershed, the differences between the two times of concentration on the triangular parameters were significant, i.e. 77.13%, 337.57%, and 338.20% for peak discharge, peak time, and base time, respectively. On the other hand, in Temon Sub-watershed, the differences were 33.88%, 28.63%, and 25.09% for peak discharge, peak time, and base time, respectively. In terms of the GIUH-based flood hydrograph estimation, the results show that the utilization of the Ventura equation generally led to better estimates (closer to the observed flood hydrograph) than that of Kirpich equation.

Exploring the Application of Flood Scaling Property in Hydrological Model Calibration

AbstractModel calibration has always been one major challenge in the hydrological community. Flood scaling properties (FS) are often used to estimate the flood quantiles for data-scarce catchments based on the statistical relationship between flood peak and contributing areas. This paper investigates the potential of applying FS and multivariate flood scaling properties [multiple linear regression (MLR)] as constraints in model calibration. Based on the assumption that the scaling property of flood exists in four study catchments in northern China, eight calibration scenarios are designed with adopting different combinations of traditional indicators and FS or MLR as objective functions. The performance of the proposed method is verified by employing a distributed hydrological model, namely, the Soil and Water Assessment Tool (SWAT) model. The results indicate that reasonable performance could be obtained in FS with fewer requirements of observed streamflow data, exhibiting better simulation of flood peaks than the Nash–Sutcliffe efficiency coefficient calibration scenario. The observed streamflow data or regional flood information are required in the MLR calibration scenario to identify the dominant catchment descriptors, and MLR achieves better performance on catchment interior points, especially for the events with uneven distribution of rainfall. On account of the improved performance on hydrographs and flood frequency curve at the watershed outlet, adopting the statistical indicators and flood scaling property simultaneously as model constraints is suggested. The proposed methodology enhances the physical connection of flood peak among subbasins and considers watershed actual conditions and climatic characteristics for each flood event, facilitating a new calibration approach for both gauged catchments and data-scarce catchments.Significance StatementThis paper proposes a new hydrological model calibration strategy that explores the potential of applying flood scaling properties as constraints. The proposed method effectively captures flood peaks with fewer requirements of observed streamflow time series data, providing a new alternative method in hydrological model calibration for ungauged watersheds. For gauged watersheds, adopting flood scaling properties as model constraints could make the hydrological model calibration more physically based and improve the performance at catchment interior points. We encourage this novel method to be adopted in model calibration for both gauged and data-scarce watersheds.

Evolution of historical sediment yield using check-dam systems as carriers: A case study in a restored agricultural catchment on the Loess Plateau, China

It is critical to determine soil erosion rates and sediment yields under ecological restoration conditions on the Loess Plateau. Sediments retained in check-dams document the soil erosion history in a small watershed. We selected the check-dam systems to reconstruct the long-term sediment yield in a restored agricultural catchment on the Loess Plateau. The storage capacity curve method was applied to estimate the total sediment deposition of check-dam systems, and 725.82 thousand tons of sediment was trapped in the check-dams systems from 1975 to 2013. Two abrupt change points were detected in 1987 and 2003 using the double mass curve method; the average catchment sediment yields were 5,273.63 t/(km2·a) in Stage-1 (1975–1987), 1,734.09 t/(km2·a) in Stage-2 (1988–2003), and 2,653.10 t/(km2·a) in Stage-3 (2004–2013), represented intense erosion, mild erosion, and moderate erosion, respectively. In Stage-2, the sediment yield variation in the Fangta catchment was mainly affected by human activities, and its contribution reached 97%. According to the hierarchical cluster analysis, the soil erosion intensity changed gradually from mild to moderate and then to intense when the average annual erosive rainfall reached approximately 300, 350, and 400 mm in the Fangta catchment of the loess hilly-gully region. Approximately 6.1–42.6 % of the sediment was deposited along the way since 2003 when the sediments generated by erosion in small watersheds were gradually transported to the downstream river network. Therefore, check-dam systems are important carriers for reconstructing the historical sediment yield in small and ungauged watersheds. The results of this study not only serve as a basis for obtaining reliable information on erosion and sediment yield in small dam-controlled catchments, but also provide a useful reference for implementing sustainable ecological restoration in the future.

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.