Direct Runoff Hydrograph Research Articles

Investigating Parameters of Geomorphic Direct Runoff Hydrograph Models

Formulation of rainfall runoff models and identification of their parameters is difficult step especially for catchments having scanty or no data. Parameters of geomorphic instantaneous unit hydrograph (GIUH) models have been investigated in this research. Recorded data of Shahpur Dam Catchment, Pakistan was used for developing direct runoff hydrograph model. Satellite imageries of the catchment were processed using ArcGIS 10.1 to estimate geomorphologic parameters. The rainfall and runoff data for 10 events was collected from Meteorological Department Lahore and from Small Dam Organization, Rawalpindi, Pakistan. Rainfall data was analyzed and excess rainfall was estimated using Percent Runoff Method. Using estimated geomorphologic parameters the ordinates of GIUH- Nash Model were obtained by standard equations of Nash instantaneous unit hydrograph (IUH). These ordinates of Nash-IUH, were converted into the ordinates of direct runoff hydrographs through their convolution with the excess rainfall. The results of model were evaluated on the basis of their deviation from the observed runoff data. Statistical parameter Nash-Sutcliffe Coefficient and percent error between observed and simulated direct runoff were used for this purpose. The impact of using digital elevation models (DEM) of two different resolutions; 30 and 90 m, was then investigated. It is observed that the geomorphic parameters are affected due to DEM’s resolution. Hence the resolution of DEM impacts the direct runoff as well.

Spatial discretization influence on flood modeling using unit hydrograph theory

ABSTRACT The Unit Hydrograph (UH) is the most popular method for flood related applications. There are several conceptual and geomorphological models based on UH and coupled with different spatial discretizations. However, there are few studies concerning the evaluation of UH models according to semi-distributed approaches. This study aimed to assess the influence of lumped and semi-distributed modeling on the applicability of Soil Conservation Service UH (SCS UH) and Clark’s Instantaneous UH (Clark’s IUH) for estimation of flood hydrographs. The methodological procedures were conducted in the Hydrological Modeling System (HEC-HMS) using rainfall-runoff events of a gauged watershed in Southern Brazil. The main conclusions were: a) CIUH under the semi-distributed approach provided slightly superior performance; b) CIUH was able to effectively estimate the direct surface runoff hydrographs even for long duration rainfall events; c) SCS UH presented more accurate hydrographs for lumped modeling; d) the Nelder and Mead algorithm may have limited application.

A spatially distributed Clark’s unit hydrograph based hybrid hydrologic model (Distributed-Clark)

ABSTRACTA hybrid hydrologic model (Distributed-Clark), which is a lumped conceptual and distributed feature model, was developed based on the combined concept of Clark’s unit hydrograph and its spatial decomposition methods, incorporating refined spatially variable flow dynamics to implement hydrological simulation for spatially distributed rainfall–runoff flow. In Distributed-Clark, the Soil Conservation Service (SCS) curve number method is utilized to estimate spatially distributed runoff depth and a set of separated unit hydrographs is used for runoff routing to obtain a direct runoff flow hydrograph. Case studies (four watersheds in the central part of the USA) using spatially distributed (Thiessen polygon-based) rainfall data of storm events were used to evaluate the model performance. Results demonstrate relatively good fit to observed streamflow, with a Nash-Sutcliffe efficiency (ENS) of 0.84 and coefficient of determination (R2) of 0.86, as well as a better fit in comparison with outputs of spatially averaged rainfall data simulations for two models including HEC-HMS.

KESESUAIAN HIDROGRAF SATUAN SINTETIK TERHADAP HIDROGRAF SATUAN TERUKUR(STUDI KASUS SUB SUB DAERAH ALIRAN SUNGAI PEDINDANG BAGIAN TENGAH)

A widely used method for analyzing river flow for flood forecasts is hydrograph unit. The hydrograph unit is a direct runoff hydrograph that can be created when there are AWLR record data, debit measurements and rainfall data. Synthetic Unit Hydrograph (SUH) is a unit hydrograph derived based on river data in the same watershed or nearby watershed but has the same characteristics, ie HSS Gama I, HSS Nakayasu, Limasan HSS, HSS Snyder and HSS SCS. Of the two hydrographs, there will be suitability of the hydrograph form that is going to be made. Sub territory of Pedindang River Basin has four flood incidents, namely, date 23-24 February 2016; March 2-3, 2016; March 3-4, 2016; and date 5-6 March 2016. In the analysis of each flood event, the peak discharge of synthetic unit hydrograph is very different from the peak discharge of the measured unit hydrograph. The average peak discharge of synthetic unit hydrograph occurs in the range of 2 or 3 hours, while the measured unit hydrograph of Pedindang River occurs in the range of 7 or 8 hours. In four flood events it is stated that, HSS Gama I approaches RMSE value (validation <10%) to HST form of Pedindang River with value: RMSE incidence I (23,601%); RMSE incidence II (16.315%); RMSE incidence III (50,400%); RMSE incidence IV (22.322%). With this result, it is stated that there is no synthetic unit hydrograph model that has compatibility with the measured unit hydrograph of Pedindang River.

Effect of using multiple stream gauging stations on calibration of hydrologic parameters and estimation of hydrograph of ungauged neighboring basin

In this study, Hydrologic Engineering Center-Hydrologic Modeling System is used to simulate hydrologic processes in a watershed in Western Black Sea Region that frequently experiences flooding. The region is mountainous with steep hill slopes and receives high precipitation throughout the year. There are three stream gauging stations in the basin whose data are available for calibration and validation of hydrologic parameters. Simulations are performed for different scenarios to investigate the effect of using multiple stream gauging stations’ data on catchment wide calibration and validation of hydrologic parameters. Furthermore, performance of using calibrated internal stream gauging stations’ flow data in the estimation of hydrologic parameters in an assumed neighboring ungauged basin was assessed. It is found that using data of multiple stream gauging stations for calibration and validation gives satisfactory results for direct runoff hydrograph but the peak discharge predictions are not improved. The study results suggest that using data of internal stream gauging stations enables improved understanding of internal dynamics and transport in the basin and better predicted direct runoff hydrograph for the assumed neighboring ungauged basin.

Implication of Directly Connected Impervious Areas to the Mitigation of Peak Flows in Urban Catchments

The existence of impervious areas is one of the most distinguishing characteristics of urban catchments. They decrease infiltration and increase direct runoff in urban catchments. The recent introduction of green infrastructure in urban catchments for the purpose of sustainable development has contributed to the decrease in directly connected impervious areas (DCIA) by isolating existing impervious areas, and consequently, has also contributed to flood risk mitigation. This study coupled the width function-based instantaneous hydrograph (WFIUH), which is able to handle the spatial distribution of the impervious areas, with the concept of the DCIA to assess the impact of decreasing DCIA on the shape of direct runoff hydrographs. Using several scenarios for typical green infrastructure and the corresponding changes of DCIA for a test catchment in Seoul, South Korea, this study evaluated the effect of green infrastructure on the shape of the resulting direct runoff hydrographs and reducing peak flows. The results showed that the changes in the DCIA immediately affect the shape of the direct runoff hydrograph, and decrease peak flows by up to 12% depending on spatial implementation scenarios in the test catchment. This study demonstrates the importance of the DCIA concept for the evaluation of green infrastructures in urban catchments, enabling quantitative assessment of the spatial distribution of impervious areas, and also changes to the DCIA by various types of green infrastructure. The results of this study also suggest that more effective and well-planned green infrastructures could be introduced in urban environments for the purpose of flood risk management.

Effect of watershed partitioning on hydrologic parameters and estimation of hydrograph of an ungauged basin: a case study in Gokirmak and Kocanaz, Turkey

The main goal of this study is to investigate the effect of the size of the subbasins of a watershed on the hydrologic parameters and their spatial variability in an estimation of the hydrologic parameters and hydrograph of a neighbouring ungauged basin. In this paper, Hydrologic Engineering Center-Hydrologic Modelling System (HEC-HMS), a semi-distributed hydrologic model, is used to calibrate and cross-validate two flood events occurred in 1998 and then validate four other flood events occurred in 1991, 1994, 2002, and 2009 in Gokirmak Basin in Western Black Sea Region, Turkey. The basin is divided into seven different subbasins to investigate the effect of watershed partitioning on calibrated hydrologic parameters of each subbasin using the peak-weighted root mean square error method as an objective function and the hydrograph at the outlet of the whole basin. It is found out that as the geometric magnitudes of the subbasins changed, the calibrated values of the hydrologic parameters of those subbasins changed as well. Then, a neighbouring basin, Kocanaz, is considered as an assumed neighbouring ungauged basin to investigate the effect of watershed partitioning of a gauged basin on the estimation of hydrograph of a neighbouring ungauged basin. Hydrologic parameters and direct runoff hydrograph of assumed ungauged neighbouring basin are estimated from the hydrologic parameters of the HEC-HMS calibration results of Gokirmak. Statistical indicators of the simulation results for each basin partitioning were graded with respect to the boundary values of the simulation outputs to find the best alternative. The grading results show that the simulation results with a single basin gave better representation among all other partitioning except two flood events.

ESTIMATING STORM DIRECT RUNOFF USING SYNTHETIC UNIT HYDROGRAPH METHODS BY (WMS) MODEL (SOLAG BASIN AS A CASE STUDY)

The Storm Runoff Volume and its Peak are important for designing and operating dams and different hydraulic structures. Watershed Modeling System (WMS) version (8.1) and The HEC-1 v (4.1) hydrological models were used in this study to estimate the Synthetic runoff hydrograph resulting from two storm events over Solag Basin which is located in Sinjar District north of Iraq, making use of available recorded storms rainfall and Synthetic unit hydrograph technique. Direct runoff hydrographs are determined using Natural Resources Conservation Service (NRCS), Snyder Synthetic Unit Hydrograph and Clark methods.Nash model was used to compare the efficiency of the Synthetic runoff Hydrograph using both Synthetic and recorded discharge data for Solag basin which were available for two storms within water years 1991-1992. Synthetic unit hydrographs are important in flood studies and water resources management.

Assessment of the Flow at the Chatara on Koshi River Basin using Semi Distributed Model

Hydrological modeling is a physical phenomenon to incorporate rainfall-runoff process in watershed area at particular point and analysis the behaviors of the flow through outlet. There are various types of models such as black box, conceptual and physical based model depends upon the availability of required data, parameters and description of the physical process. Semi distributed model is a conceptual and physical based model which has been developed to bridge the gap between the lumped and distributed model. It have more advantages than other models with respect to time of calculation, less number of parameters, comparatively low calibration needs and have high efficiency of model. The model performance may be achieved upon the qualitative and quantitative available rainfall data, land use data and size of the watershed. In this study, Koshi river basin was taken for the study area. Kinematic wave method was used for overland routing and Muskingum conge method was applied for channel routing to describe the discharge on Saptakoshi river and peak flow attenuation and dispersion observed in the direct runoff hydrograph. From this study result, Annual runoff, Peak flow and time of peak at the outlet are similar to the observed flow in calibration and verification period using trapezoidal channel. The nash efficiency obtained from the semi distributed model with kinematic wave method is more than 90%in verification periods and nearly 90% in calibration periods Hence Hydrological modeling is a powerful technique in the planning and development of integrated approach for management of water resources.Journal of the Institute of Engineering, 2016, 12(1): 184-198

Runoff prediction in ungauged catchments using the gamma dimensionless time-area method

Time-area (TA), which constitutes the basis for rainfall-runoff transformation in the Clark model, is conventionally derived from the tedious procedure of delineating isochrones. In the present study, by combining the Nash instantaneous unit hydrograph (IUH), in terms of the gamma function, and the Clark model, a new TA relationship (TAR) is introduced. This equation involves the Nash models’ parameters (i.e., the number of reservoirs, n, and the storage coefficient, k). Considering that n = 5 for ungauged catchments, the following equation for estimating k was obtained: k = t c /4.24, with t c being the time of concentration of the catchment. Finally, a gamma time-area (GTA) function was derived for estimating the time-area diagram (TAD) of catchments. The TADs derived from the GTA function were compared to the GIS-based TADs and those derived from the US Army Corps of Engineers (USACE) method and the kinematic wave (KW) model in four catchments, namely, Kasilian, Ajay, Jafarabad, and Shourandika. In the Kasilian catchment, the direct runoff hydrograph (DRH) was simulated using the Clark model based on the GTA and USACE methods and compared with the observed hydrographs. Results indicate that the coefficient of efficiency (CE) in the Kasilian catchment for the two methods is approximately 0.8, while the errors in the peak discharge prediction are 9 and 11.2% in the GTA and USACE methods, respectively.

Direct estimation of catchment response time parameters in medium to large catchments using observed streamflow data

AbstractIn single‐event deterministic design flood estimation methods, estimates of the peak discharge are based on a single and representative catchment response time parameter. In small catchments, a simplified convolution process between a single‐observed hyetograph and hydrograph is generally used to estimate time parameters such as the time to peak (TP), time of concentration (TC), and lag time (TL) to reflect the “observed” catchment response time. However, such simplification is neither practical nor applicable in medium to large heterogeneous catchments, where antecedent moisture from previous rainfall events and spatially non‐uniform rainfall hyetographs can result in multi‐peaked hydrographs. In addition, the paucity of rainfall data at sub‐daily timescales further limits the reliable estimation of catchment responses using observed hyetographs and hydrographs at these catchment scales. This paper presents the development of a new and consistent approach to estimate catchment response times, expressed as the time to peak (TPx) obtained directly from observed streamflow data. The relationships between catchment response time parameters and conceptualised triangular‐shaped hydrograph approximations and linear catchment response functions are investigated in four climatologically regions of South Africa. Flood event characteristics using primary streamflow data from 74 flow‐gauging stations were extracted and analysed to derive unique relationships between peak discharge, baseflow, direct runoff, and catchment response time in terms of TPx. The TPx parameters are estimated from observed streamflow data using three different methods: (a) duration of total net rise of a multipeaked hydrograph, (b) triangular‐shaped direct runoff hydrograph approximations, and (c) linear catchment response functions. The results show that for design hydrology and for the derivation of empirical equations to estimate catchment response times in ungauged catchments, the catchment TPx should be estimated from both the use of an average catchment TPx value computed using either Methods (a) or (b) and a linear catchment response function as used in Method (c). The use of the different methods in combination is not only practical but is also objective and has consistent results.

Evaluating the impact of lower resolutions of digital elevation model on rainfall-runoff modeling for ungauged catchments.

Geomorphological instantaneous unit hydrograph (GIUH) usually uses geomorphologic parameters of catchment estimated from digital elevation model (DEM) for rainfall-runoff modeling of ungauged watersheds with limited data. Higher resolutions (e.g., 5 or 10m) of DEM play an important role in the accuracy of rainfall-runoff models; however, such resolutions are expansive to obtain and require much greater efforts and time for preparation of inputs. In this research, a modeling framework is developed to evaluate the impact of lower resolutions (i.e., 30 and 90m) of DEM on the accuracy of Clark GIUH model. Observed rainfall-runoff data of a 202-km2 catchment in a semiarid region was used to develop direct runoff hydrographs for nine rainfall events. Geographical information system was used to process both the DEMs. Model accuracy and errors were estimated by comparing the model results with the observed data. The study found (i) high model efficiencies greater than 90% for both the resolutions, and (ii) that the efficiency of Clark GIUH model does not significantly increase by enhancing the resolution of the DEM from 90 to 30m. Thus, it is feasible to use lower resolutions (i.e., 90m) of DEM in the estimation of peak runoff in ungauged catchments with relatively less efforts. Through sensitivity analysis (Monte Carlo simulations), the kinematic wave parameter and stream length ratio are found to be the most significant parameters in velocity and peak flow estimations, respectively; thus, they need to be carefully estimated for calculation of direct runoff in ungauged watersheds using Clark GIUH model.

NEXRAD Quantitative Precipitation Estimations for Hydrologic Simulation Using a Hybrid Hydrologic Model

AbstractA hybrid hydrologic model (lumped conceptual and distributed feature model), Distributed-Clark, is introduced to perform hydrologic simulations using spatially distributed NEXRAD quantitative precipitation estimations (QPEs). In Distributed-Clark, spatially distributed excess rainfall estimated with the Soil Conservation Service (SCS) curve number method and a GIS-based set of separated unit hydrographs are utilized to calculate a direct runoff flow hydrograph. This simple approach using few modeling parameters reduces calibration complexity relative to physically based distributed (PBD) models by only focusing on integrated flow estimation at watershed outlets. Case studies assessed the quality of NEXRAD stage IV QPEs for hydrologic simulation compared to gauge-only analyses. NEXRAD data validation against rain gauge observations and performance evaluation with model simulation result comparisons for inputs of spatially distributed stage IV and spatially averaged gauged data for four study watersheds were conducted. Results show significant differences in NEXRAD QPEs and gauged rainfall amounts, with NEXRAD data overestimated by 7.5% and 9.1% and underestimated by 15.0% and 11.4% accompanied by spatial variability. These differences affect model performance in hydrologic applications. Rainfall–runoff flow simulations using spatially distributed NEXRAD stage IV QPEs demonstrate relatively good fit [direct runoff: Nash–Sutcliffe efficiency ENS = 0.85, coefficient of determination R2 = 0.89, and percent bias (PBIAS) = 3.92%; streamflow: ENS = 0.91, R2 = 0.93, and PBIAS = 1.87%] against observed flow as well as better fit (ENS of 3.7% and R2 of 6.0% increase in direct runoff) than spatially averaged gauged rainfall for the same model calibration approach, enabling improved estimates of flow volumes and peak rates that can be underestimated in hydrologic simulations for spatially averaged rainfall.

Maximum Design Flow Estimates for Large Basins Using the Local Frequency Analysis (LFA) and the Most Probable Maximum Hydrograph (MPMH) Methods – a Critical Analysis

Statistical and deterministic methods have been severely criticized regarding their ability to calculate peak flows. The recently proposed method denominated as Most Probable Maximum Hydrograph (MPMH), applicable to large hydrographic basins, promises to overcome the difficulties faced in the most traditional methods (statistical and deterministic). The method not only uses the peak flow as in the statistical methods, and not requires detail information about the physical and meteorological characteristics of the hydrographic basin as in the deterministic method. The MPMH method takes information from observed hydrographs as base time, maximum flow and its respective volume. Using simple linear regression equations or simple exponential distribution adjusted to mean hydrograph volumes, the MPMH method is used to calculate design hydrograph, based on observed hydrographs volumes and the respective return periods. This paper assesses the applicability of the MPMH method by evaluating the influence of data characteristics as size of data series, removal of outliers and lowest values of the flow series on the peak flow estimates. In this paper it is shown that the MPMH method has great potential for estimation of design flows when compared to traditional methods. Besides that, it is shown that the difficulty in the use the MPMH method lies in deciding the base time (duration of the direct runoff hydrograph) of the standard hydrograph for the basin, which proved to be a decisive factor to estimate the peak flow and hydrograph volumes.

HYDROLOGICAL MODELLING OF UNGAUGED ARID VOLCANIC ENVIRONMENTS AT UPPER BATHAN CATCHMENT, MADINAH, SAUDI ARABIA

Hydrological modelling of ungauged catchments is still a challenging task especially in arid regions with a unique land cover features such as highly fracture volcanic basalt rocks. In this study, upper Bathan catchment (103 km2) in Madinah, western of Saudi Arabia is selected. The aim of this paper is to simulate the hydrological responses of volcanic catchment to daily design storm events. The weighted areal average of two daily design rainfall depth scenarios are computed, which are 50 years and 100 years return period and correspondent predicted rainfall are 80.6 mm and 94.1 mm, respectively. SCS Type II temporal synthetic distribution of daily rainfall is selected to disaggregate the daily rainfall into smaller time interval. Excess rainfall is computed using Soil Conservation Services Curve Number (SCS-CN) method based on Land Cover and Land Use (LCLU) and hydrological soil groups (HSG) maps, while direct runoff hydrograph is developed using Soil Conservation Services dimensionless unit hydrograph (SCS-UH) method using lag time equation. HEC-HMS software is used, and it showed that the runoff volumes of the two rainfall scenarios are 50% and 54% of the total rainfall depth, and the peak discharges are 123 m3/sec and 158 m3/sec. This study provided an indication of the hydrograph characteristics of basaltic catchments and the result of this paper can be used for further flood studies in arid ungauged volcanic catchments.

STUDI TENTANG MODEL HIDROGRAF SATUAN SINTETIK PADA SUB DAS BAYUR SAMARINDA, KALIMANTAN TIMUR

The present research entitled Study of Synthetic Unit Hydrograph Model of Bayur River Basin at Samarinda, East Kalimantan was aimed to analyze and identify the best-suited synthetic unit hydrograph to apply in Bayur river basin. The research was carried out by developing models, and then those models were compared to main unit hydrograph based on 2004 field measurement. Comparative analysis was performed on two stages. The first was to calculate and compare three-unit hydrograph parameters (Tp, Qp and Tb) of two synthetic unit hydrograph to main unit hydrograph. Second stage was to develop direct runoff hydrograph of two synthetic unit hydrograph models and of the main unit hydrograph using rainfall data of April 9, 2004. Then, deviation percentage averages of the two synthetic direct runoff hydrograph were calculated. Through the two analysis stages, the best-suited synthetic unit hydrograph model to apply in Bayur river basin presumably was obtained by identifying smallest deviation percentage average on main unit hydrograph. By calculating deviation averages of direct runoff hydrograph in the two models of synthetic unit hydrograph, it was found that GAMA I model provided smallest deviation average compared Snyder model. Based on the two analysis, it was showed that GAMA I was the closest model to main unit hydrograph compared to Snyder model. Key word: synthetic unit hydrograph, river basin, direct runoff, Snyder model, GAMA I model.

Comparison of methods for estimation of flood hydrograph characteristics

This research has been carried out for investigation and comparison of the accuracy and reliability of different methods of unit hydrograph estimation, including geomorphologic (GIUH) and geomorphoclimatic (GCIUH) methods as well as methods by Nash (Nash-IUH), Rosso (Rosso-IUH) and the Soil Conservation Service (SCS); the methods simulated the rainfall-runoff process over the Manshad River basin located in central Iran. The first six equivalent rainfall-runoff events were selected, and a hydrograph of outlet runoff was calculated for each event. Compared were peak time, peak discharge, base time, W50 and W75 parameters (hydrograph widths at 50% and 75% of peak discharge) and the volume of outlet runoff simulated by the models; then determined was the model that most efficiently estimated the hydrograph of outlet flow. The comparison of calculated and observed hydrographs showed that the Nash model was more efficient in estimating peak discharge, peak time, outlet runoff volume and the shape of direct surface runoff (DSRO) hydrographs, though it could not precisely simulate base time and W50 and W75 parameters. The other methods were more accurate in simulating outlet runoff volume of the hydrographs. The Rosso-IUH and SCS models could estimate the base time parameter better than the others. GIUH performance was comparable to the Nash method and was relatively suitable. In spite of these results, the GIUH, GCIUH, Rosso-IUH and SCS models had weak performance for estimating other characteristics of outlet DSRO hydrographs.

Two-dimensional continuous simulation of spatiotemporally varied hydrological processes using the time-area method

Distributed, continuous hydrologic models promote better understanding of hydrology and enable integrated hydrologic analyses by providing a more detailed picture of water transport processes across the varying landscape. However, such models are not widely used in routine modelling practices, due in part to the extensive data input requirements, computational demands, and complexity of routing algorithms. We developed a two-dimensional continuous hydrologic model, HYSTAR, using a time-area method within a grid-based spatial data model with the goal of providing an alternative way to simulate spatiotemporally varied watershed-scale hydrologic processes. The model calculates the direct runoff hydrograph by coupling a time-area routing scheme with a dynamic rainfall excess sub-model implemented here using a modified curve number method with an hourly time step, explicitly considering downstream ‘reinfiltration’ of routed surface runoff. Soil moisture content is determined at each time interval based on a water balance equation, and overland and channel runoff is routed on time-area maps, representing spatial variation in hydraulic characteristics for each time interval in a storm event. Simulating runoff hydrographs does not depend on unit hydrograph theory or on solution of the Saint Venant equation, yet retains the simplicity of a unit hydrograph approach and the capability of explicitly simulating two-dimensional flow routing. The model provided acceptable performance in predicting daily and monthly runoff for a 6-year period for a watershed in Virginia (USA) using readily available geographic information about the watershed landscape. Spatial and temporal variability in simulated effective runoff depth and time area maps dynamically show the areas of the watershed contributing to the direct runoff hydrograph at the outlet over time, consistent with the variable source area overland flow generation mechanism. The model offers a way to simulate watershed processes and runoff hydrographs using the time-area method, providing a simple, efficient, and sound framework that explicitly represents mechanisms of spatially and temporally varied hydrologic processes. Copyright © 2015 John Wiley & Sons, Ltd.

Changeability of simulated hydrograph from a steep watershed resulted from applying Clark’s IUH and different time–area histograms

Reflecting the shape and drainage characteristics of the watershed, time–area histogram (TAH) seems to be the most important parameter for derivation of the transformation hydrograph. In this study, a semi-distributed instantaneous unit hydrograph was established and applied to the steep 103 km2-Galazchai watershed in Iran to improve the results of the rainfall–runoff modelling. Towards this attempt, twenty-three runoff producing events with wide different characteristics were used for the analysis. The direct surface runoff hydrographs (DSRHs) were obtained and consequently compared for the study area using Clark’s instantaneous unit hydrograph (IUH) and through applying different TAHs calculated based on channel profile, dimensionless TAH, average velocity and spatially distributed travel time methods. A weight grid developed from kinematic wave travel time equation for overland flow was prepared and used as input to derive the spatial TAH of the watershed. The results indicated that the different TAHs had noticeable impacts on the estimated hydrographs. The results also proved that the spatial TAH method performed well with efficiency criteria of 0.75 and 0.69 in calibration and validation steps. The implemented method also offered the advantages of flexibility, efficiency and physically powerful links to the spatial data set and GIS software.

Comparison between two different conceptual mathematical models in prediction of direct runoff hydrographs from a small watershed

In the present study, two mathematical models namely (i) Lag and route model and (ii) Muskingum model which are based on unit-step and transfer functions approach were developed for runoff prediction from Shenda park watershed treating the watershed as lumped, linear and time-invariant system. The hydrological data of the study watershed were collected from zonal station of NationalAgricultural Research Project, Shenda Park, Kolhapur (M.S.) for the years 2000 to 2008. Out of twelve single storm events, nine storm events were included in the analysis for parameters estimation and remaining three storm events were considered for prediction purposes. The model parameters, viz. , lag time and (,) and storage co-efficient (K) of Lag and route model were estimated by the methods of cumulants (Singh, 1988) and moments (Nash, 1957) whereas the model parameter storage constant (K) for Muskingum model was estimated by using method suggested by Jawed (1973). Performance evaluation of these two developed model in determining direct runoff hydrograph ordinates were evaluated using various statistical indices such as correlation co-efficient (R), special correlation co-efficient (Rs.), co-efficient of efficiency (CE) and root mean square error (RMSE). The results showed that both the developed model can be used for prediction of the direct run off hydrograph from the study watershed, however, direct runoff hydrographs obtained through Muskingun models are much closer to actual observed direct runoff hydrograph than that of Lag and route model.