مجله مهندسی و مدیریت آبخیز
سال هفدهم شماره 1 (بهار 1404)

Estimation of erosion and sedimentation using radionuclides is a one of the methods used to provide erosion/sedimentation maps and calculate sediment budget components at time scales ranging from storm to annual and even long-term annual averages. Comparing the results of long-term average annual erosion/sedimentation with observed sediment values over recent decades can be used to investigate trends in erosion and sediment yield. This study compares the average intensity of soil erosion over two periods: 100–120 years and the last 60 years, using the isotopes and , respectively. By analyzing changes in erosion and sediment production trends over the past century, this research was conducted in the reference sub-watershed of the Khamsan representative-paired watershed.

Soil sampling was performed at 57 systematically random points across various land uses within the reference sub-watershed. Erosion and deposition intensities were estimated using the diffusion and transfer model for non-agricultural lands and Mass Balance Model II for agricultural lands. These calculations were based on the soil inventories of and (Bq m⁻²) along with other relevant parameters. Erosion, stable, and deposition area maps were generated by comparing isotope inventories with reference areas. The spatial distribution of erosion and deposition intensities was then mapped using the working unit approach, and sediment budget components were quantified for the 100–120-year and 60-year periods.

The sediment budget analysis indicated that total soil erosion increased from 292.87 to 526.87 tons year⁻¹, total deposition rose from 7.07 to 20.26 tons year⁻¹, and net soil erosion (sediment yield) grew from 285.79 to 506.61 tons year⁻¹ over the two periods. The estimated average erosion intensity increased from 2.92 to 5.25 tons ha⁻¹ year⁻¹, while sediment production rose from 2.85 to 5.05 tons ha⁻¹ year⁻¹. Additionally, the sediment delivery ratio (SDR) was 0.98 for the 100–120-year period and 0.96 for the last 60 years, indicating that the proportion of eroded soil retained within the watershed increased from 2% to 4% in recent decades.

The findings demonstrate a significant increase in soil erosion intensity over the past 60 years compared to the 100–120-year period. The average total erosion, total deposition, and net erosion have approximately doubled, tripled, and doubled, respectively. Field observations suggest that this rise in erosion and sediment production is primarily due to intensified tillage operations in rain-fed agricultural lands, particularly on steep slopes, and the prevalence of contour-plowing practices. In addition to understanding the trend of erosion and sediment yield changes in the last century, the results of this study can also be used to predict future soil erosion condition in the watersheds.

In recent years and decades, due to minor changes and challenges in sufficient and high-quality water resources, sustainable water resources have been the subject of various studies and research. The lack of safe and high-quality water resources is a major obstacle to sustainable development. For this reason, understanding the processes of the water cycle is very important and requires accurate information about hydrological phenomena. Runoff from water transfer is one of the main sources meeting various water demands, including agriculture, industry, and domestic use. The allocation of water resources to these sectors is planned based on runoff data at different times. A significant portion of precipitation in the hydrologic cycle is converted into runoff due to watershed characteristics. Considering the issue that the Lake Urmia Basin is shrinking, identifying the water resources of this basin and its sub-basins is crucial.

The Ajichai Basin is one of the sub-basins of Lake Urmia. In this study, rainfall data from the Tabriz synoptic station and runoff data from the Nahand hydrometric station were used. The aim of this research is to model the daily rainfall-runoff of the Ajichai Basin using intelligent machine learning models, including Artificial Neural Network (ANN), Support Vector Machine (SVM), Gene Expression Programming (GEP), and Random Forest (RF). Seventy percent of the data was used for training, and 30% was used for testing the models. Statistical measures such as the Coefficient of Determination (R²), Root Mean Square Error (RMSE), Nash-Sutcliffe Efficiency (NSE), and Willmott Index (WI) were used to evaluate the performance of the models.

The results of this research showed that all models performed very well in simulating rainfall-runoff in the Ajichai Basin. According to the obtained results, the GEP model, with R² equal 0.84, RMSE equal 0.024 m³/s, NSE equal 0.864, and WI equal 0.968, was the most accurate in modeling rainfall-runoff in the Ajichai Basin. Based on scatter plots and time series analysis, the GEP model demonstrated higher accuracy than other models in predicting rainfall-runoff values with a high correlation.

According to the results, all the investigated models showed good capability in modeling daily rainfall-runoff in the Ajichai Basin. The findings of this research highlight the strong performance of machine learning models in rainfall-runoff modeling. In general, due to the high accuracy of intelligent models, particularly the GEP model, in predicting daily rainfall-runoff, it is recommended to apply these methods to hydrological problems. Additionally, for future research, it is suggested that intelligent methods and data mining techniques be used to model the precipitation-runoff process in different basins separately for drought-affected and wet years.

Today, the natural resources of the country face serious problems and threats such as water scarcity, drought, climate change, water pollution, soil and air pollution, desertification, soil erosion, and issues arising from unsustainable and inappropriate land use practices such as deforestation, destruction of rangelands, overgrazing, and unsustainable agriculture. This underscores the urgent need for a more serious focus on the issue of research and sustainable development in the management of the country's natural resources and environment to provide solutions to problems and improve the health and sustainability of the country's ecosystems. Despite the abundance of research articles in the field of watershed management with an emphasis on its technical and managerial aspects published in domestic and international journals, the categorization and thematic analysis of scientific research articles published in domestic journals have received less attention. In this regard, this study aims to investigate the thematic analysis and classification of various types of articles published in the Watershed Engineering and Management Journal, as well as to examine the thematic trends of each category over three five-year periods.

This study was conducted with the aim of examining and evaluating the articles published in the "Watershed Engineering and Management" journal during the period from 1388 to the end of 1402 (Volume 15, Issue 4). In this regard, the first step was to classify the types of published articles thematically. Additionally, to examine the frequency of various types of articles published in the relevant specialized categories, as well as the frequency of keywords presented in these articles, three time periods were considered, including the periods from 2009 to 2013 (Volumes 1 to 5), 2014 to 2018 (Volumes 6 to 10), and 2019 to 2023 (Volumes 11 to 15). Furthermore, in this study, an investigation was conducted into the perspectives of faculty members and research experts of the five research groups of the Soil and Watershed Conservation Research Institute regarding the submission of articles to the "Watershed Engineering and Management" journal, as well as the quality, characteristics, challenges, and functions of articles published in this journal during the period from 2009 to the end of 2023 (Volume 15, Issue 4). In this regard, after categorizing the types of published articles thematically by the five research groups and designing relevant questionnaires, a survey was conducted regarding the role of selected factors in submitting articles, as well as the types of published articles categorized by each group, based on a questionnaire consisting of 11 qualitative ordinal variables and scoring according to the Likert pentagon spectrum. After examining the validity and reliability of the measurement tool and determining the sample size using the Cochran formula, descriptive statistical analysis was employed in this study to analyze the results.

In this study, articles were categorized into 12 thematic classes based on their titles. The results indicate that the thematic categories of "Hydrology" and "Horticulture" have the maximum and minimum frequencies, respectively. The findings reveal that the number of articles presented in the categories of "Climate", "Geomorphology", "Management", "Hydrology", and "Hydrogeology" shows an ascending trend over the triennial periods of 2009 to 2013 (Volumes 1 to 5), 2014 to 2018 (Volumes 6 to 10), and 2019 to 2023 (Volumes 11 to 15). The results also show that the five keywords "Geographic Information System", "Runoff", "Soil Erosion", "Sediment", and "Remote Sensing" have the highest frequency in the articles published during the period from 2009 to the end of 2023 (Volume 15, Issue 4). Furthermore, the results indicate that all research groups have given higher than average scores for selecting this journal as the first choice for publishing their articles. Additionally, factors such as "Review and Publication Time", "Review and Publication Costs", and "Ease of Working with the Journal System (Sina Web)" play an important role in encouraging researchers to submit articles to this journal, with average scores above average and more than average. This is while the roles of editorial board members, chief editors, editors-in-chief, internal, and executive managers in submitting articles to this journal have varying degrees of importance, ranging from low to less than high. Moreover, the survey results show that the five research groups have given scores higher than average to less than very high to the alignment of article topics with the journal's focuses.. In this regard, articles related to the expertise of the Watershed Management research group have the highest alignment (from high to very high) with the journal's focuses. This is while the innovation of articles is mostly average, which is evaluated as less than average to average for articles related to the expertise of the Drought and Climate Change research group. Furthermore, the results indicate that the level of application of innovative research tools and methods in the articles of this journal is mostly average to less than high. Additionally, the subject matter of the articles published in this journal is mostly scored higher than less than average to less than high for addressing the country's needs and challenges. This is less than average for articles related to the River Engineering and Coastal Protection group and between average to less than high for other groups. Except for the Drought and Climate Change group, the application of the results and achievements of these articles in the description of detailed-executive watershed studies is less than average. This highlights the urgent need for a review of the description of detailed-executive watershed studies by the Natural Resources and Watershed Management Organization.

The results indicate that the thematic categories of "Hydrology" and "Horticulture" have the maximum and minimum frequencies, respectively. Furthermore, the number of articles presented in the categories of "Climate", "Geomorphology", "Management", "Hydrogeology", and "Hydrology" shows an ascending trend over the three five-year periods. Additionally, the keywords "Geographic Information System", "Runoff", "Soil Erosion", "Sediment", and "Remote Sensing" have the highest frequency in the articles published in this journal during the period from 2009 to 2023. Regarding the inclination and influential factors in submitting articles by the faculty members of the Soil and Watershed Conservation Research Institute to this journal, the results indicate that all research groups have given scores higher than average for selecting this journal as their first choice for publishing their articles. Additionally, factors such as "Review and Publication Time", "Review and Publication Costs", and "Ease of Working with the Journal System (Sina Web)" play an important role in encouraging researchers to submit articles to this journal, with average scores above average and more than average. The results indicate that the five research groups have given scores ranging from above average to less than very high for the alignment of article topics with the journal's focuses. This is while the innovation of articles and the level of application of innovative research tools and methods in the articles of this journal are respectively at an average and less than high level. Additionally, the results of evaluating the quality of article publication in this journal, in terms of aspects such as editing, formatting, and illustration clarity, suggest a relatively good status (above average to less than very high) for the journal. Furthermore, the subject matter of the articles published in this journal is mostly scored higher than less  to less than high for addressing the country's needs and challenges. Also, except for the Drought and Climate Change group, the application of the results and achievements of these articles in the description of detailed-executive watershed studies is less than average. The current evaluation can serve as a model for examining and analyzing other journals and improving the quality of various scientific content published in them.

Human life is constantly affected by various natural hazards, some of which result from climate events and global warming. This issue is so significant that the Intergovernmental Panel on Climate Change (IPCC) was established to study climate change on a global scale and assess the role of human activity in this matter. One of the consequences of climate change is the occurrence of extreme events such as drought. In fact, with climate change, the intensity and frequency of droughts are becoming more complex. Given the vital role of water in human life, agricultural production, and the environment, it is crucial to assess its adverse effects on the occurrence and severity of droughts. The primary cause of droughts is fluctuations in climatic components, such as decreased precipitation and increased temperature.

This study aimed to predict drought conditions in Ardabil, Ahar, Parsabad, Jolfa, Khoi, and Maku stations, located within the Aras River watershed. To achieve this, the accuracy of the General Circulation Models (GCMs) from the IPCC’s CMIP6 dataset was evaluated. The climate models used for forecasting precipitation in the future period (2024–2043) include CanESM5, NorESM2-MM, and MPI-ESM1-2-HR under both optimistic (SSP1-2.6) and pessimistic (SSP5-8.5) emission scenarios. Then, using the Standardized Precipitation Index (SPI), drought characteristics on an annual scale during the historical period (1985–2014) were compared with those in the future period. Model validation was performed using R², RMSE, and NSE parameters. Finally, the SPI drought index was analyzed in terms of intensity and frequency based on the generated precipitation data.

Model evaluation using different error metrics indicated that the CanESM5 model outperformed the others in simulating precipitation for the studied stations. A comparison of the models and scenarios revealed that, according to the MPI-ESM1-2-HR model, drought severity is projected to increase, while the CanESM5 model suggests a rise in drought frequency. The results also indicate that under the most pessimistic scenario, both the number and intensity of dry periods will increase compared to the optimistic scenario. Furthermore, normal conditions are expected to decrease, while the number and frequency of dry periods will rise due to the consequences of climate change in the study area. Additionally, the frequency and intensity of wet years have shown a slight increase compared to the historical period. Overall, climate change is expected to have a significant impact on the future drought conditions in the Aras River watershed.

In summary, understanding changes in precipitation and water resources is crucial due to their various applications in agriculture, drinking water supply, and industry. The accuracy of future climate models varies depending on the climate of each region, the month, and the predicted climate variable. The Aras River Basin is facing a decline in water resources due to agricultural activities exceeding capacity and high water consumption in different agricultural sectors. This study aimed to assess the impact of climate change on drought severity and frequency in the Aras River Basin, providing insights for developing intelligent policies for sustainable water and land resource management. The findings confirm the occurrence of climate change and, in particular, the recurrence of droughts in the studied stations. Using multiple climate models, as the most reliable tool for generating climate scenarios, not only allows for more accurate forecasts but also helps evaluate uncertainties in future climate conditions. These insights assist planners and decision-makers in better monitoring the adverse effects of drought on water resources and agriculture, enabling proactive planning to mitigate regional losses before a crisis occurs.

Watershed management measures are implemented to maintain or restore the ecological functions of watersheds by reducing the impacts of events such as floods, landslides, and erosion in downstream areas, while also enhancing resource productivity and improving local livelihoods. These measures become effective when they identify, restore, or reinforce areas of high ecological value that must be protected from degradation or conversion to other uses. Therefore, it is essential to assess the effectiveness of watershed management measures (both in research and implementation phases) to develop a model of their positive or negative impacts, guiding decision-makers in planning and executing successful watershed management projects. Project reports often emphasize the strengths and positive outcomes of these measures, while their inefficiencies are less frequently highlighted. This study reviews and evaluates recent watershed management projects to examine both their strengths and weaknesses.

In this study, the effectiveness of several recent watershed management projects implemented in different watersheds of Iran was assessed based on the latest available documents. The evaluated watersheds include Behshahr-Galoogah, Maragheh (Markazi Province), Khaveh-Delijan, Asiaborud (Chalus City), and Faryab-Golashgerd (Kerman Province). The state of water resources in these areas was also analyzed.

The findings indicate both positive and negative outcomes for each project. In the Behshahr-Galoogah watershed, sub-basins where a combination of different structures was implemented along the stream network experienced reduced peak discharge and increased concentration time. However, in sub-basins with few check dams relative to their area and stream length, the measures did not significantly alter stream length, concentration time, or peak discharge, but they did help stabilize the stream's longitudinal profile. In the Maragheh watershed (Markazi Province), specific erosion and sediment rates before and after the implementation of watershed measures were recorded as 10.78 and 3.14 (Ton/km²), and 3.1 and 66.9 (Ton/km²), respectively. The effectiveness of watershed measures in the Khaveh-Delijan watershed was also evident. The construction of check dams led to a reduction in peak discharge for different return periods (5, 10, 20, 50, and 100 years), with values decreasing from 3.9 to 0.6, 4.9 to 2.7, 9.4 to 5.1, 14.6 to 8.0, and 22.4 to 12.4 m³/s, respectively. Additionally, base time and lag time of the hydrograph increased by approximately 3.5 hours. The Asiaborud watershed project demonstrated that, following the implementation of watershed measures, specific erosion was recorded at 3.89 Ton/ha/year, and specific sedimentation was estimated at 1.87 Ton/ha/year. Hydrological studies from 2000 to 2019 indicated a decrease in curve number (CN), an increase in concentration time, and reductions in both peak discharge and flood volume (by 1078.49 mm³). Consequently, financial losses caused by floods in this watershed decreased due to the reduction in flood volume. Similarly, findings from the Faryab-Golashgerd watershed project indicated a significant increase in concentration time and a reduction in peak discharge across all studied return periods.
 

Overall, the results show that most of the implemented watershed management projects had significant positive effects, including reduced peak flood/runoff discharge, decreased erosion and sedimentation, and increased concentration time. However, maintaining the long-term effectiveness of these measures depends on proper care and maintenance. Additionally, integrating suitable biological measures alongside mechanical interventions in watersheds plays a crucial role in sustaining the effectiveness of these measures.

In the transition from an introverted traditional society to an extroverted modern one, many places cannot withstand the tensions of modernity and the conflicts of dominant culture. The effort to create reliable, safe, economical, and resilient infrastructure against climatic hazards is an undeniable principle in the present age. Ancient hydraulic structures, as a water heritage, contain successful structural, functional, and managerial solutions for sustainability and resilience against harsh ecological conditions. Therefore, deriving ancient perspectives regarding the human-nature relationship and nature-based solutions, to develop practical patterns for addressing natural and human crises, as well as applying them in the planning processes of urban landscapes and watersheds, can enhance the resilience of contemporary cities. Considering that the ancestors of each region developed management strategies in response to social, ecological challenges, and human needs, taking into account cultural, economic, and geographical characteristics, and that these strategies, along with their functions, experiences, values, and beliefs, have been tested over thousands of years, they have laid the foundations for the formation of ecological wisdom. In this study, the main research question is posed as follows: How will the development of principles and rules governing the structure and function of water heritage affect the realization of resilient cities against flooding? Therefore, by selecting and examining a sample of ancient water structures in ancient China, the ancient thoughts regarding the relationship between humans and nature were inferred, and a practical pattern for achieving resilient cities against flooding was outlined.

This study is a descriptive-analytical investigation aimed at analyzing the drainage system pattern of Tuancheng in ancient China to achieve resilience against urban flooding. Using an inductive-comparative method, the structural-functional principles and rules governing the Tuancheng drainage system, as an ancient urban flood management experience, were compared with modern experiences at multiple scales. Based on this comparison, a practical pattern for achieving resilience against flooding and water resource management was developed. The steps of the study can be described as follows:Examining the drainage system of ancient China, a symbol of ecological wisdom in ancient heritage, Clarifying resilience criteria in water structures and drainage, Inferring the principles and rules governing the drainage system of ancient China, Developing a practical pattern for resilience against floods and runoff in contemporary cities.

Ancient hydraulic structures engineered by past cultures are based on the realization of fundamental hydrological functions and provide valuable insights into preserving water resources, managing runoff, maintaining ecological integrity, managing watersheds, protecting soil and biodiversity, etc. Despite the vast differences in complexity, technological development, and resource demands between ancient indigenous communities and the modern world, these insights can inspire contemporary societies in land management. The design principles governing these ancient structures effectively preserve water and natural systems, enhance ecological processes and functions, and increase ecosystem services. Notably, they offer efficient utilization of rainwater resources to mitigate the challenge of water scarcity in the present age. Furthermore, by reducing the occurrence of natural disasters, flooding, and economic losses, these structures offer a reliable method for managing water resources and saving financial costs for local communities. They also create investment opportunities in upgrading infrastructure, engineering products, and new technologies. The combination of green and gray infrastructure, clustered design, and multifunctional performance significantly improves efficiency, performance, stress resistance, and reduces damages and costs. Common technologies in the formation of sponge cities include the use of green roofs, open and green spaces, green parking lots, artificial wetlands for collecting rainwater and runoff, permeable ponds, facilities to maintain biological processes, permeable pavements, reducing rigid and impermeable surfaces, integrated ecological management, protecting aquatic ecosystems, and utilizing rainwater resources. These approaches facilitate the harmonious development of urban and natural environments, the development of green-blue infrastructure, and the enhancement of resilience against water crises.

The results of the study indicate that the manifestation of the principles and rules of ecological wisdom through various policies and strategies, such as conservation plans, infiltration practices, runoff storage, runoff transfer, runoff filtration, and landscaping at multiple structural-functional scales, can provide a fundamental pattern for achieving sponge cities that are resilient to flooding. This concept reflects an environmental civilization capable of reducing the impacts of urban development on natural ecosystems, integrating green-blue and gray infrastructures, managing aquifers, and addressing various social, economic, and ecological dimensions. It aims to manage water resources and resolve the water crisis in many countries, including Iran.

Water collection and storage with minimal losses are among the most effective measures for water resource development in dry and semi-arid regions. Underground dam technology is a suitable solution for managing water resources in such areas. These subterranean structures influence hydraulic flow in porous environments and are constructed to block, divert, or reduce groundwater movement. Underground dams serve various purposes, including water resource control, management, protection, and development. The success and sustainability of these structures largely depend on selecting the most appropriate location, considering project goals, operational efficiency, and intended use. One of the primary reasons for failure in dam projects is the lack of comprehensive studies on site selection.

The study area is located in the western sub-basins of Semnan Province, geographically positioned between 53°15' to 54°20' East longitude and 35°20' to 36° North latitude. Geologically, it lies within the Alborz and Central Iran zones. The basin spans 104.91 square kilometers, with elevations ranging from 1,643 meters to 1,413 meters. The region has a semi-arid Mediterranean climate, receiving approximately 100 millimeters of annual rainfall. The current geomorphology and topography of the basin are shaped by geological structures, rock types, and erosion susceptibility. Due to the mountainous terrain, the area lacks significant alluvial groundwater aquifers, with water resources being limited to subsurface flow in alluvial deposits along streambeds or within hard rock formations. Consequently, underground dams present a viable solution for water supply in such conditions. This study assesses the feasibility of constructing underground dams in the western sub-basins of Semnan Province using a hierarchical analysis approach within a Geographic Information System (GIS). Relevant resources were reviewed to determine the essential criteria and conditions for selecting suitable locations for underground dam construction. Informational layers were generated and integrated in a GIS environment, employing a hierarchical approach to facilitate decision-making.

Controlling and blocking subsurface flows in alluvial deposits of streambeds through underground dams can create reservoirs within the porous environment, enhancing water resource development. This strategy is particularly relevant for dry and semi-arid regions, including the northwestern and western sub-basins of Semnan Province. To identify suitable locations for these structures, key factors such as geology, hydrology, water resources, topography, and stream geometry were analyzed. Areas that failed to meet the required standards were deemed unsuitable and excluded from consideration. Most of the necessary criteria were derived from satellite imagery, topographic and geological maps, and expert knowledge. The site selection criteria were classified into two groups: absolute criteria and decision-making criteria. Absolute criteria provided a binary assessment (favorable/unfavorable), whereas decision-making criteria were prioritized within a suitability range, from highly suitable to relatively suitable. These factors were incorporated into the site selection process using a hierarchical methodology in GIS. The application of GIS tools for overlay analysis, function execution, and weighted scoring significantly improved the speed, accuracy, and efficiency of the process.

The findings indicate that using a step-by-step hierarchical methodology within modern techniques, such as GIS, enhances the accuracy and efficiency of regional-scale site selection studies. This approach improves reliability by refining criteria determination, weighting, and scoring. The hierarchical methodology systematically eliminates unsuitable areas, progressively narrowing the study region to the most viable locations. The results showed that approximately 20% of the streambeds in the western sub-basins of Semnan Province have the potential for underground dam construction, providing effective water storage and management solutions. The most suitable locations are found in streambeds of categories 4 and 5, particularly on sandstone, limestone, and marl formations.

Flood estimation in ungauged watersheds is crucial for designing hydraulic structures. The total flood flow in watersheds consists of surface and subsurface flows. In highly permeable watersheds, subsurface flow significantly contributes to total runoff, yet limited studies have addressed this aspect. Runoff mechanisms in watersheds generally follow two models: the Hortonian mechanism, where the soil saturates from the top, and the Dunne mechanism, where saturation occurs from below. This study adopts the Dunne mechanism for runoff generation.

One method for predicting surface and subsurface hydrographs in ungauged watersheds is the Geomorphologic Instantaneous Unit Hydrograph (GIUH), which utilizes geomorphological data. These data were derived using ArcGIS and hydrological extensions. The GIUH model can separate surface and subsurface flow components. While previous studies have primarily used GIUH for surface flow estimation, the model equations were expanded in this study to estimate the total watershed subsurface hydrograph. The GIUH model was applied to estimate surface and subsurface runoff in two watersheds: Kasillian in Iran and Gagas in India.

The proposed GIUH model was used to analyze surface and subsurface flows in the Kasillian (Iran) and Gagas (India) watersheds. Simulation results for four rainfall-runoff events in each watershed demonstrated that the model effectively estimated total hydrographs and their components. A comparison of estimated and observed peak discharges in the Kasillian watershed showed that the simulated peak discharge on May 10, 1992, was 10.1 m³/s, whereas the observed value was 11.8 m³/s. The error margin across different events ranged from 3% to 16%, indicating an acceptable model accuracy in runoff estimation. In the Gagas watershed, total peak discharge varied between 44 and 110 m³/s, with subsurface flow contributing approximately 5%–6% of the total flow. In the Kasillian watershed, total discharge ranged from 1.6 to 12 m³/s, while peak subsurface discharge was estimated between 35 and 60 L/s. The relationship between rainfall and subsurface peak discharge revealed that lower rainfall led to reduced subsurface peak discharge. Sensitivity analysis showed that hydraulic conductivity was one of the most influential parameters in subsurface flow simulation. In soils with high hydraulic conductivity, subsurface flow accounted for a larger portion of total flow, and the hydrograph lag time increased. For example, reducing the hydraulic conductivity in the Kasillian watershed from 0.0025 to 0.0009 m/s increased subsurface peak discharge from 0.35 to 1.3 m³/s. Additionally, reducing the Manning’s roughness coefficient from 0.2 to 0.4 resulted in a 31% decrease in flood peak discharge. These findings highlight the importance of hydrological and geomorphological characteristics in accurate runoff estimation and flood control structure design.

This study evaluated the GIUH model for estimating surface and subsurface runoff in the Kasillian (Iran) and Gagas (India) watersheds. Results showed that the model provided accurate peak discharge estimates, with estimation errors ranging from 3% to 16% in Kasillian and 1.6% to 12% in Gagas. The subsurface flow played a significant role in highly permeable watersheds, contributing 5%–6% of total runoff in the Gagas watershed. Sensitivity analysis revealed that increasing hydraulic conductivity led to higher subsurface peak discharge, whereas reducing the Manning’s coefficient increased flood peaks. These findings confirm the importance of geomorphological and hydrological characteristics in runoff modeling. Ultimately, the GIUH model can serve as a useful tool for flood management and watershed hydrological response assessment.