Excess Stormwater Quantification in Ungauged Watersheds Using an Event-Based Modified NRCS Model

Quantifying runoff from a storm event is a crucial part of rainfall-runoff model development. The objective of this study is to illustrate inconsistencies in the initial abstraction (I _a) and curve number (CN) in the Natural Resources Conservation Service (NRCS) model for ungauged steep slope watersheds. Five alternatives to the NRCS model were employed to estimate stormwater runoff in 39 forest-dominated mountainous watersheds. The change to the parameterization (slope-adjusted CN and I _a) leads to more efficient modified NRCS models. The model evaluations based on root mean square error (RMSE), Nash-Sutcliffe coefficient E, coefficient of determination (R ² ), and percent bias (PB) indicated that our proposed model with modified I _a, consistently performed better than the other four models and the original NRCS model, in reproducing the runoff. In addition to the quantitative statistical accuracy measures, the proposed I _a modification in the NRCS model showed very encouraging results in the scatter plots of the combined 1799 storm events, compared to other alternatives. This study’s findings support modifications to the CN and the I _a in the NRCS model for steep slope ungauged watersheds and suggest additional changes for more accurate runoff estimations.     

Regionalizing Flood Magnitudes using Landscape Structural Patterns of Catchments

Emerging as an important issue in the disciplines of landscape ecology and landscape hydrology which inspired it, defining the concept of landscape metrics in a hydrological context has become a challenge to both landscape planners and engineers. Accordingly, the present study addresses the relationships existing between flooding phenomena and landscape metrics (shape index, fractal dimension index, perimeter-area ratio, related circumscribing circle, and contiguity index) of land use/land cover, hydrological soil groups and geological permeability classes. A regionalization approach was adopted employing 39 select catchments (33—4800 km² in area, 0.47—21 m³ s^?1 in mean discharge), located within the southern basin of the Caspian Sea. These catchments were predominantly covered by forest (57.4%), while rangeland, farmland and urban areas accounted for 25.9%, 11.7%, and 1.6%, respectively. Class-level landscape structural metrics of land use/land cover, hydrological soil groups and geological permeability classes have then been served as inputs to stepwise multiple linear regression analysis in an attempt to explain the flood magnitudes. The regression models (0.69?≤?r² ≤?0.84) suggested that the catchments’ flood magnitude could explicitly be predicted using average measure of the shape and related circumscribing circle indices for the land use/land cover classes and those of hydrologic soil groups and geological permeability classes of the catchments. This indicated that regularity (vs. irregularity) of the landscape, pedoscape, and lithoscape, as represented by the shape index as well as the circumscribing circle index (for elongation and convolution), explained 69–84% of the variation in the flood magnitudes in the catchment.     

Experimental Verification of the Effect of Slope and Land Use on SCS Runoff Curve Number

The present study investigates experimentally the effect of watershed (i.e. field plot of 22 m?×?5 m) slope on rainfall-generated runoff and, in turn, the runoff curve numbers (CN) resulting from field plots (with land use of maize and sugarcane on the soil falling in Hydrologic Soil Group C) located in Roorkee, (India). The results indicated that the plot of 5 % slope yielded the largest runoff and, in turn, CN compared to those due to the plots of 3 and 1 % grades, for the same rainfall, soil, and land use. The CN derived from the present study were quite close to the NEH-4 CN-values, indicating satisfactory match between the two. The use of slope adjusted CN-values in the standard CN method significantly (at 5 % level) improved the otherwise underestimated large runoff events, i.e. model efficiency E improved from 0.47 to 0.78, and R² from 0.71 to 0.83.     

Application of GIS-based SCS-CN method in West Bank catchments,Palestine

Among the most basic challenges of hydrology are the prediction and quantification of catchment surface runoff.The runoff curve number(CN)is a key factor in determining runoff in the SCS(Soil Conservation Service)based hydrologic modeling method.The traditional SCS-CN method for calculating the composite curve number is very tedious and consumes a major portion of the hydrologic modeling time.Therefore,geographic information systems(GIS)are now being used in combination with the SCS-CN method.This paper assesses the modeling of flow in West Bank catchments using the GIS-based SCS-CN method.The West Bank,Palestine,is characterized as an arid to semi-arid region with annual rainfall depths ranging between 100 mm in the vicinity of the Jordan River to 700 mm in the mountains extending across the central parts of the region.The estimated composite curve number for the entire West Bank is about 50 assuming dry conditions.This paper clearly demonstrates that the integration of GIS with the SCS-CN method provides a powerful tool for estimating runoff volumes in West Bank catchments,representing arid to semi-arid catchments of Palestine.     

A new empirical model for stormwater TSS event mean concentrations (EMCs)

An empirical model for TSS event mean concentrations in storm weather discharges has been derived from the analysis of data sets collected in two experimental catchments (Chassieu, separate system and Ecully, combined system) in Lyon, France. Preliminary tests have shown that the values of TSS EMCs were linked to the variable X =TP ×ADWP (TP rainfall depth, ADWP antecedent dry weather period) with two distinct behaviours under and above a threshold value of X named λ: EMCs are increasing if X < λ and are decreasing if X > λ. An empirical equation is proposed for both behaviours. A specific calibration method is used to calibrate λ while the 4 other parameters of the model are calibrated by means of the Levenberg-Marquardt algorithm. The calibration results obtained with 8 events in both sites indicate that the model calibration is satisfactory: Nash Sutcliffe coefficients are all above 0.7. Monte Carlo simulations indicate a low variability of the model parameters for both sites. The model verification with 5 events in Chassieu shows maximum levels of uncertainty of approximately 20%, equivalent to levels of uncertainty observed in the calibration phase.     

潘家口水库流域土地利用变化的水文响应分析

以潘家口水库流域为研究区域,采用SWAT分布式水文模型定量评估土地利用变化对流域径流过程的影响。相对于1980年土地利用情景,2006年土地利用变化主要表现为耕地减少,林地和草地面积增大,而水域和城镇用地变化相对较小。利用3个水文站1986年1月至1999年12月的月径流实测数据进行模型的校准和验证。在不同土地利用情景下,对流域的天然径流过程进行模拟分析,模拟结果显示,与1980年土地利用情景相比,2006年土地利用情景下的多年平均径流减少11%;对于不同典型水文年份而言,土地利用变化对枯水年的影响最大,对丰水年和平水年的影响相当;土地利用变化对年径流的影响主要表现为汛期径流减少。潘家口水库流域水资源涵养能力有所增加。模拟研究土地利用变化对潘家口水库流域径流的影响对合理规划土地利用和流域水资源的综合管理具有现实意义。     

Long-Term Spatio-Temporal Variation in Runoff Curve Number under Consistent Cover Conditions: a Southeastern US Case Study

Runoff, erosion and pollutant transport are influenced by spatial and temporal variation in soil hydraulic properties. Hydrologic models contain parameters that are related to these properties. Model parameters can be anticipated to exhibit similar variation which, when characterized, can improve runoff estimates. This study used plot-scale data from a 0.07 ha grassed study area in the southeastern US to characterize spatio-temporal variation in the NRCS curve number (CN) parameter. Based on over 200 rainfall-runoff data pairs collected over a 19-year period, CN exhibited highly variable (coefficient of variation up to 0.54) spatial behavior. Within-plot variation also demonstrated discernible spatial structure, with high variation associated with low CN. While preserving some similarities, spatial structure of CN and connectedness of regions of similar CN varied with runoff event date. Temporal trends were detected, with some regions demonstrating increasing trends and others decreasing. Rainfall-runoff plot characteristics and experimental protocols were consistent during the period of data collection; based on spatial distribution of CN values and trends as well as observation, traffic patterns and mammalian bioturbation are hypothesized as major sources of observed spatio-temporal CN variation.     

Impacts of Land-use Changes on the Hydrology of the Grande River Basin Headwaters,Southeastern Brazil

Land-use changes affect soil water balance. The Upper Grande River Basin (UGRB) headwaters have undergone intense modifications in land use. This study was conducted to simulate, using the LASH model, the impacts on the hydrological regime in the UGRB with five land-use trends: S₁ and S₂ – reforestation with eucalyptus covering 20 % and 50 %, respectively, from the current grassland area; S₃ – reforestation with eucalyptus covering 100 % of the current grassland area only in the sub-basins where this trend is predominant; S₄ and S₅ deforestation of 30 % and 70 % of the forest remnants in the Mantiqueira Range region for the cultivation of grasslands, respectively. Results demonstrate that runoff would be reduced due to the land-use changes by 51.65 mm yr^?1, 110.29 mm yr^?1 and 59.48 mm yr^?1 for scenarios S₁, S₂ and S₃, respectively. However, scenarios S₄ and S₅ could increase streamflow by 57.63 mm year^?1 and 156.78 mm year^?1, respectively. This indicates that land-use changes might make the basin more prone to flooding and other hazards associated with increased runoff.     

北庙水源地土地利用变化对水文要素影响的研究

选用保山市日气象观测数据和北庙水源地水库坝上径流资料,结合北庙水源地内的土地利用数据、土壤数据和DEM数据,构建其SWAT分布式水文模型,研究水源地三期不同土地利用方式变化对水文要素的影响。结果表明:在模型率定期和验证期,R2数分别为0.70和0.72,模型效率系数均大于0.55,径流模拟值与实测值相对误差在15%以内,SWAT水文模型在北庙水源地有较好的适用性。1986-2009年,北庙水源地土地利用类型以林地、耕地、草地为主;草地面积减少,耕地、未利用土地和居民用地增加是这一时期土地利用方式变化的主要特征。水源地水文要素发生相应的变化主要表现为坡面径流深、径流深和土壤侵蚀模数增加,而实际蒸散减少。     

Estimation of the Runoff Curve Number via Direct Rainfall Simulator Measurements in the State of Iowa, USA

This study was the first to provide detailed methodological steps to estimate in-situ runoff curve number (CN) for selected agricultural fields in the State of Iowa via rainfall simulators. Representative fields in six counties were chosen to identify the effects of the following variables on runoff CN: rainfall intensity, soil type, soil moisture condition, tillage practice, and residue cover. The study also re-evaluated the range of the existing CN values for the different hydrologic soil groups in Iowa, and revised the equations describing the CN method to consider variables such as residue cover and soil moisture in a more detailed manner than the existing USDA method. The findings of this investigation showed that rainfall simulators are useful instruments for estimating in-situ runoff CN because rainfall intensity was adjustable during an experimental run. Further, the simulators eliminate the need of natural storm events. The range of the estimated CN values in summer agreed well (deviation less than 6%) with the reported CN values. However, the range of the estimated CN values in fall was generally less the reported CN values (deviation of about 40%) due to the high residue levels found in the fields after harvest. The effects of tillage practice and crop type were insignificant compared to residue cover and soil moisture. The study has also shown that the initial abstraction I _a is not linearly proportional to the potential maximum retention S, which agrees with the available literature.     

SCS-CN-based Continuous Simulation Model for Hydrologic Forecasting

A new lumped conceptual model based on the Soil Conservation Service Curve Number (SCS-CN) concept has been proposed in this paper for long-term hydrologic simulation and it has been tested using the data of five catchments from different climatic and geographic settings of India. When compared with the Mishra et al. (2005) model based on variable source area (VSA) concept, the proposed model performed better in all applications. Both the models however exhibited a better match between the simulated and observed runoff in high runoff producing watersheds than did in low runoff producing catchments. Using the results of the proposed model, dominant/dormant processes involved in watershed’s runoff generating mechanism have also been identified. The presented model is found useful in the continuous simulation of rainfall–runoff process in watersheds.     

Application of a Hydrologic Model Considering Rainwater Storage to Analyze Storm-induced Landslides in a Forest Catchment

In this study, a conceptual hydrologic model was applied for the analysis of storm-induced landslides, which were affected by typhoons in 2004, in the forest catchments of Ehime Prefecture, Japan. Through the model, hydrologic conditions such as rainfall intensity, rainwater storage, and soil moisture behavior in 2004 were obtained and compared with other 19 years (1985 ∼ 2003). Furthermore, the relationship between landslide occurrence and hydrologic phenomena in the catchments were assessed. Results of this study indicated that soil moisture, excess rainfall storage combined with rainfall intensity, and its duration in the forest were the key elements for the occurrence of landslides. The hydrologic model in this study was able to represent hydrologic phenomena reasonably and it can be used for the estimation of discharge, soil moisture content, and water storage in catchments. The concept of this model is available to be applied in other areas and can be expected to provide important information on soil moisture behavior for forecasting and preventing landslide disasters.     

Incorporating Influences of Shallow Groundwater Conditions in Curve Number-Based Runoff Estimation Methods

Runoff generation process in any watershed is mainly affected by precipitation, land use and land cover, existing soil moisture conditions and losses. Shallow groundwater table conditions that occur in many regions are known to affect the soil moisture retention capacity, infiltration and ultimately the runoff. A methodology that links soil moisture capacity to the shallow groundwater table or High-Water Table (HWT) using a nonlinear functional relationship within a curve number (CN)-based runoff estimation method, is proposed and investigated using single and continuous event simulation models in this study. The relationship is used to obtain an adjusted CN that incorporates the effect of change in soil moisture conditions due to HWT. The CN defined for average conditions is replaced by this adjusted CN and is used for runoff estimation. A single event model that uses Soil Conservation Service (SCS) CN approach is used for evaluation of variations in runoff depths and peak discharges based on different HWT conditions. A real-life case study from central Florida region in the USA was adopted for application and evaluation of the proposed methodology. Results from the case study application of the models indicate that HWT conditions significantly influence the magnitudes of peak discharge by as much as 43% and runoff depth by 48% as the water table height reaches the land surface. The magnitudes of increases in peak discharges are specific to case study region and are dependent on the functional form of the relationship linking HWT and soil storage capacity. Also, for specific values of HWT, an equivalency between HWT-based CN and wet antecedent moisture condition (AMC)-based CN can be established.     

TOPMODEL for Streamflow Simulation of a Tropical Catchment Using Different Resolutions of ASTER DEM: Optimization Through Response Surface Methodology

The unavailability of proper hydrological data quality combined with the complexity of most physical based hydrologic models limits research on rainfall-runoff relationships, particularly in the tropics. In this paper, an attempt has been made to use different resolutions of DEM generated from freely available 30 m-based ASTER imagery as primary input to the topographically-based hydrological (TOPMODEL) model to simulate the runoff of a medium catchment located in the tropics. Response surface methodology (RSM) was applied to optimize the most sensitive parameters for streamflow simulation. DEM resolutions from 30 to 300 m have been used to assess their effects on the topographic index distribution (TI) and TOPMODEL simulation. It is found that changing DEM resolutions reduces the TOPMODEL simulation performance as the resolutions are varied from 30 to 300 m. The study concluded that the ASTER 30 m DEM can be used for reasonable streamflow simulation of a data scarce tropical catchment compared with the resampled DEMs.     

Evidence of Land-use Controlled Water Storage Depletion in Hai River Basin,North China

Although critical for human sustenance, irrational land use could cause land resources depletion, environmental degradation, food insecurity and social instability. This study uses Landsat images (06/09/1979 and 12/08/2009) with historical precipitation data (1955–2012) to analyze land use change in relation to surface water storage change in Hai River Basin, North China. Based on analysis in ENVI (Environment for Visualizing Images) and ArcMap, land area under water in 1979 is 1.8 % and that in 2009 is 0.6 % of the 23 826 km² study area. Although the rate of precipitation decline in 1955–2012 is 4.41 mm/year, it is almost the same for 1979 (582 mm) and 2009 (564 mm). This suggests that drought or flooding has no significant effect on the water storage change. For 1979–2009, land area under forest and grass decreases respectively by 54.2 % and 70.7 %. Then that under settlement, farmland and others increases respectively by 64.8 %, 56.0 % and 63.6 %. The loss of land area under water is 64.6 %, which is more the effect of land use change and the related water use in the region. Irrespectively, more water-efficient land use measures could ensure the sustainability and availability of water resources for future use in the predominantly agrarian semiarid region.     

淮河流域历史覆被变化及其对水文过程的影响

以淮河流域蚌埠集水区为研究区域,利用淮河流域1700、1800和2000年3种历史覆被情景,结合陆面水文耦合模型(CLHM)定量评价了流域土地利用/覆被变化的水文效应,并分析了流域径流与主要覆被类型变化方式的定量关系。结果表明:CLHM模型可以较好地模拟淮河流域的流量过程,在研究区具有较好地适用性;三期覆被情景下主要覆被类型变化为林地转变为耕地或草地以及草地转变为耕地,此种变化导致流域总蒸发量减少了6.5%,流域出口研究期平均径流量增加了6.1%;极端覆被情况下,年均蒸发量由多到少依次为林地、草地和耕地;林地对流域径流过程的影响主要体现在洪峰上,相对于耕地,林地具有削减洪峰的作用;通过分析覆被类型变化与流域多年平均径流的相关关系,林地转变为草地以及林地转变为耕地是近400年来影响淮河流域水文过程及水资源分配的主要覆被变化因子。因此,合理规划土地利用格局对流域水资源高效利用具有重要意义。     

Estimation of a Unique Pair of Nash Model Parameters: An Optimization Approach

Water resources planning and management requires hydrologic models to estimate runoff from a catchment. For catchments with limited data, the choice of model and identification of its parameters is very important for development of a direct runoff hydrograph. A method is presented to determine a unique pair of hydrologic parameters of the Nash Model, number of linear cascade (n) and storage coefficient (k), using optimization based on Downhill Simplex technique. In this study physical parameters of the catchment are derived from (SPOT) satellite imageries of the basin using ERDAS software. Four different objective functions of varying complexity are tested to find the best solution. Weighted root mean square error (RMSE) and Model Efficiency (Nash-Sutcliffe coefficient) are used to evaluate the model performance. Using the NASH model, a direct surface runoff hydrograph (DSRH) is developed. Kaha catchment is part of Indus river system and is located in the semi-arid region of Pakistan. This catchment is dominated by hill torrent flows and is used in this work to demonstrate the applicability of the proposed method. Ten randomly selected rainfall-runoff events are used for calibration and five events are used for validation. Model results during validation are very promising with model efficiency exceeding 93% and error in peak discharge under 8%. The sensitivity of the Nash model output in response to variation in hydrologic parameters n and k is also investigated. When evaluating the hydrologic response of large catchments, model output is more sensitive to n as compared to k indicating that the runoff diffusion phenomenon is dominant compared to translation flow effects.     

Analyzing the influence of changes in land use and management practices on the lag time of peak flows for tropical watersheds of Ethiopia

Changes in peak flow response time governed by rainfall and physical configurations of watersheds have been the topic of many studies, but other factors are also important. We aimed to analyze the dynamics in the lag time (T_L) of peak flow for a tropical watershed (Kecha) and its sub-watersheds (Dokmit, Zenjero Maderia, and Wotit Minch) in Ethiopia, as influenced by land-use changes from 1982 to 2017 and the implementation of soil and water conservation (SWC) practices since 2011. Layered maps of land use, soil type, and SWC practices were used to determine the curve number, and the Natural Resource Conservation Service hydrologic model was used to estimate T_L. We compared the estimated values against measured median values of T_L for 30 rainfall–runoff events. The estimated T_L of 1982, 2005, and 2017 varied from 9 to 19 min, 8 to 18 min, and 10 to 22 min, respectively, in the Dokmit, Zenjero Maderia, and Wotit Minch sub-watersheds. The smallest (8 min) and greatest (22 min) values of T_L were observed during 2005 and 2017, respectively. These results are likely attributable to the increased amount of cultivated area at the expense of bushland and forest in 2005 as compared to 1982, and the implementation of SWC practices in 2011–2017. Dokmit had the lowest T_L values of the three sub-watersheds, most likely because of its greater coverage of grazing and cultivated lands and degraded Nitic Luvisol. The variation in T_L values among the sub-watersheds was related to changes in both land use and SWC practices but also to the presence of degraded areas inherited from past human activities. Overall, such spatiotemporal flow response time analysis can provide useful information for the proper design of sustainable development strategies for particular niches in tropical regions of Ethiopia and elsewhere.     

Desirable Low Flow Releases below Dams: Fish Habitats and Reservoir Costs

ABSTRACT

Streamflowpattern and catcbment characteristics are examined in Guyana, a tropical country located on the northeastern part of South America. Only streamflow data of ten major rivers are considered, because long-term records are not available for all the rivers. The catchments are grouped into three hydrologic environments based on rainfall and vegetation. A number of characteristics such as annual variability of flows, seasonal variability of flows, flow duration curves, flood flows and low flows were compared. The study showed that these were variations in hydrologic characteristics between the three environments. It also indicated that apart from rainfall several other factors, particularly geology play key roles in influencing the hydrologic characteristics. In order to study this phenomenon and other hydrologic characteristic particularly floodflow a rigorous data analysis is required. The need for additional hydrologic data for a detailed analysis of streamflow characteristics in a tropical country such as Guyana is emphasised throughout the study.