Factors affecting the runoff coefficient (original) (raw)
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Analyses of event runoff coefficients provide essential insight on catchment response, particularly if a range of catchments and a range of events are compared by a single indicator. In this study we examine the effect of climate, geology, land use, flood types and initial soil moisture conditions on the distribution functions of the event runoff coefficients for a set of 14 mountainous catchments located in the eastern Italian Alps, ranging in size from 7.3 to 608.4 km 2 . Runoff coefficients were computed from hourly precipitation, runoff data and estimates of snowmelt. A total of 535 events were analysed over the period 1989-2004. We classified each basin using a ''permeability index" which was inferred from a geologic map and ranged from ''low" to ''high permeability". A continuous soil moisture accounting model was applied to each catchment to classify 'wet' and 'dry' initial soil moisture conditions. The results indicate that the spatial distribution of runoff coefficients is highly correlated with mean annual precipitation, with the mean runoff coefficient increasing with mean annual precipitation. Geology, through the 'permeability index', is another important control on runoff coefficients for catchments with mean annual precipitation less than 1200 mm. Land use, as indexed by the SCS curve number, influences runoff coefficient distribution to a lesser degree. An analysis of the runoff coefficients by flood type indicates that runoff coefficients increase with event snowmelt. Results show that there exists an intermediate region of subsurface water storage capacity, as indexed by a flow-duration curve-based index, which maximises the impact of initial wetness conditions on the runoff coefficient. This means that the difference between runoff coefficients characterised by wet and dry initial conditions is negligible both for basins with very large storage capacity and for basins with small storage capacity. For basins with intermediate storage capacities, the impact of the initial wetness conditions may be relatively large.
The effects of soil properties on floods in the Agri basin (southern Italy)
2003
The spatial distribution of soil properties is directly involved in the fluctuation of soil moisture conditions and in the runoff generation mechanisms. In this work, the influences of the physical characteristics of the contributing area to the peak flow were widely investigated with the aim of understanding the influences of each physical parameter. The study was conducted by means of a hydrological distributed model (Manfieda et al. [7]), applied on the study area of the Agri river basin at Tarangelo (507 km2), in the region of Basilicata, Italy. Two different time scales were used: the first with higher temporal resolution (l-hour) dedicated to the superficial routing, the latter at daily scale used for local water balances and subsurface flow evaluation. Using the model at daily scale, daily soil moisture maps and the river discharge at the outlet are obtained. These results were used as input for the model at the hourly scale in order to describe the initial conditions in the ...
Empirical methods to determine average annual runoff coefficient in Sicilian basins
Runoff estimation in ungauged basin is a challenge for the hydrological engineers and planners. For any hydrological study on an ungauged basin, a methodology has to be appropriately selected for the determination of runoff at its outlet. Several methods have been used to estimate the basin runoff production. In this work the empirical Kennessey method to determine average annual runoff coefficient, RC, is tested on 61 Sicilian basins characterized by different climate conditions, surface permeability, mean slope and vegetation cover. A comparison between observed and calculated RC showed that a calibration of the Kennessey model could be necessary. The slight and not satisfying improvement of the calibrated model suggested that the main factors accounted for the Kennessey method could not be enough to describe mean runoff production. So the analysis has been focused on researching empirical relations between RC and other variables which could play a significant role on RC estimation. Finally, the best result on RC estimate was obtained by a simple linear regression for two Sicilian sub-zones, by considering only two main climatic parameters, average annual rainfall depth and average annual temperature.
In the present paper, an analytical work for the description of the soil water balance and runoff production was adopted over a significant number of river basins belonging to a humid region of Southern Italy. The model is based on a stochastic differential equation, where the spatial heterogeneity of a basin is incorporated by a parabolic function describing the distribution of soil water storage capacity at the basin scale. The model provides an analytical description of the probability density function (PDF) of relative saturation of a basin as well as the PDF of daily runoff production. The proposed model includes five parameters that depend on climatic and soil characteristics. In particular, two parameters describe the rainfall process (α and λ), two characterize the distribution of soil water storage capacity (w max and b), and the last is the soil water loss coefficient (V). Application of the model allowed the regionalization of model parameters based on physically consistent characteristics of the river basins. In particular, it was found that the soil water loss coefficient is strongly controlled by the fraction of forest cover of the river basin, while the parameter b, controlling the shape of the distribution of soil water storage capacity, is influenced by the basin topography.
Predicted and measured soil retention curve parameters in Lombardy region north of Italy
International Soil and Water Conservation Research, 2016
Water retention characteristics are fundamental input parameters in any modeling study on water flow and solute transport. These properties are difficult to measure and for that reason, we usually need to use direct and indirect methods to determine them. An extensive comparison between measured and estimated results is needed to determine their applicability for a range of different soils. However this study attempts to make a contribution specifically in this connection. These properties were determined in two representative sites located in Landriano field, in Lombardy region, northern Italy. In the laboratory we used the pressure plate apparatus and the tensiometric box. Field soil water retention was determined including measurements of soil water content with SENTEK probes and matric potential with tensiometers. The soil waer retention curves (SWRC) were also settled on with some recently developed pedo-transfert functions (PTFs). Field retention curves were compared against those obtained from PTFs estimations and laboratory measurements. The comparison showed that laboratory measurements were the most accurate. They had the highest ranking for the validation indices (RMSE ranging between 2.4% and 7.7% and bias between 0.1% and 6.4%). The second best technique was the PTF Rosetta (Schaap et al. 2001). They perform only slightly poorer than the laboratory measurements (RMSE ranging between 2.7% and 10% and bias between 0.3% and 7.7%). The lowest prediction accuracy is observed for the Rawls and Brakensiek (1985) PTF (RMSE ranging between 6.3% and 17% and bias between 5% and 10%) which is in contradiction with previous finding (Calzolari et al., 2001), showing that this function is well representing the retention characteristics of the area. Due to time and cost investments of laboratory and field measurements, we conclude that the Rosetta PTF developed by Schaap et al. (2001) appears to be the best to predict the soil moisture retention curve from easily available soil properties in the Lombardy area and further field investigations would be useful to support this finding.
Empirical Determination of the Average Annual Runoff Coefficient in the Mediterranean Area
American Journal of Applied Sciences, 2014
Runoff estimation in ungauged basin is a challenge for the hydrological engineers and planners. For any hydrological study on an ungauged basin, a methodology has to be appropriately selected for the determination of runoff at its outlet. Several methods have been used to estimate the basin runoff production. In this study the empirical Kennessey method to determine average annual runoff coefficient, RC, is tested on 61 Sicilian basins characterized by different climate conditions, surface permeability, mean slope and vegetation cover. A comparison between observed and calculated RC showed that a calibration of the Kennessey model could be necessary. The slight and not satisfying improvement of the calibrated model suggested that the main factors accounted for the Kennessey method could not be enough to describe mean runoff production. So the analysis has been focused on researching empirical relations between RC and other variables which could play a significant role on RC estimation. Finally, the best result on RC estimate was obtained by a simple linear regression for two Sicilian sub-zones, by considering only two main climatic parameters, average annual rainfall depth and average annual temperature.
Controls on event runoff coefficients in the eastern Italian Alps
Journal of Hydrology, 2009
Analyses of event runoff coefficients provide essential insight on catchment response, particularly if a range of catchments and a range of events are compared by a single indicator. In this study we examine the effect of climate, geology, land use, flood types and initial soil moisture conditions on the distribution functions of the event runoff coefficients for a set of 14 mountainous catchments located in the eastern Italian Alps, ranging in size from 7.3 to 608.4 km 2 . Runoff coefficients were computed from hourly precipitation, runoff data and estimates of snowmelt. A total of 535 events were analysed over the period 1989-2004. We classified each basin using a ''permeability index" which was inferred from a geologic map and ranged from ''low" to ''high permeability". A continuous soil moisture accounting model was applied to each catchment to classify 'wet' and 'dry' initial soil moisture conditions. The results indicate that the spatial distribution of runoff coefficients is highly correlated with mean annual precipitation, with the mean runoff coefficient increasing with mean annual precipitation. Geology, through the 'permeability index', is another important control on runoff coefficients for catchments with mean annual precipitation less than 1200 mm. Land use, as indexed by the SCS curve number, influences runoff coefficient distribution to a lesser degree. An analysis of the runoff coefficients by flood type indicates that runoff coefficients increase with event snowmelt. Results show that there exists an intermediate region of subsurface water storage capacity, as indexed by a flow-duration curve-based index, which maximises the impact of initial wetness conditions on the runoff coefficient. This means that the difference between runoff coefficients characterised by wet and dry initial conditions is negligible both for basins with very large storage capacity and for basins with small storage capacity. For basins with intermediate storage capacities, the impact of the initial wetness conditions may be relatively large.
Hydrological analysis of the Upper Tiber River Basin, Central Italy: a watershed modelling approach
Hydrological Processes, 2012
The quantification of the various components of hydrological processes in a watershed remains a challenging topic as the hydrological system is altered by internal and external drivers. Watershed models have become essential tools to understand the behaviour of a catchment under dynamic processes. In this study, a physically based watershed model called Soil Water Assessment Tool was used to understand the hydrologic behaviour of the Upper Tiber River Basin, Central Italy. The model was successfully calibrated and validated using observed weather and flow data for the period of 1963-1970 and 1971-1978, respectively. Eighteen parameters were evaluated, and the model showed high relative sensitivity to groundwater flow parameters than the surface flow parameters. An analysis of annual hydrological water balance was performed for the entire upper Tiber watershed and selected subbasins. The overall behaviour of the watershed was represented by three categories of parameters governing surface flow, subsurface flow and whole basin response. The base flow contribution has shown that 60% of the streamflow is from shallow aquifer in the subbasins. The model evaluation statistics that evaluate the agreement between the simulated and the observed streamflow at the outlet of a watershed and other three different subbasins has shown a coefficient of determination (R 2) from 0.68 to 0.81 and a Nash-Sutcliffe efficiency (E NS) between 0.51 and 0.8 for the validation period. The components of the hydrologic cycle showed variation for dry and wet periods within the watershed for the same parameter sets. On the basis of the calibrated parameters, the model can be used for the prediction of the impact of climate and land use changes and water resources planning and management.
Landslides, 2011
Rainfall thresholds represent the main tool for the Italian Civil Protection System for early warning of the threat of landslides. However, it is well-known that soil moisture conditions at the onset of a storm event also play a critical role in triggering slope failures, especially in the case of shallow landslides. This study attempts to define soil moisture (estimated by using a soil water balance model) and rainfall thresholds that can be employed for hydrogeological risk prevention by the Civil Protection Decentrate Functional Centre (CFD) located in the Umbria Region (central Italy). Two different analyses were carried out by determining rainfall and soil moisture conditions prior to widespread landslide events that occurred in the Umbria Region and that are reported in the AVI (Italian Vulnerable Areas) inventory for the period 1991-2001. Specifically, a "local" analysis that considered the major landslide events of the AVI inventory and an "areal" analysis subdividing the Umbria Region in ten sub-areas were carried out. Comparison with rainfall thresholds used by the Umbria Region CFD was also carried out to evaluate the reliability of the current procedures employed for landslide warning. The main result of the analysis is the quantification of the decreasing linear trend between the maximum cumulated rainfall values over 24, 36 and 48h and the soil moisture conditions prior to landslide events. This trend provides a guideline to dynamically adjust the operational rainfall thresholds used for warning. Moreover, the areal analysis, which was aimed to test the operational use of the combined soil moisture-rainfall thresholds showed, particularly for low values of rainfall, the key role of soil moisture conditions for the triggering of landslides. On the basis of these results, the Umbria Region CFD is implementing a procedure aimed to the near real-time estimation of soil moisture conditions based on the soil water balance model developed ad hoc for the region. In fact, it was evident that a better assessment of the initial soil moisture conditions would support and improve the hydrogeological risk assessment.
Indicators for evaluation of surface runoff potential in critical land use areas
In several research projects at the BFW the runoff behavior of different units of land cover, including intensively used areas has been investigated: During these investigations two codes of practice for assessment of surface runoff disposition have been developed. One manual for assessment of runoff potential in case of the recurrent design event, valid for mountainous regions of the Eastern Alps. The second code of practice focused on assessment of changes in runoff disposition due to sealing in intensively settled areas of the Upper Austrian Salzkammergut in the frame of the INTERREG-project MONITOR. Both manuals are based on three indicator groups: Soil (physical) characteristics, way and intensity of actual and historic land use and indication of soil humidity / density by plants. The paper gives a short overview on the information which can be gained from such indicators and describes the way to the assessment of surface runoff coefficients and surface roughness coefficients.