Single Storm Event Research Articles

Regulations and patterns of soil moisture dynamics and their controlling factors in hilly regions of lower reaches of Yangtze River basin, China

Time-domain reflectometry was used to make continuous measurements of soil moisture to 18 sample points with depth of 2 m for 36 months in a typical artificial secondary oak forest located in a hilly area on Zijin Mountain in the suburbs of Nanjing, China. The data were then used to examine the patterns of soil moisture variations on temporal and spatial scales and predict the relationships between soil moisture and major factors of both meteorology and topography. Water in the topsoil was active, and the upper 30 cm of soil supplied about 43% of the water content variation during the whole year. This difference of water content changes among layers could be due to the distribution conditions of some soil physical properties. When initial soil moisture was in the range from 10% to 40%, the impact of a single storm event on soil moisture was extremely significant, especially on sunny slope. Both climate and slope condition were related to soil moisture change, and the impact of slope gradient on soil moisture was higher that on shady slope. Moreover, root uptake was another important path of soil water consumption.

Channel adjustments to a succession of water pulses in gravel bed rivers

AbstractGravel bed rivers commonly exhibit a coarse surface armor resulting from a complex history of interactions between flow and sediment supply. The evolution of the surface texture under single storm events or under steady flow conditions has been studied by a number of researchers. However, the role of successive floods on the surface texture evolution is still poorly understood. An experimental campaign in an 18 m‐long 1 m‐wide flume has been designed to study these issues. Eight consecutive runs, each one consisting of a low‐flow period of variable duration followed by a sudden flood (water pulse) lasting 1.5 h, have been conducted. The total duration of the experiment was 46 h. The initial bed surface was created during a 280 h‐long experiment focused on the influence of episodic sediment supply on channel adjustments. Our experiments represent a realistic armored and structured beds found in mountain gravel bed rivers. The armor surface texture persists over the duration of the experiment. The experiment exhibits downstream fining of the bed‐surface texture. It was found that sorting processes were affected by the duration of low‐flow between flood pulses. Since bed load transport is influenced by sediment sorting, the evolution of bed load transport is impacted by the frequency of the water pulses: short interpulse durations reduce the time over which fine material (transported as bed load) can be winnowed. This, in turn, contributes to declining reduction of the bed load transport over time while the sediment storage increases.

Impacts of storm chronology on the morphological changes of the Formby beach and dune system, UK

Abstract. Impacts of storm chronology within a storm cluster on beach/dune erosion are investigated by applying the state-of-the-art numerical model XBeach to the Sefton coast, northwest England. Six temporal storm clusters of different storm chronologies were formulated using three storms observed during the 2013/2014 winter. The storm power values of these three events nearly halve from the first to second event and from the second to third event. Cross-shore profile evolution was simulated in response to the tide, surge and wave forcing during these storms. The model was first calibrated against the available post-storm survey profiles. Cumulative impacts of beach/dune erosion during each storm cluster were simulated by using the post-storm profile of an event as the pre-storm profile for each subsequent event. For the largest event the water levels caused noticeable retreat of the dune toe due to the high water elevation. For the other events the greatest evolution occurs over the bar formations (erosion) and within the corresponding troughs (deposition) of the upper-beach profile. The sequence of events impacting the size of this ridge–runnel feature is important as it consequently changes the resilience of the system to the most extreme event that causes dune retreat. The highest erosion during each single storm event was always observed when that storm initialised the storm cluster. The most severe storm always resulted in the most erosion during each cluster, no matter when it occurred within the chronology, although the erosion volume due to this storm was reduced when it was not the primary event. The greatest cumulative cluster erosion occurred with increasing storm severity; however, the variability in cumulative cluster impact over a beach/dune cross section due to storm chronology is minimal. Initial storm impact can act to enhance or reduce the system resilience to subsequent impact, but overall the cumulative impact is controlled by the magnitude and number of the storms. This model application provides inter-survey information about morphological response to repeated storm impact. This will inform local managers of the potential beach response and dune vulnerability to variable storm configurations.

Stochastic Urban Pluvial Flood Hazard Maps Based upon a Spatial-Temporal Rainfall Generator

It is a common practice to assign the return period of a given storm event to the urban pluvial flood event that such storm generates. However, this approach may be inappropriate as rainfall events with the same return period can produce different urban pluvial flooding events, i.e., with different associated flood extent, water levels and return periods. This depends on the characteristics of the rainfall events, such as spatial variability, and on other characteristics of the sewer system and the catchment. To address this, the paper presents an innovative contribution to produce stochastic urban pluvial flood hazard maps. A stochastic rainfall generator for urban-scale applications was employed to generate an ensemble of spatially—and temporally—variable design storms with similar return period. These were used as input to the urban drainage model of a pilot urban catchment (~9 km2) located in London, UK. Stochastic flood hazard maps were generated through a frequency analysis of the flooding generated by the various storm events. The stochastic flood hazard maps obtained show that rainfall spatial-temporal variability is an important factor in the estimation of flood likelihood in urban areas. Moreover, as compared to the flood hazard maps obtained by using a single spatially-uniform storm event, the stochastic maps generated in this study provide a more comprehensive assessment of flood hazard which enables better informed flood risk management decisions.

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.

Exorheism growth as an explanation of increasing flooding in the Sahel

For two decades, the Niamey area, in Niger, has undergone the creation of several new wadis (“koris” in hausa, the most spoken language in West Africa). The significant runoff increase in the Sahelian reach of Niger river makes us interested in the behavior of the basins of the tributary koris of Niger River in the Niamey area, in Niger. These koris generally formed during a single storm event, within depressions previously occupied by ponds; these ponds are overflown creating a new “kori”. This study examines in detail the causes of this new exorheism mechanism. The main explanation of this evolution has been determined as being the strong runoff increase, related to an extension of crusted soils due to agricultural practices, mostly the reduction of fallow duration. The degradation of their structural stability leads to crusting and a strong reduction of their hydraulic conductivity. This is linked to water and sediment balance at the catchment scale.

A Simplified Approach to Pollutant Load Modeling

The combined sewer overflow (CSO) control requirements imposed by the United States Environmental Protection Agency (USEPA) on approximately 1 100 sewer district utilities within the United States initially focused solely on the volumetric reduction of CSOs, with the assumption that a corresponding reduction in pollutant loads to a combined sewer system’s (CSS) receiving stream would result. As the development of CSO long term control plans for addressing the agency’s CSO control policies has progressed, the focus of CSO control has shifted, and assessing water quality benefits through quantitative analysis is becoming more common. Development of CSO improvements typically involves the consideration of several alternatives, and the benefits provided by each are evaluated in addition to its cost. While evaluating CSO control alternatives in Cincinnati, Ohio a simplified approach for com paring the relative water quality benefits achieved by each alternative was developed. Pollutant load event mean concentrations (EMCs) were developed for the pollutants of concern, based on available national average information. Within the existing conditions and alternatives models being evaluated utilizing USEPA’s Stormwater Management Model 5 (SWMM5), EMC assignments were applied to the rainfall derived infiltration and inflow, sanitary baseflow and individual subareas based on land use character istics. The treatment effectiveness of both grey and green CSO and stormwater treatment facilities were simulated using estimated pollutant removal efficiencies. A single design storm event as well as continuous annual simulation modeling over an entire year was performed using design storm rainfall and historical rainfall data. Pollutant loadings to the receiving stream were quantified and compared to assess the water quality benefit of each alternative. This study presents these results and provides an approach for making relative comparisons of the water quality benefits offered by CSO control alternatives within any CSS.

Evaluation of urban growth effects on surface runoff using SCS-CN method and Green-Ampt infiltration model

Evaluation of land cover/use change and its impact on hydrological design aspects plays a vital role in assessing the capacity of existing storm water conveyance systems and its capability towards the anticipated peak flow. This paper attempts to illustrate an application of Soil Conservation Service (SCS-CN) method for runoff estimation using continuous time series rainfall. And also, the present work proposes a Land use factor based on the Curve Number to correct the infiltration rate according to the prevailing land cover/use. Indian remote sensing (IRS) satellite data for the years 1989, 1992, 1995, 1998, 2001, 2004, 2007, and 2010 are used to evaluate the growth of Tiruchirapalli city, India city and to assess its impact on surface runoff. The Geographical information system (GIS) is used to prepare the different layers belonging to various land covers identified from remotely sensed data. The sub-watersheds are created using Digital Elevation Model (DEM) prepared by ARC GIS software. The single storm event and 50-year return period rainfall data are used as a hydrological input to the model to evaluate the increase in runoff over the years due to change in the land cover of the study area. The study reveals that the impact of land cover/use change is more significant for longer duration storm than short duration storm at the same magnitude. Further, it is noticed from the study that the runoff depth and runoff co-efficient is large for short duration rainfall than long duration rainfall at the same magnitude.

Taphonomic implications from Upper Triassic mass flow deposits: 2-dimensional reconstructions of an ammonoid mass occurrence (Carnian, Taurus Mountains, Turkey)

Abstract Ammonoid mass occurrences of Late Triassic age were investigated in sections from A şağlyaylabel and Yukarlyaylabel, which are located in the Taurus Platform-Units of eastern Turkey. The cephalopod beds are almost monospecific, with > 99.9 % of individuals from the ceratitic genus Kasimlarceltites, which comprises more than hundreds of millions of ammonoid specimens. The ontogenetic composition of the event fauna varies from bed to bed, suggesting that these redeposited shell-rich sediments had different source areas. The geographical extent of the mass occurrence can be traced over large areas up to 10 km2. Each of the Early Carnian (Julian 2) ammonoid mass occurrences signifies a single storm (e.g. storm-wave action) or tectonic event (e.g. earthquake) that caused gravity flows and turbidity currents. Three types of ammonoid accumulation deposits are distinguished by their genesis: 1) matrix-supported floatstones, produced by low density debris flows, 2) mixed floatstones and packstones formed by high density debris flows, and 3) densely ammonoid shell-supported packstones which result from turbidity currents. Two-dimensional calculations on the mass occurrences, based on sectioning, reveal aligned ammonoid shells, implying transport in a diluted sediment. The ammonoid shells are predominantely redeposited, preserved as mixed autochthonous/parautochnonous/ allochthonous communities based on biogenic and sedimentological concentration mechanisms ( = in-situ or post-mortem deposited). This taphonomic evaluation of the Kasimlarceltites beds thus reveals new insights into the environment of deposition of the Carnian section, namely that it had a proximal position along a carbonate platform edge that was influenced by a nearby shallow water regime. The Kasimlarceltites-abundance zone is a marker-zone in the study area, developed during the drowning of a shallow water platform, which can be traceable over long distances.

Fine sediment sources in Conservation Effects Assessment Project watersheds

Two naturally occurring radionuclides, Beryllium-7 (⁷Be) and Lead-210 (²¹⁰Pb_xs) were used as tracers to discriminate eroded surface soils from channel-derived sediments in the fine suspended sediment loads of eight Conservation Effects Assessment Project (CEAP) benchmark watersheds. Soil, bank, and suspended sediment samples, as well as rainfall samples, were collected in the watersheds from single storm events and analyzed for the radionuclide activities, which were then applied to a two end-member mixing model to determine the relative proportions from the two source areas. In larger watersheds where the transport length of the sediment was longer, the suspended sediment load contained lower proportions of eroded upland soils compared to smaller systems. The longer transport paths contained more depositional areas in which fine sediment could settle, and hence, less eroded surface soils reached the stream channel, resulting in higher proportions of sediment from the channels. This study showed that more than 50% of the fine sediment in six of the eight watersheds originated from channel sources that included stream banks, the riverine bed, and gullies. These results underscore the need to consider channel and gully processes when management practices are designed to reduce sediment yield in agricultural watersheds.

Large-scale wind disturbances promote tree diversity in a Central Amazon forest.

Canopy gaps created by wind-throw events, or blowdowns, create a complex mosaic of forest patches varying in disturbance intensity and recovery in the Central Amazon. Using field and remote sensing data, we investigated the short-term (four-year) effects of large (>2000 m2) blowdown gaps created during a single storm event in January 2005 near Manaus, Brazil, to study (i) how forest structure and composition vary with disturbance gradients and (ii) whether tree diversity is promoted by niche differentiation related to wind-throw events at the landscape scale. In the forest area affected by the blowdown, tree mortality ranged from 0 to 70%, and was highest on plateaus and slopes. Less impacted areas in the region affected by the blowdown had overlapping characteristics with a nearby unaffected forest in tree density (583±46 trees ha−1) (mean±99% Confidence Interval) and basal area (26.7±2.4 m2 ha−1). Highly impacted areas had tree density and basal area as low as 120 trees ha−1 and 14.9 m2 ha−1, respectively. In general, these structural measures correlated negatively with an index of tree mortality intensity derived from satellite imagery. Four years after the blowdown event, differences in size-distribution, fraction of resprouters, floristic composition and species diversity still correlated with disturbance measures such as tree mortality and gap size. Our results suggest that the gradients of wind disturbance intensity encompassed in large blowdown gaps (>2000 m2) promote tree diversity. Specialists for particular disturbance intensities existed along the entire gradient. The existence of species or genera taking an intermediate position between undisturbed and gap specialists led to a peak of rarefied richness and diversity at intermediate disturbance levels. A diverse set of species differing widely in requirements and recruitment strategies forms the initial post-disturbance cohort, thus lending a high resilience towards wind disturbances at the community level.

Lakes as sensors in the landscape: Optical metrics as scalable sentinel responses to climate change

As the lowest point in the surrounding landscape, lakes act as sensors in the landscape to provide insights into the response of both terrestrial and aquatic ecosystems to climate change. Here a novel suite of climate forcing optical indices (CFOI) from lakes across North America is found to respond to changes in air temperature, precipitation, and solar radiation at timescales ranging from a single storm event to seasonal changes to longer‐term interdecadal trends with regression r2 values ranging from 0.73 to 0.89. These indices are based on two optical metrics of dissolved organic carbon (DOC) quality: DOC specific absorbance (a*320) and spectral slope (S275–295), where the ratio a*320 to S275–295 gives a composite climate forcing index. These indices of DOC quality are more responsive to climate forcing than is DOC concentration. A similar relationship between the component indices a*320 and S275–295 is observed across a wide range of lake types. A conceptual model is used to examine the similarities and differences in DOC‐related mechanisms and ecological consequences due to increased temperature vs. precipitation. While both warmer and wetter conditions increase thermal stratification, these two types of climate forcing will have opposite effects on water transparency as well as many ecological consequences, including oxygen depletion, the balance between autotrophy and heterotrophy, and depth distributions of phytoplankton and zooplankton.

Towards understanding river sediment dynamics as a basis for improved catchment, channel, and coastal management: the case of the Motueka catchment, Nelson, New Zealand

ABSTRACTThis paper brings together work in the Motueka catchment that has focused on both suspended sediment data and bedload transfers to provide a more holistic understanding of sediment dynamics in the catchment to inform effective river management. The annual suspended sediment load averages 349,000 t and shows considerable temporal variability (49,000 t to 1.7 Mt). Event yields may increase by an order of magnitude in response to single high magnitude storm events. Much of the sediment is generated from high rainfall areas of the catchment under indigenous forest and grassland. Short-term studies show pasture areas have a higher specific sediment yield than production forest, but that forest harvesting leads to a short-term increase in yield. Bedload transfers assessed via morphological budgeting from digital elevation model (DEM) differencing in selected reaches of the upper Motueka reveal similarly highly variable transfers at an annual scale, reflecting the magnitude and frequency of competent flow events. Longer term mean bed-level (MBL) changes demonstrate a high degree of spatial variability in the upper Motueka. Overall, DEMs of difference and longer term MBL changes both reveal a net channel degradation and export of bedload in the mainstem of the upper Motueka. Suspended sediment data also suggest an overall reduction in sediment yield from the catchment, suggesting a catchment-wide limitation of sediment supply, or a period of lower flows reducing sediment mobilization. This understanding has informed on issues such as the role of river channel management and catchment land use on in-stream ecosystems, coastal erosion, and shelf water quality and fisheries. Future river management, if it is to be effective, needs to recognize the history of this system, its likely longer term trajectory, and its linkages with the coast.

Comparison and Assessment of Success of Models in Watershed Simulation and Management

In Jordan, Zarqa River Basin (ZRB) has been taken as a case study for applying water management models because of its limited water resources and due to the fact that the basin is dwelling with about 52% of Jordan’s population. The surface water resources are mainly used for agriculture because they are mixed with treated water and cannot be used for domestic purposes. This paper aims to demonstrate the contributions of Models in watershed management that provide indirect ways of assessing and confirming the success of models in water flow simulation. The method includes transferring the computed hydrologic parameters for Zarqa basin’s sub-catchments within Watershed Modeling System (WMS) into Water Resources Model (WRM) and HEC-1 models. Then the results of the HEC-1 and WRM models are compared according to their basin’s simulation with the real basin. The study includes description of the HEC-1, WRM models philosophy, the models representation, and simulation results and analysis of the Zarqa River Basin. Comparing the results of WRM and HEC-1 models proved their simulation efficiency in predicting the flow of Zarqa River Basin. Nevertheless, the philosophy of HEC-1 is a single storm event and is based on values of curve number, while WRM philosophy describes the water flow and availability, and demand and supply balance on a daily basis across the basin. The models’ predictions for the real flow definitely establish the modeling certainty and help the water resources’ developers to incorporate different basin features for watershed representation, simulation, and management. Hence, the certainty of the results in modeling provides indirect ways of assessing the success of models’ simulations.

Reply to Ogden and Stallard: Phenomenological runoff models in the Panama Canal watershed

Ogden and Stallard (1) argue that our estimates of the effect of reforestation on runoff in the Panama Canal watershed (2) are invalid because we use a particular phenomenological runoff model: the US Department of Agriculture–Natural Resources Conservation Service Curve Number (CN) model. The authors claim that the CN model, originally developed to describe runoff from single storm events in temperate watersheds, does not “work” in humid tropical watersheds and cannot be applied at other temporal or spatial scales. We disagree. The test of any phenomenological model is whether it predicts well at the scale on which data are observed. The CN model does not offer the same insights into hydrological processes as physics-based models, but it has been applied at various temporal (3) and spatial scales (4, 5) with reasonable predictive power. We calibrated our model using precipitation and hydrograph data for the Candelaria subbasin and validated against observed flows for six subbasins in the watershed. At the subbasin scale the model predicted flows within . The predicted margin of error for the whole watershed was found to be 1%. Sensitivity analysis showed that the estimated hydrological flows were relatively stable with respect to CN variations.

Cultivos herbáceos, viñedos y olivares : el manejo tradicional del suelo y sus consecuencias en la erosión hídrica

The loss of productivity and the frequent use of fertilizers and pesticides in Spanish agricultural fields makes us reflect on the adequacy of the soil management used in our land. Soil erosion is one of the processes involved in this loss of productivity, showing the need/importance to evaluate its severity. Such an evaluation is essential for decision-making by society, consumers and/or farmers, and to guide the competent authorities to implement suitable measures. During the last 16 years we have been studying soil loss in the main crops in Spain (cereal, olive groves and vineyards) using different techniques. So, we can know the impact of different land management in our agricultural lands and whether the erosion rate is tolerable or not by today’s standards. It has been tested in different settings that the managements with bare soil, such as tilling and/or the use of herbicides, threatens the soil conservation with average erosion rates up to 20 t ha-1 yr-1. These erosion rates vary from 1.3 t ha-1 yr-1 with rains of low erosivity, to 93.5 t ha-1 that can be produced in a single storm event highly erosive. In the case of woody crops (olives and grapes), we have studied different soil managements. The spontaneous vegetation reduced vineyard erosion rates to 1.76 t ha-1 yr-1, value limit of what is considered admissible in our circumstances. The soil managements more effective to control erosion were permanent grass cover and mown cereal cover, with soil losses of 0.24 and 0.32 t ha-1 yr-1 respectively. Thus, we recommend the use of groundcovers, the management of which will vary depending on the characteristics of the area.

Exploring the role of rainfall variability and extreme events in long-term landscape development

An obvious timescale gap exists between a single storm event and long term landscape development. In this study the event- and physically based OpenLISEM soil erosion model was compared to the landscape evolution model LAPSUS, deliberately extending and shortening the timescales for which each model was developed. Calibration of OpenLISEM using average erosion rates derived from long-term simulations with LAPSUS and, vice versa, calibration of LAPSUS on event-scale did not give satisfactory results, suggesting that the gap between the different timescales of both models is too large to be bridged directly. However, calibration of LAPSUS on annual basis using the summed OpenLISEM erosion and deposition values for each year resulted in a good reproduction of these values by LAPSUS. Thus, when keeping to the timescale that the model was originally intended for, but calibrating the model using simulation results from the event-based model, short-term variability could successfully be introduced in longer-term modelling of landscape development. Subsequently, the erosion effects of rainfall variability, climate and land use change were explored on a centennial timescale. Results show non-linear behaviour between rainfall input and simulated net erosion. Simulated net erosion for increased rainfall erosivity was compared to rainfall variability, showing that mean annual net erosion of up to 15% increased erosivity is not significantly different from annual mean net erosion of the original simulations. Single events must be of very high magnitude and/or frequency to leave a signal in the landscape that is beyond the scope of natural rainfall variability. Scenarios of human impact show that land use changes can have a potentially larger effect on erosion dynamics than climate variability and change. This is the first time that an event-based erosion model and a landscape evolution model were calibrated for the same area and compared in terms of erosion and deposition dynamics.

Dynamics of upwelling in the Alaskan Beaufort Sea and associated shelf–basin fluxes

Data from a high-resolution mooring array deployed across the Alaskan Beaufort shelfbreak and slope, together with an idealized numerical model, are used to investigate the dynamics of wind-driven upwelling and the magnitude of the resulting shelf–basin exchange. The analysis focuses on a single storm event in November 2002 when the sea-ice concentration was 50–70%. The normally eastward-flowing shelfbreak jet was reversed to the west, and the secondary circulation near the shelfbreak was characterized by offshore flow in the upper layer and a nearly equal amount of onshore flow at depth. Ekman theory accurately predicts the strength of the secondary circulation when one takes into account the ice–ocean stress. The depth-integrated alongstream momentum balance reveals that, near the shelf edge, the reversed jet is driven by a combination of the surface stress and divergence of cross-stream momentum flux. The reversed jet is primarily spun-down – before the winds subside – by the alongstream pressure gradient that likely results from the variation in sea surface height. The shelf–basin fluxes of heat, freshwater, and nitrate resulting from the storm are substantial. Much of the yearly supply of heat to the Beaufort shelf from the inflowing Pacific water through Bering Strait was fluxed offshore, and the amount of freshwater transported into the basin represents a substantial fraction of the year-to-year variation in the freshwater inventory of the Beaufort Gyre. The on-shelf flux of nitrate from 4 to 5 such storms could account for most of the net annual primary production that occurs on the Beaufort shelf.

Major coastal impact induced by a 1000-year storm event

Extreme storms and storm surges may induce major changes along sandy barrier coastlines, potentially causing substantial environmental and economic damage. We show that the most destructive storm (the 1634 AD storm) documented for the northern Wadden Sea within the last thousand years both caused permanent barrier breaching and initiated accumulation of up to several metres of marine sand. An aggradational storm shoal and a prograding shoreface sand unit having thicknesses of up to 8 m and 5 m respectively were deposited as a result of the storm and during the subsequent 30 to 40 years long healing phase, on the eroded shoreface. Our results demonstrate that millennial-scale storms can induce large-scale and long-term changes on barrier coastlines and shorefaces, and that coastal changes assumed to take place over centuries or even millennia may occur in association with and be triggered by a single extreme storm event.

A sheet erodibility parameter for water erosion modeling in regions with low intensity rain

Soil erodibility reflects the soil effect on the detachment process by rainfall and runoff; an evaluation of this parameter for single storm events was carried out using natural runoff plot data collected for two rainfall seasons in northern Iraq. The region is characterized by a semiarid Mediterranean-type climate with normal rainfall intensity below 20 mm/h and dominant sheet erosion on agricultural land. The plots were three 30 × 3 m and three 10 × 3 m, in fallow, situated on a 6% uniform slope; the soil at the site has a silty clay loam texture and belongs to the Calciorthid suborder. Sheet erosion rate was assumed linearly proportional to the storm power and the sheet flow power; a steady-state turbulent and kinematic sheet flow was also assumed. The results indicated a dominant detachment by rainfall with a substantial variability in storm by storm calculated sheet erodibility. The two-parameter lognormal probability distribution fitted the obtained sheet erodibility values reasonably well. Using this probability distribution, a representative sheet erodibility value of 0.056 × 10−3kg/J was obtained for use at the experimental site.