High-intensity Rainstorms Research Articles

Ant mounds as a source of sediment on citrus orchard plantations in eastern Spain. A three-scale rainfall simulation approach

Abstract Ants are widely found in Mediterranean soils, where they increase water infiltration rates by forming soil macropores during nest construction. While higher water infiltration usually results in lower soil erosion rates, new soil brought to the surface by ant activity could increase sediments available for erosion. This could be especially important in intensively-managed citrus orchards, where surface mineral soil is exposed due to the lack of vegetation cover as a consequence of herbicide treatments. In the summer of 2009 rainfall simulations of low frequency–high intensity rainstorms were conducted in an orange orchard in eastern Spain on plots that contained ant nests and adjacent paired-plots without ant nests. Since soil erosion is a scale-dependent process, we used three plot sizes (0.25 m 2 , 1 m 2 , and 12 m 2 ) to determine the effect of ant burrowing and nesting on soil and water losses. Ant nests decreased water losses from 22.5% at 0.25 m 2 to 10.6% at 12 m 2 , but soil erosion rates were nearly double in areas with ant activity (0.56 to 0.59 Mg ha − 1 h − 1 ), as compared to soil with no ants (0.31 to 0.36 Mg ha − 1 h − 1 ). Our results indicate that the presence of ants can increase soil erosion when rainfall intensity is greater than the infiltration capacity of the ant macropores.

Debris Flows and Floods Following the 2003 Wildfires in Southern British Columbia

A number of post-wildfire debris flow and debris flood events occurred in the two years following the extreme fire season of 2003 in the southern interior of British Columbia. Such events had not been previously documented in Canada. Rainstorms following five fires caused significant events in the Okanagan and Kootenay regions of the province, including the Okanagan Mountain Park, Cedar Hills, Kuskonook, Lamb Creek, and Ingersoll fires. Damage to residential property and infrastructure downstream of the fires occurred in three cases. Most of the damaging events were debris flows, although several large debris floods occurred on lower gradient streams. At four of the fires, the events were triggered by short-duration, high-intensity rainstorms. At the fifth fire, a long-duration, low-intensity rainfall triggered the events. In all cases, severe burns with water repellent soils were identified or suspected as contributing to the events. The debris flow and debris flood events described here illustrate three initiation mechanisms: runoff-triggered debris flows caused by erosion of channel bed and banks by critically high discharge; debris flows and floods caused by progressive sediment bulking of runoff with material eroded from headwater slopes; and landslide-triggered debris flows caused by a landslide that enters a steep channel. In the burned areas studied, the first mechanism is the most common, whereas in unburned forested landscapes in the region, the third mechanism is the most common.

Behavior of debris flows monitored on test slopes of Kamikamihorizawa Creek, Mount Yakedake, Japan

Kamikamihorizawa Creek on the slopes of Mount Yakedake, Nagano Prefecture, was selected as an observation site for debris flows due to the high frequency of debris flow and instrumented with monitoring equipments in 1970: seven years after the last phreatic explosion of this volcano. The monitoring system was improved by adding speedometers, stage meters, seismometers and so on. Over the last 30 years, data were obtained from 88 debris-flow events that contained more than 200 episodes of debris-flow surges. The data indicated the following common features related to debris flows. 1) High-intensity rainstorms with a short duration of about 10 min were more likely to trigger debris flows than storms with large rainfall for longer durations. 2) The threshold of rainstorm intensities for debris flows increased due to the changes in hillslope hydrology. 3) The hydrograph of debris-flow surges showed mass and boulder focusing on the front due to mechanical processes. 4) Pulsation and superelevation were common features that were significant as hazard-inducing factors. 5) Monitoring of seismic signals from debris flows is useful for hazard warning as well as for estimating debris-flow hydrographs. 6) Evaluation of hazard-prone locations downstream in the fan is possible by analyzing the interaction of the flows and the fan morphology. 7) Moderation of debris-flow hydrographs is achievable using new net dam or flat-board breakers in the lower reaches of gullies.

Rocky Headwaters in the Dolomites, Italy: Field Observations and Topographic Analysis

Rock outcrops cover large areas of alpine headwaters and are entrenched by chutes and couloirs, which are controlled by faults in bedrock. These widespread landforms play an important role in delivering sediment to lower basin slopes. High-resolution topographical data from LiDAR surveys allow investigation of morphometric characteristics and sediment transport processes in these features. Using aerial photo interpretation, field surveys, and topographic analyses of LiDAR data, this paper quantifies the morphological characteristics of rocky couloirs and their drainage basins, and the relationship between these features and the structural setting, in a study area in the Dolomites (northeastern Italy). Rock basins are characterized by small sizes (surface area < 0.066 km2) and high average basin slopes (up to 2.1 m m−1). The analysis of contributing area and local slope outlines the difference between these rock basins, and even smaller and steeper rock faces entrenched by very shallow chutes, which were defined as interbasin areas. We consider rocky couloirs and rock basins in the headwaters of the Dolomites to be part of the channel network, since channeled flow occurs in the couloirs during storms. High-intensity rainstorms trigger debris flows as evidenced from local scouring, especially in the lower parts of the couloirs. The longitudinal profiles of the couloirs are overall linear, but the high-resolution data display distinct high-slope and low-slope stretches forming steps, that may function as localized sources and sinks for debris flows. The cross-sectional widths of the couloirs do not appear related to upslope area; this may be due to both structural control on cross-sectional geometry and complex erosion of the couloir by debris flows.

Estimation of Soil Erosion Using Field and Modelling Approaches in an Undisturbed Arnhem Land Catchment, Northern Territory, Australia

AbstractAn assessment of slope erosion at Tin Camp Creek catchment, Arnhem Land, Northern Territory, Australia, was carried out using the fallout environmental radioisotope caesium‐137 (137Cs) as an indicator of soil erosion status, two numerical models (SIBERIA and the Revised Universal Soil Loss Equation (RUSLE)) and erosion pins. This undisturbed drainage basin is situated in the seasonally wet‐dry tropics, with high energy storms and a mean annual rainfall of 1389 mm. Tin Camp Creek catchment is unaffected by European agriculture or pastoral activities, but often experiences fire during the dry season. Two transects were sampled for 137Cs in 2002 and 2004, and two models were used to convert 137Cs measurements into soil loss estimates. Two methods using the theoretical Profile Distribution Model (PDM) gave net soil redistribution rates between +2.72 and –22.19 t ha−1 yr−1 and +2.95 and –24.06 t ha−1 yr−1, respectively, while an Australian empirical model (AEM) for uncultivated soils produced estimates between +1.84 and –7.00 t ha−1 yr−1 (negative values indicate soil erosion, positive, deposition). The RUSLE gave estimated soil losses for the two transects of approximately 10 t ha−1 yr−1, while the SIBERIA model produced values between 0.5 and 2 t ha−1 yr−1 for the transects and between 3.5 and 11 t ha−1 yr−1 for the total catchment. Average net soil losses of 14 and 15 t ha−1 yr−1 for the total catchment and slopes, respectively, were measured by erosion pins. The soil losses in the catchment are similar to those for some other transects in the Northern Territory and the Kimberley region of Western Australia (measured by the 137Cs AEM), even though these areas are affected by pastoral activities. This may be at least partly explained by erosion in Tin Camp Creek catchment during high intensity rainstorms at the commencement of the wet season, especially if the slopes have been affected by fire during the previous dry season.

The stability of vegetation boundaries and the propagation of desertification in the American Southwest: A modelling approach

A process-based approach to spatially distributed, overland-flow modelling is employed to assess the impact of water and nutrient redistribution at the landscape scale caused by short, high-intensity rainstorm events across grassland–shrubland vegetation boundaries of a semi-arid ecosystem in the south-western United States. The modelling scenarios showed that simulated fluxes from shrubland into grassland lead to a gain of water resources but to a loss of nutrient resources in the grassland areas close to the boundary. Simulated fluxes from grasslands into shrublands do not lead to a gain of water resources, but to an increase of nutrient resources for the shrubland areas close to the boundary. On the basis of the modelling results, a new hypothesis for the on-going desertification process in the south-western United States is proposed. It is hypothesised that a vegetation boundary is stable when two conditions prevail to balance the lower resistance of grassland within the existing environmental setting with the higher resistance of shrubland: that the depletion of soil nutrients by the action of overland flow in the grassland zone close to the boundary is in balance with the replenishment rates of grassland by other nutrient cycling, and that the grassland gains enough water resources from the upslope shrublands. In contrast, a vegetation boundary potentially becomes unstable when the grassland acquires a competitive disadvantage towards shrubland regarding water benefit and nutrient depletion due to the combined effects of overland-flow dynamics and some external forces such as extensive overgrazing or climate change. The modelling results suggest that landscape linkages through the redistribution of water and soil resources across vegetation-transition zones at the landscape scale and feedback dynamics of overland-flow processes play a significant role in the persistence of land degradation in the US Southwest.

Application of physical modelling of debris flow triggering to field conditions: Limitations posed by boundary conditions

Debris flows are often triggered by Hortonian overland flow during high-intensity rainstorms. Data derived from debris flow trigger zones in the southern French Alps were fed into a physical model of debris flow triggering based on Takahashi. Using a Monte Carlo approach with 1000 runs, the results show a wide distribution of safety factor values, indicating that physical modelling based on actual field measurements may not always be practical. As all safety factor values obtained are well below 1 even though debris flows only occur during very high-intensity rainstorms, the model used must be inappropriate. Apparently, the composition of the overland flow plays an important role: during high-intensity rainstorms it usually has a very high sediment content and contains stones. This prevents it from flowing through the pores of coarse debris accumulations in the central gully of a trigger zone; it will rather run over the debris. This situation is more stable than with the fluid flowing through the pores. The behaviour switch of the fluid above a certain sediment and stone content thus drastically changes the triggering conditions for debris flows and it is concluded that debris flow triggering in the area requires the occurrence of both overland flow and landsliding.

The cost of soil erosion in vineyard fields in the Penedès–Anoia Region (NE Spain)

On-site and off-site environmental impacts of runoff and erosion are usually stressed in order to bring to the public's attention the importance and implications of soil erosion. However, few studies are aimed at calculating the economic implications of erosion, this being the message that farmers and/or policy makers understand best. In this current work we estimated the cost of erosion in vineyards in the Penedès–Anoia region (NE Spain), in which high intensity rain storms (> 80–100 mm h − 1 ) are frequent. Modern plantations in the region consist of trained vines, usually planted perpendicular to the maximum slope direction. Broadbase terraces are interspersed between vine rows to intercept surface runoff and convey it out of the field. Part of the sediment generated above these terraces is deposited in them and other parts are either deposited beyond the boundaries of the fields or are exported to the main drainage network. High intensity rainfall produces heavy soil losses (up to 207 Mg ha − 1 computed in an extreme event in June 2000, which had a maximum intensity in 30-min periods of up to 170 mm h − 1 ). To estimate the cost of erosion in vineyard fields of this region, two important aspects were considered. These were a) the cost incurred by the maintenance of the broadbase terraces, drainage channels and filling of ephemeral gullies and b) the cost incurred by the loss of fertilisers (mainly N and P) caused by erosion. According to farmers' records, the former was estimated at 7.5 tractor-hour ha − 1 year − 1 (as average), which comprises 5.4% of the income from grape sales. Regarding N and P losses, nutrients exported by runoff were 14.9 kg ha − 1 N and 11.5 kg ha − 1 of P, which, if compared to the annual intakes, represent 6% and 26.1% of the N and P respectively. In economic terms, the replacement value of the N and P lost represents 2.4% for N or 1.2% for P of the annual income from the sale of the grapes.

EFFECT OF WHEEL TRAFFIC COMPACTION ON RUNOFF AND SOIL EROSION IN NO-TILL

A seven-year no-tillage study was conducted at the North Carolina A&T State University Farm, Greensboro, NorthCarolina, from 1995 to 2001. The treatments were no-tillage controlled traffic (NC) and no-tillage full traffic (NF). No-tillageconsists of opening a small slit in the soil by means of a coulter running ahead of a planter with openers. Wheel-traffic iscontrolled in NC by using alternate interrow areas such as traffic lanes. In the spring of each year, a 4.5-tonnes tractor wasused to compact the soil surface in NF plots. The plots were planted on a corn-corn-soybean-soybean rotation.<br><br>NF runoff and soil loss were not significantly different (p > 0.05) from NC in six of the seven years. The one year ofdifference occurred in 1997 when NF runoff and soil loss were significantly higher than NC. This was due to a series of highintensity rainstorms in July of 1997, which had return periods of 50 and 100 years. Without this event, there are no significantdifferences in NF and NC runoff and soil loss in all of the years. Total runoff and soil loss of NF is respectively, 1.2 and 2 timesmore than NC. This may indicate that NC represents a better management practice than NF. However, these differences aremostly caused by the single event (July 1997) of very low probability. Therefore, the long-term differences in runoff and soilloss between NF and NC may not be critical. Hence, for this case, farmers may use whichever system fits their managementpattern; and runoff and soil erosion should not be significantly impacted.

A Preliminary Study of the Role of Cover Crops in Improving Soil Fertility and Yield for Potato Production

This study examined the potential of cover crops (fodder rape, oats, and white lupin) to improve phosphorus (P) availability in a high P-fixing Red Ferrosol soil at Robertson, New South Wales. Of particular interest was a fourth cover crop that consisted of white lupin which has P releasing characteristics, grown in combination with fodder rape, which scavenges P with its extensive fine root system. The cover crops were grown for 17 weeks and then mulched at 30 cm above ground to simulate grazing. The cultivar of white lupin used (Kiev Mutant) made poor regrowth after mulching. At 27 weeks after sowing, the cover crops were slashed at soil level and incorporated into the soil. After 7 weeks of decomposition, each cover crop plot was split, and seed potatoes were sown with and without fertilizer. Levels of Olsen-extractable P in the fertilized subplots previously sown to lupin and the lupin/fodder rape combination were significantly higher (16 and 38%, respectively) than in the unfertilized plots at 8 weeks after sowing the potatoes. However, Olsen-extractable P levels in the fertilized oat and fodder rape subplots were similar to those in the unfertilized subplots. Fertilizer increased the yield of potato tubers, but prior cover crop had no effect on tuber yield. The mean apparent recoveries of fertilizer P and N in tubers from the fertilized plants were 4 and 52%, respectively. Although tuber yield did not reflect differences in extractable P among the fertilized subplots, the results suggest that cover crops containing white lupin have the potential to improve P availability to the following potato crop. However, the cultivar of white lupin must be capable of recovering after grazing and match the growth of a companion cover crop. The cover crops containing fodder rape significantly reduced the incidence of Rhizoctonia, presumably due to the biofumigation effect of the rape. The high biomass of the fodder rape and lupin/fodder rape cover crops provided excellent protection for the soil surface against erosion during high intensity rainstorms, as well as providing grazing for stock.

Systematic variation of bedrock channel gradients in the central Oregon Coast Range: implications for rock uplift and shallow landsliding

In tectonically active regions, bedrock channels play a critical role in dictating the pace of landscape evolution. Models of fluvial incision into bedrock provide a means of investigating relationships between gradients of bedrock channels and patterns of active deformation. Variations in lithology, orographic precipitation, sediment supply, and erosional processes serve to complicate tectonic inferences derived from morphologic data, yet most tectonically active landscapes are characterized by these complexities. In contrast, the central Oregon Coast Range (OCR), which is situated above the Cascadia subduction zone, has experienced rock uplift for several million years, did not experience Pleistocene glaciation, boasts a relatively uniform lithology, and exhibits minor variations in precipitation. Although numerous process-based geomorphic studies suggest that rates of erosion across the OCR are relatively constant, it has not been demonstrated that bedrock channel gradients in the region exhibit spatially consistent values. Analysis of broadly distributed, small drainage basins (∼5–20 km) in the central OCR enables us to explore regional variability in bedrock channel gradients resulting from differential rock uplift or other sources. Consistent with previous studies that have documented local structural control of deformed fluvial terraces in the western portion of our study area, our data reveal a roughly 20-km-wide band of systematically elevated channel slopes (roughly twice the background value), roughly coincident with the strike of N–S-trending mapped folds. Although many factors could feasibly generate this pattern, including variable rock strength, precipitation gradients, or temporal or spatial variations in forearc deformation, the elevated bedrock channel slopes likely reflect differential rock uplift related to activity of local structures. Importantly, our analysis suggests that rock uplift and erosion rates may vary systematically across the OCR. Although our calculations were focused on the fluvial-dominated portion of study basins, our results have implications for upstream areas, including unchanneled valleys that often serve as source areas for long-runout debris flows. Zero-order basins (or topographic hollows) within the N–S-trending band of elevated channel slopes tend to be steeper than adjacent areas and may experience more frequent evacuation by shallow landsliding. Thus, this region of the OCR may be highly sensitive to land use practices and high-intensity rainstorms.

Sediment generation and delivery from large historic landslides in the Southern Alps, New Zealand

There is much variability of sediment yield in mountain river systems because it is influenced by both periodic (e.g., snowmelt, monsoonal circulation) and episodic (high-intensity rainstorms, earthquakes) controls. In the Southern Alps of New Zealand, landslides are dominant mechanisms of conveying large amounts of debris to river channels. This paper attempts to quantify catastrophic modes of sediment delivery by presenting three recent examples of large landslides on the western slopes of the Southern Alps. A combination of ground surveys, DEM-based volume calculations, and air photo interpretation attributes immediate postfailure sediment yields in excess of 70,000 t km −2 a −1 in the short term. Such landslide-derived sediment pulses pose significant hazards to downstream settlements and infrastructure on alluvial fans, where they cause massive aggradation, increased flooding frequency, and large-scale channel avulsion. In addition to intramontane valley floor obliteration or channel metamorphosis, these geomorphic impacts may thus occur far beyond the initial locations of slope failure. These catastrophic off-site effects involve apparent net sediment delivery rates in the order of 10 5–10 6 m 3 a −1. The accurate quantification of the relative contribution of debris from the disturbing landslide to overall sediment flux is extremely difficult. Nonetheless are quantitative estimates of landslide-derived sediment production and delivery important data to develop a more comprehensive understanding of the variability in alpine sediment flux. Such data are essential not only for use in hydraulic engineering or catchment management, but also in gauging potential future impacts from rainstorm- and earthquake-induced landsliding in the Southern Alps.

Hydrophobicity and aggregate stability in calcareous topsoils from fire-affected pine forests in southeastern Spain

Soil hydrophobicity is known to enhance runoff responses to rainstorms and to increase soil aggregate stability (AS). It has been widely reported for acidic soils particularly under burnt, but also unburnt pine forests following dry periods. Few studies have reported hydrophobicity from alkaline soils, but they have not established whether hydrophobicity also occurs in burnt or unburnt pine forests on alkaline soil. This study examines the wettability and stability of air-dry aggregates and their size fractions (<0.25, 0.25–0.5, 0.5–1 and 1–2 mm) taken from surface layers (0–2.5 and 2.5–5 cm depth) of alkaline, calcareous loamy soils. Four sites in southeastern Spain were sampled with comparable vegetation (>30-year-old Aleppo pine { Pinus halepensis} and associated shrub community), geology (limestone), soil type (Lithic Xerorthents), slope angle and aspect (5–8°SW). Included were three sites (A, B, C) burned, respectively, in 1998, 1999 and 2000, and one unburnt for >30 years (D). Hydrophobicity was detected in samples from all sites. Both spatial frequency and persistence of hydrophobicity (Water Drop Penetration Times (WDPT) ranged from 10 to 600 s), however, was lower than reported from studies of acidic soils under pine. This might be associated with a lower susceptibility of alkaline soils to hydrophobicity development and/or the comparatively low biomass production in the region. Probably because it had been most recently affected by severe fire, spatial frequency of hydrophobicity was higher at site B (53% of samples), compared to A, C and D (6%, 33% and 10% of samples, respectively). In contrast to some previous studies, the finest size fraction of the samples consistently had the highest degree of hydrophobicity. Degree of hydrophobicity was positively correlated with organic matter (OM) content ( r=0.714). It is speculated that fine, interstitial hydrophobic organic matter accumulating in the finest sieve fraction contributes to this enhanced hydrophobicity. As shown in previous studies on acidic soils, aggregate stability increased with hydrophobicity ( r=0.897 in the fraction 0.25–2 mm) for the samples investigated here. This elevated stability occurs despite an already relatively high level of aggregate stability amongst all samples investigated. Hydrophobicity observed at the study sites was not spatially contiguous and it may therefore enhance overland flow and slope wash over only short distances for most, except the very high intensity rainstorms that occur in the region. The increased stability of hydrophobic soil aggregates against slaking, however, may counter an otherwise enhanced susceptibility to erosion.

Discharge and sediment measurements at the outlet of a watershed on the Loess plateau of China

A dam and weir system was constructed to measure the discharge of water and sediment from a selected small catchment on the Loess plateau in China. The aim of the system described here was to collect data on discharge and sediment content during occasional summer storms. These data can be used for calibrating and validating the LISEM erosion model. A V-notch weir was selected since it can measure a wide range of discharges. The measurement structure was equipped with an ultrasonic sensor to measure the water level. The system automatically switched on as soon as rain had been detected and the water level data were stored when a certain level threshold was surpassed. As a back-up system a flow meter was used, while a local farmer had also been hired to manually record water level during events. Sediment content of the runoff was determined on samples taken by an automatic sampler and a local farmer took additional samples. The system operated from April 1998 until September 2000. In this period, six events occurred and data could be collected during five of those. The data were corrected to make them useful for comparison with erosion simulation results. The collected data show that runoff only occurs during high-intensity rainstorms that produce more than about 11 mm of rain.

Rain-generated overland flow as a factor in the development of some stratified slope deposits: A case study from the Pays du Buëch (Préalpes, France)

A large exposure of stratified slope deposits, and a partly vegetated active talus, both situated near Veynes (Préalpes du Sud, France) were studied. On the active slope, dry grain flows, erosion by rain-generated overland flow, and 'aggregate' flows caused by high-intensity rainstorms are most typical. The term 'aggregate' flow was chosen because of the abundant presence of silty aggregates (diameter 0.5-3 mm) within these deposits which closely resemble small debris flows. These three processes operate in a fixed sequence thus producing a characteristic stratification pattern. Forms and sediments caused by the processes mentioned are described in detail. Subsequently, the results are applied in the interpretation of the sections of the relict deposits. The correlation and the interpretation problems with regard to processes involved between deposits on the active slope and within the relict sediment mass are discussed, together with the traces of other processes recognised within the latter. The results are placed in a wider context of stratified slope deposits and their palaeoclimatic significance.

Scale effects of Hortonian overland flow and rainfall‐runoff dynamics: laboratory validation of a process‐based model

AbstractHortonian runoff was measured in the laboratory from uniform slopes of lengths of 1·5, 3·0, and 6·0 m for steady, high‐intensity rainstorms with durations of 1·0 to 7·5 min. A clear reduction in runoff per unit slope length was found as slope lengths were increased. This effect becomes more pronounced with decreasing storm duration. The runoff data were used to validate a simple process‐based model that combines the Philip‐two‐term infiltration equation with the kinematic wave overland flow principle. The predicted and experimental results agreed well. Laboratory findings were extrapolated with the aid of the model to slopes and rainfall durations similar to those found under West African conditions. The calculated reduction of runoff per unit length is similar to reported observations. Thus, this process‐based model can largely explain the phenomenon of runoff reduction with increasing slope length. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Deforestation, Wood Famine, and Environmental Degradation in Ethiopia's Highland Ecosystems: Urgent Need for Action

One of the major challenges facing Ethiopia in striving for development is environmental degradation, manifested in the degradation of land and water resources as well as loss of biodiversity. Land degradation is expressed in terms of soil erosion and loss of soil fertility. Deforestation/devegetation is one of the major factors contributing to land degradation by exposing the soil to various agents of erosion. With high-intensity rainstorms and extensive steep slopes, Ethiopia is highly susceptible to soil erosion, especially in the highlands. The organic content of soils is often low due to the widespread use of dung and crop residues for energy. Land degradation in turn greatly affects agricultural productivity and production. In 1990 alone, for instance, reduced soil depth caused by erosion resulted in a grain production loss of 57,000 (at 3.5 mm soil loss) to 128,000 tons (at 8 mm soil depth). It has been estimated that the grain production lost due to land degradation in 1990 would have been sufficient to feed more than four million people. The availability of land suitable for agriculture is shrinking, while at the same time the amount of land required to feed the growing population is steadily increasing. With agricultural productivity increases lagging behind population growth rates, the gap between availability and demand for agricultural land continues to grow, resulting in severe land-use conflicts between crop farming, animal grazing, and forestry. Natural high forests and plantations are encroached upon and cleared for cultivation or grazing by local people. State and community forest interests collide with local grazing interests on hillside land, and grazing and fuel wood/charcoal interests confront each other in the woodlands and bushlands. Forestry can play a role in reducing land pressure and land degradation, but forestry alone cannot solve the problem. Even if the management of existing forest resources is improved and new trees and forests are established, this may well prove futile if high population growth rates continue to increase the need for crop and grazing land. Using the land for forestry to improve soil fertility or to rehabilitate and conserve the environment will be viewed as secondary to using the land for cropping and grazing to meet immediate survival needs. Attempts to alleviate land degradation are therefore critically dependent on efforts to deal with the three main underlying causes of land degradation, namely population growth, low agricultural productivity, and high dependence on fuel wood, dung, and crop residue as sources of household energy. Considering the magnitude of the land degradation problem, conservation programs implemented so far are inadequate. The policy, institutional, planning, and technical constraints responsible for the inadequacy of past conservation efforts are presented, and any future initiatives to overcome the escalating land degradation problem in Ethiopia should first address these constraints realistically. There are no universal formulae or solutions that can work across the board; rather, solutions should be locality-specific and closely tied to the socioeconomic setup of the communities. In this regard, forestry can play a significant role in either preventing or arresting land degradation by avoiding or reducing soil erosion through reduced surface runoff and maintenance of organic matter and soil fertility. It can help not only in addressing off-farm and on-farm dimensions of soil erosion but also in maintaining the fertility of the soil, thereby alleviating land degradation and the destruction of natural resources. The various means by which forestry can be used to address problems of land degradation are discussed as outlined in the Ethiopian Forestry Action Program.

The impact of tropical rainstorms on sediment and runoff generation from bare and grass‐covered surfaces: a plot study from Singapore

The characteristically short, intense rainstorms of Singapore occur throughout the year. The impact of such rainstorms on runoff and sediment generation from bare and grass-covered slopes of urban Singapore is examined through observations made on an experimental plot for a representative sample of natural rainstorms. While the responses of the bare surfaces to the incoming rainfall are instantaneous and significant (runoff: up to 91 per cent of total rainfall), irrespective of the dimension of the rainfall, the grass-covered slopes respond more slowly and gently (runoff: 4·8 per cent–17 per cent of the total rainfall). The sediment generating potential under the two types of landcover is closely linked to the short-term intensities of the incoming rainfall, particularly on bare surfaces where high-intensity rainstorms of less than 10 minute duration are capable of generating measurable sediment and runoff. The sediment discharge from the bare surfaces during the long construction period is not only controlled by the episodic heavy storms, but also to a considerable extent by the more frequent, low-magnitude rainstorms, which contribute to a more sustained sediment movement. With a higher frequency of occurrence, such short-duration, high-intensity rainstorms have a significant impact on the sediment and runoff generation potential of the extensive bare surfaces at the construction sites in urban Singapore. © 1998 John Wiley & Sons, Ltd.

The impact of tropical rainstorms on sediment and runoff generation from bare and grass-covered surfaces: a plot study from Singapore

The characteristically short, intense rainstorms of Singapore occur throughout the year. The impact of such rainstorms on runoff and sediment generation from bare and grass-covered slopes of urban Singapore is examined through observations made on an experimental plot for a representative sample of natural rainstorms. While the responses of the bare surfaces to the incoming rainfall are instantaneous and significant (runoff: up to 91 per cent of total rainfall), irrespective of the dimension of the rainfall, the grass-covered slopes respond more slowly and gently (runoff: 4·8 per cent–17 per cent of the total rainfall). The sediment generating potential under the two types of landcover is closely linked to the short-term intensities of the incoming rainfall, particularly on bare surfaces where high-intensity rainstorms of less than 10 minute duration are capable of generating measurable sediment and runoff. The sediment discharge from the bare surfaces during the long construction period is not only controlled by the episodic heavy storms, but also to a considerable extent by the more frequent, low-magnitude rainstorms, which contribute to a more sustained sediment movement. With a higher frequency of occurrence, such short-duration, high-intensity rainstorms have a significant impact on the sediment and runoff generation potential of the extensive bare surfaces at the construction sites in urban Singapore. © 1998 John Wiley & Sons, Ltd.

A MAJOR DEBRIS FLOW ALONG THE WASATCH FRONT IN NORTHERN UTAH, USA

A high-intensity rainstorm focused above Slide Canyon in North Ogden, Utah, released a total of 21.3 cm of rainfall in a 24-hour period during September 1991. This storm exceeded all previous records for the state and resulted in a debris flow that discharged onto an existing fan and into a residential neighborhood (Camarren Cove Subdivision). Poorly sorted deposits of cobbles and boulders with saturated sand-, silt-, and clay-sized fractions issued 500 m beyond the canyon mouth and deposited debris 1.8 m deep. Water and mud also penetrated an additional 100 m into Camarren Cove. Flow depths exceeded 4.2 m in at least two bedrock reaches of the canyon. One home was dislodged 0.6 m off its foundation and destroyed. Approximately 400 homes were affected by the storm in North Ogden, and another 350 in neighboring Harrisville. High-intensity rainfall on a sparsely vegetated, high-gradient mountain canyon containing abundant unconsolidated deposits provided an ideal combination for this disaster. This investigation discusses the event in light of other historical debris flows occurring along the Wasatch Front and illustrates the need for more stringent building-code policies at the base of active fans in mountainous regions. [Key words: debris flow, rock levee, Wasatch Front, Utah.]