Werner Nel

Global rainfall erosivity assessment based on high-temporal resolution rainfall records

The exposure of the Earth’s surface to the energetic input of rainfall is one of the key factors controlling water erosion. While water erosion is identified as the most serious cause of soil degradation globally, global patterns of rainfall erosivity remain poorly quantified and estimates have large uncertainties. This hampers the implementation of effective soil degradation mitigation and restoration strategies. Quantifying rainfall erosivity is challenging as it requires high temporal resolution(<30 min) and high fidelity rainfall recordings. We present the results of an extensive global data collection effort whereby we estimated rainfall erosivity for 3,625 stations covering 63 countries. This first ever Global Rainfall Erosivity Database was used to develop a global erosivity map at 30 arc-seconds(~1 km) based on a Gaussian Process Regression(GPR). Globally, the mean rainfall erosivity was estimated to be 2,190 MJ mm ha−1 h−1 yr−1, with the highest values in South America and the Caribbean countries, Central east Africa and South east Asia. The lowest values are mainly found in Canada, the Russian Federation, Northern Europe, Northern Africa and the Middle East. The tropical climate zone has the highest mean rainfall erosivity followed by the temperate whereas the lowest mean was estimated in the cold climate zone.

Rainfall and temperature attributes on the Lesotho–Drakensberg escarpment edge, southern Africa

Abstract. Located near the southeastern limit of Africa, the Lesotho‐Drakensberg and associated escarpment is the highest range of African mountains south of the massifs in Tanzania. At the escarpment summit and on the adjacent high peaks, the climate is generally interpreted as marginal periglacial yet few data, specifically rainfall and temperature, exist on record at these altitudes. Climatic data from two temporary field stations on the escarpment edge, one of which is the highest rainfall station yet on record in southern Africa, provide contemporary surface‐climate conditions. Mean annual rainfall recorded between 2001 and 2005 averages 767.8 mm at Sani Pass summit (three complete years), and 753.2 mm on Sentinel Peak (two complete years); these values are less than those recorded for the same period in the mountain foothills. Even though rainfall is slightly below long‐term rainfall averages for the area due to a marginally dry spell, the data show that earlier estimates of between 1000 mm and 2000 mm rainfall per annum on the escarpment are too high. A measured mean air temperature of 5.8°C at Sani Pass, however, falls within the range estimated for the escarpment summit. Frost cycles in air and at the soil surface are frequent in winter, but absent in soil for summer, and no long‐duration surface‐soil freeze was measured. Temperatures thus confirm the marginal periglacial nature as postulated for previously, but precipitation data indicate a dryer environment than anticipated. Palaeoenvironmental scenarios, notably arguments for former glaciation based on extrapolations from somewhat exaggerated contemporary precipitation values, thus require re‐consideration.

TRENDS IN RAINFALL TOTAL AND VARIABILITY (1970-2000) ALONG THE KWAZULU-NATAL DRAKENSBERG FOOTHILLS

ABSTRACT South Africas most valuable source of water is the eastern escarpment region of the KwaZulu-Natal Drakensberg and Lesotho highlands. These upper catchments supply much of KwaZulu-Natal, feed the Vaal River in the interior through two inter-basin transfer schemes and are important conservation areas. Analysis of rainfall and rainfall variability trends can thus contribute to a better understanding and management of the area and yet no recent studies have investigated these aspects. This study assesses the 1970–2000 rainfall period using 13 stations to the east of the escarpment in KwaZulu-Natal and presents a spatial perspective on annual rainfall totals and intra- and inter-annual variability. Altitude and distance from the escarpment eastward are found to influence total annual rainfall with an increase of 41.5 mm per 100m in altitude between approximately 1100m and 2100m a.s.l., and a corresponding decrease of 54 mm for every 10 km eastward from the escarpment. Neither inter- nor intra-annual rainfall variability is influenced directly by altitude or the relation to the escarpment. Latitudinal position is found to have no significant affect on station totals but variability increases from south to north in the Drakensberg, possibly related to the greater seasonal contribution by frontal rains in the south, or more variable annual storm activity in the north.

Thermal Attributes of Rock Weathering: Zonal or Azonal? A Comparison of Rock Temperatures in Different Environments

Recent investigations into mechanical weathering in cold environments have highlighted products similarities to those of hot deserts. Although general temperature conditions between these two settings are obviously different on the basis of absolute air temperatures, the zonality with respect to thermal changes affecting the rock is less apparent. Data are presented here from four diverse environmental settings with particular emphasis on the fluctuating temperature regime as applicable to rock thermal stress fatigue and thermal shock. The data focus on diurnal oscillations and short-duration rapid changes on rock surfaces at sites in the Antarctic, sub-Antarctic, and at two southern African sites. Comparisons show that different climatic regimes may not be distinctive with respect to rock thermal changes. The azonality is strongly apparent when contrasting the two African sites, a hot desert and cooler alpine setting, in terms of diurnal fluctuations where very similar values are recorded. Overall temperature ranges measured at the Antarctic site approach the magnitude of those in southern Africa, and all sites show a high potential for thermal shock under rapid temperature changes. These findings highlight potential azonality with respect to thermally induced rock weathering and shift the emphasis in cross-climate comparisons to detailed considerations of the moisture regime.

Aspect-controlled Weathering Observed on a Blockfield in Dronning Maud Land, Antarctica

Abstract Investigations of openwork block deposits have the potential to improve understanding of the production of substrates for biological colonisation, the control of geological structure and the impacts of climate change on landforms. As part of research into the morphology and characterisation of a blockfield located on the orthern uttress of the esleskarvet unataks, estern ronning aud and, ntarctica (2°, 71°), this paper investigates the influence that aspect has on weathering for a blockfield using data collected during the 2009–2012 ustral summer seasons. Proxy data recorded for weathering included rock hardness rebound values as well as visual evidence of flaking, pitting, surface roughness and lichen presence. Aspect was found to affect weathering of blockfield samples across the study site, with the south‐facing sides being the least weathered of the directional aspects. This paper argues that a higher radiation budget associated with slope aspect and angle could drive thermal regimes and subsequent weathering of clast faces. Furthermore, in the absence of abundant precipitation, ice and snow cover become the primary source of moisture in this high‐latitude ntarctic environment.

Temporal sensitivity analysis of erosivity estimations in a high rainfall tropical island environment

Abstract The Erosivity ndex (EI) and the odified ournier ndex (MFI) are two commonly used methods in calculating the R factor of the universal soil loss equation/revised universal soil loss equation formula. Using auritius as a case study, the value of high‐resolution data versus long‐term totals in erosivity calculations is investigated. A limited number of four auritius eteorological ervices stations located on the west coast and the entral lateau provided the study with detailed rainfall data for 6 years at 6‐min intervals. Rainfall erosivity for erosive events was calculated using different set interval data. In this study, within the , the use of 6‐min rainfall intervals during erosive rainfall gave estimates of around 10% more erosivity than the 30‐min time intervals and 33% more rainfall erosivity than the 60‐min rainfall measurements. When the was used to determine erosivity through annual and monthly rainfall totals, substantially higher erosivity than the method was calculated in both regions. This stems from the large amount of non‐erosive rainfall that is generated on auritius. Even when the was used to calculate erosivity through monthly and annual rainfall totals derived purely from erosive rainfall, erosivity calculations were not comparable to those from high‐resolution data within the . We suggest that for the computation of erosivity, rainfall data with the highest possible resolution should be utilised if available and that the application of annual and monthly rainfall totals to assess absolute soil erosion risk within a high rainfall tropical environment must be used with caution.

Intra-Storm Attributes of Extreme Storm Events in the Drakensberg, South Africa

Intra-storm rainfall attributes were analyzed for 49 extreme storm events at five locations in the KwaZulu-Natal Drakensberg eastern South Africa. Three stations located in the mountain foothills and two stations sited on the escarpment edge above 2800 m a.s.l. provide the first detailed intra-storm data for the Drakensberg. Extreme rain-fall events were found to vary in duration and in depth, but all stations measure a clear exponential distribution of cumulative kinetic energy content of storm rainfall over time. The first 300 minutes of storm duration generate more than 90% of the total energy content available as well as 80% of the total rainfall. When rainfall generation and high rainfall intensity (exceeding 25 mm/h) are plotted as a function of storm duration, most storms at the stations indicate a high proportion of rainfall being generated within the first and second quartile of storm duration. More than half the storms generate their maximum peak intensity within the first quartile of the duration of the storm and 84% of the storms show maximum intensity within the first half of the storm duration. Even though these common tendencies are evident, the data show that the structure of erosive rain is site specific, and that the within-storm distribution of rainfall should be incorporated into soil-loss modeling in the region. Further research is needed to ascertain the association between storm structure and synoptic conditions, as well as the actual effect that within-storm distribution of rainfall has on runoff and soil detachment in the KwaZulu-Natal Drakensberg.

THE NATURE OF EROSIVE RAINFALL ON A TROPICAL VOLCANIC ISLAND WITH AN ELEVATED INTERIOR

Mauritius is a typical tropical volcanic island with a distinct elevated central plateau above 550 m.a.s.l. Rainfall depth, duration, intensity, kinetic energy, and erosivity were analysed for 385 erosive rainfall events at five locations over a five-year period (2004 to 2008). Two Mauritius Meteorological Services stations located on the west coast and three sited on the Central Plateau provide detailed rainfall data at 6-minute intervals. Erosive storm events are found to differ markedly between the coastal lowlands and the elevated interior with regard to the frequency, the total rainfall generated, the duration, total kinetic energy, and total erosivity of individual events. However, mean kinetic energy, mean and maximum rainfall erosivity (EI30), and maximum intensities (I30) from individual erosive events do not show this distinct differentiation. The distribution of kinetic energy and erosivity generated by individual events at the two altitudes are also significantly different. Although erosivity measured during summer exceeds that recorded in winter, the data indicate that large percentages of winter rainfall events on Mauritius are erosive and rainfall from non-tropical cyclones can pose a substantial erosion risk. Soil erosion risk occurs from storm-scale to synoptic-scale events, and extreme rainfall events generate the bulk of the erosivity. This paper also highlights that the use of rainfall records at an event scale in soil erosion risk assessments on tropical islands with a complex topography increases the effectiveness of erosivity estimates.

The effect of synoptic scale weather systems on sub-surface soil temperatures in a diurnal frost environment: preliminary observations from sub-Antarctic Marion Island.

Abstract. Marion Island in the South Indian Ocean has a maritime climate dominated by diurnal frost processes in the landscape. We test the hypothesis that synoptic time‐scale measurements are essential in understanding the drivers of diurnal frost processes. Preliminary results from automated microclimate measurements in a polar desert habitat show that diurnal soil surface temperatures on Marion Island are influenced by a complex interaction of radiation balance, air mass circulation, cloud cover and snow. The passage of synoptic scale weather systems influences soil thermal characteristics through changes in dominance of the radiation budget. Soil frost on Marion appears to be dependent on clear skies, while synoptic weather systems affect the duration and intensity of soil frost processes and non‐radiative heat fluxes. Air circulation patterns at Marion Island influence diurnal scale temperature fluctuations and its direct and indirect interactions with ecosystem processes. The data suggest that in a maritime sub‐Antarctic environment the climatic drivers of soil frost occur at a finertemporal resolution than for seasonal and permafrost environments and needs to be measured at a diurnal time‐scale to be meaningful.

Intra-event characteristics of extreme erosive rainfall on Mauritius

Abstract Mauritius is a volcanic island with a raised interior where extreme rainfall events dominate rainfall erosivity. Intra-event characteristics of the 120 highest erosive events at six selected locations between 2004 and 2008 were analyzed to provide the first detailed intra-storm data for a tropical island environment. On Mauritius, spatial variation is evident in the characteristics of extreme erosive rainfall recorded at the stations, with a noticeable increase in rainfall depth, duration, kinetic energy, and erosivity of extreme events with altitude. Extreme events in the raised interior (central plateau) show high variability of peak intensity over time as well as a higher percentage of events in which the greatest intensity occurs in the latter part of the event. Intra-event distribution of rainfall in the interior of the island shows that rainfall there has a higher potential to exceed infiltration rates as well as the ability to generate high peak runoff rates and cause substantial soil loss. The study suggests that even though within-event rainfall characteristics are complex, they have implications for soil erosion risk, and that, in tropical island environments, the within-storm distribution of rainfall should be incorporated in soil-loss modeling.