@phdthesis{Wulf2011,
  author    = {Wulf, Hendrik},
  title     = {Seasonal precipitation, river discharge, and sediment flux in the western Himalaya},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-57905},
  school      = {Universit{\"a}t Potsdam},
  year      = {2011},
  abstract  = {Rainfall, snow-, and glacial melt throughout the Himalaya control river discharge, which is vital for maintaining agriculture, drinking water and hydropower generation. However, the spatiotemporal contribution of these discharge components to Himalayan rivers is not well understood, mainly because of the scarcity of ground-based observations. Consequently, there is also little known about the triggers and sources of peak sediment flux events, which account for extensive hydropower reservoir filling and turbine abrasion. We therefore lack basic information on the distribution of water resources and controls of erosion processes. In this thesis, I employ various methods to assess and quantify general characteristics of and links between precipitation, river discharge, and sediment flux in the Sutlej Valley. First, I analyze daily precipitation data (1998-2007) from 80 weather stations in the western Himalaya, to decipher the distribution of rain- and snowfall. Rainfall magnitude frequency analyses indicate that 40\% of the summer rainfall budget is attributed to monsoonal rainstorms, which show higher variability in the orogenic interior than in frontal regions. Combined analysis of rainstorms and sediment flux data of a major Sutlej River tributary indicate that monsoonal rainfall has a first order control on erosion processes in the orogenic interior, despite the dominance of snowfall in this region. Second, I examine the contribution of rainfall, snow and glacial melt to river discharge in the Sutlej Valley (s55,000 km2), based on a distributed hydrological model, which covers the period 2000-2008. To achieve high spatial and daily resolution despite limited ground-based observations the hydrological model is forced by daily remote sensing data, which I adjusted and calibrated with ground station data. The calibration shows that the Tropical Rainfall Measuring Mission (TRMM) 3B42 rainfall product systematically overestimates rainfall in semi-arid and arid regions, increasing with aridity. The model results indicate that snowmelt-derived discharge (74\%) is most important during the pre-monsoon season (April to June) whereas rainfall (56\%) and glacial melt (17\%) dominate the monsoon season (July-September). Therefore, climate change most likely causes a reduction in river discharge during the pre-monsoon season, which especially affects the orogenic interior. Third, I investigate the controls on suspended sediment flux in different parts of the Sutlej catchments, based on daily gauging data from the past decade. In conjunction with meteorological data, earthquake records, and rock strength measurements I find that rainstorms are the most frequent trigger of high-discharge events with peaks in suspended sediment concentrations (SSC) that account for the bulk of the suspended sediment flux. The suspended sediment flux increases downstream, mainly due to increases in runoff. Pronounced erosion along the Himalayan Front occurs throughout the monsoon season, whereas efficient erosion of the orogenic interior is confined to single extreme events. The results of this thesis highlight the importance of snow and glacially derived melt waters in the western Himalaya, where extensive regions receive only limited amounts of monsoonal rainfall. These regions are therefore particularly susceptible to global warming with major implications on the hydrological cycle. However, the sediment discharge data show that infrequent monsoonal rainstorms that pass the orographic barrier of the Higher Himalaya are still the primary trigger of the highest-impact erosion events, despite being subordinate to snow and glacially-derived discharge. These findings may help to predict peak sediment flux events and could underpin the strategic development of preventative measures for hydropower infrastructures.},
  language  = {en}
}
@phdthesis{Wolff2011,
  author    = {Wolff, Christian Michael},
  title     = {East African monsoon variability since the last glacial},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-58079},
  school      = {Universit{\"a}t Potsdam},
  year      = {2011},
  abstract  = {The impact of global warming on human water resources is attracting increasing attention. No other region in this world is so strongly affected by changes in water supply than the tropics. Especially in Africa, the availability and access to water is more crucial to existence (basic livelihoods and economic growth) than anywhere else on Earth. In East Africa, rainfall is mainly influenced by the migration of the Inter-Tropical Convergence Zone (ITCZ) and by the El Ni{\~n}o Southern Oscillation (ENSO) with more rain and floods during El Ni{\~n}o and severe droughts during La Ni{\~n}a. The forecasting of East African rainfall in a warming world requires a better understanding of the response of ENSO-driven variability to mean climate. Unfortunately, existing meteorological data sets are too short or incomplete to establish a precise evaluation of future climate. From Lake Challa near Mount Kilimanjaro, we report records from a laminated lake sediment core spanning the last 25,000 years. Analyzing a monthly cleared sediment trap confirms the annual origin of the laminations and demonstrates that the varve-thicknesses are strongly linked to the duration and strength of the windy season. Given the modern control of seasonal ITCZ location on wind and rain in this region and the inverse relation between the two, thicker varves represent windier and thus drier years. El Ni{\~n}o (La Ni{\~n}a) events are associated with wetter (drier) conditions in east Africa and decreased (increased) surface wind speeds. Based on this fact, the thickness of the varves can be used as a tool to reconstruct a) annual rainfall b) wind season strength, and c) ENSO variability. Within this thesis, I found evidence for centennialscale changes in ENSO-related rainfall variability during the last three millennia, abrupt changes in variability during the Medieval Climate Anomaly and the Little Ice Age, and an overall reduction in East African rainfall and its variability during the Last Glacial period. Climate model simulations support forward extrapolation from these lake-sediment data, indicating that a future Indian Ocean warming will enhance East Africa's hydrological cycle and its interannual variability in rainfall. Furthermore, I compared geochemical analyses from the sediment trap samples with a broad range of limnological, meteorological, and geological parameters to characterize the impact of sedimentation processes from the in-situ rocks to the deposited sediments. As a result an excellent calibration for existing μXRF data from Lake Challa over the entire 25,000 year long profile was provided. The climate development during the last 25,000 years as reconstructed from the Lake Challa sediments is in good agreement with other studies and highlights the complex interactions between long-term orbital forcing, atmosphere, ocean and land surface conditions. My findings help to understand how abrupt climate changes occur and how these changes correlate with climate changes elsewhere on Earth.},
  language  = {en}
}