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The humid tropics are the region with the highest rate of land-cover change worldwide. Especially prevalent is the deforestation of old-growth tropical forests to create space for cattle pastures and soybean fields.
The regional water cycle is influenced by vegetation cover in various ways. Especially evapotranspiration considerably contributes to water vapor content in the lower atmosphere. Besides active transpiration by plants, evaporation from wetted plant surfaces further known as interception loss is an important supply of water vapor. Changes in interception loss due to change in land cover and the related consequences on the regional water cycle in the humid tropics of Latin America are the research focus of my thesis. (1) In an experimental setup I assess differences in interception loss between an old-growth tropical forest and a soybean plantation. (2) In a modeling study, I examine interception losses of these two vegetation types compared to a younger secondary forest with the use of the Gash interception model, including an uncertainty analysis for the estimation of the necessary model parameters. (3) Studying the water balance of a 192-km² catchment I disentangle the influences of changes in land cover and climatic factors on interception loss.
The three different research sites in my thesis represent a currently typical spectrum for land-cover changes in Latin America. In the first example I study the consequences of deforestation of transitional forest, which forms the transition from the Brazilian tree savanna (cerrado) to tropical rain forest, for the establishment of soybean fields in the southern Amazon basin. The second study site is a young secondary forest within the “Agua Salud” project area in Panama as an example of reforestation of former pastures. The third study site is the Cirí Grande river catchment which comprises a mixture of young and old forests as well as pastures, which is typical for the southern sub-catchments of the Panama Canal.
The experimental approach consists of the indirect estimation of interception loss by measuring throughfall and stem flow. For the first experimental study I measured throughfall as well as stem flow manually. Measurements of the leaf area index of the two land covers do not show distinct differences; hence it could not serve as an explanation for the differences in the measured interception loss. The considerably higher interception loss at the soybean field is attributed to a possible underestimation of stemflow but also to the stronger ventilation within the well-structured plant rows causing higher evaporation rates. This situation is valid only for two months of the rainy season, when soybean plants are fully developed. In the annual balance evapotranspiration at the soybean site is clearly less than at the forest site, accelerating the development of fast runoff components and consequently discharge. In the medium term, a reduction of water availability in the study area can be expected.
For the modeling study, throughfall in a young secondary forest is sampled automatically. The resulting temporally high-resolution dataset allows the distinction between different precipitation and interception events. The core of this study is the sensitivity and uncertainty analysis of the Gash interception model parameters and the consequences for its results. Canopy storage capacity plays a key role for the model and parameter uncertainty. With increasing storage capacity uncertainty in parameter delineation also increases. Evaporation rate as the driving component of the interception process incorporates in this context the largest parameter uncertainty. Depending on the selected method for parameter estimation, parameter values may vary tremendously.
In the third study, I analyze the influence of interception loss on the water balance of the Cirí Grande catchment, incorporating the interlinked effects of temperature, precipitation and changes of the land use mosaic using the SWAT (soil water assessment tool) model. Constructing several land-cover scenarios I assess their influence on the catchment’s discharge. The results show that land-cover change exerts only a small influence on annual discharge in the Cirí Grande catchment whereas an increase in temperature markedly influences evapotranspiration. The temperature-induced larger transpiration and interception loss balances the simultaneous increase in annual precipitation, such that the resulting changes in annual discharge are negligible.
The results of the three studies show the considerable effect of land cover on interception. However, the magnitude of this effect can be masked by changes in local conditions, especially by an increase in temperature. Hence, the results cannot be transferred easily between the different study sites. For modeling purposes, this means that measurements of vegetation characteristics as well as interception loss at the respective sites are indispensable.
Ecosystems' exposure to climate change - Modeling as support for nature conservation management
(2016)
The Lombok Island is part of the Lesser Sunda Islands (LSI) region – Indonesia, situated along the Sunda-Banda Arcs transition. It lies between zones characterized by the highest intensity geomagnetic anomalies of this region, remarkable as one of the eight most important features provided on the 1st edition of World Digital Magnetic Anomaly Map. The seismicity of this region during the last years is high, while the geological and tectonic structures of this region are still not known in detail. Some local magnetic surveys have been conducted previously during 2004–2005. However, due to the lower accuracy of the used equipment and a limited number of stations, the qualities of the previous measurements are questionable for more interpretations. Thus a more detailed study to better characterize the geomagnetic anomaly -spatially and temporally- over this region and to deeply explore the related regional geology, tectonic and seismicity is needed. The intriguing geomagnetic anomalies over this island region vis-à-vis the socio-cultural situations lead to a study with a special aim to contribute to the assessment of the potential of natural hazards (earthquakes) as well as a new natural resource of energy (geothermal potential).
This study is intended to discuss several crucial questions, including:
i. The real values and the general pattern of magnetic anomalies over the island, as well as their relation to the regional one.
ii. Any temporal changes of regional anomalies over the recent time.
iii. The relationships between the anomalies and the geology and tectonic of this region, especially new insights that can be gained from the geomagnetic observations.
iv. The relationships between the anomalies and the high seismicity of this region, especially some possible links between their variations to the earthquake occurrence.
First, all available geomagnetic data of this region and results of the previous measurements are evaluated. The new geomagnetic surveys carried out in 2006 and 2007/2008 are then presented in detail, followed by the general description of data processing and data quality evaluation. The new results show the general pattern of contiguous negative-positive anomalies, revealing an active arc related subduction region. They agree with earlier results obtained by satellite, aeromagnetic, and marine platforms; and provide a much more detailed picture of the strong anomalies on this island. The temporal characteristics of regional anomalies show a decreasing strength of the dipolar structure, where decreasing of the field intensities is faster than the regional secular variations as defined by the global model (the 10th generation of IGRF). However, some exceptions (increasing of anomalies) have to be noted and further analyzed for several locations.
Thereafter, simultaneous magnetic anomalies and gravity models are generated and interpreted in detail. Three profiles are investigated, providing new insights into the tectonics and geological evolution of the Lombok Island. Geological structure of this island can be divided as two main parts with different consecutive ages: an old part (from late Oligocene to late Miocene) in the South and a younger one (from Pliocene to Holocene) in the North. A new subduction in the back arc region (the Flores Thrust zone) is considered mature and active, showing a tendency of progressive subduction during 2005–2008. Geothermal potential in the northern part of this island can be mapped in more detail using these geomagnetic regional survey data. The earlier estimates of reservoir depth can be confirmed further to a depth of about 800 m. Evaluation of temporal changes of the anomalies gives some possible explanations related to the evolution of the back arc region, large stress accumulations over the LSI region, a specific electrical characteristic of the crust of the Lombok Island region, and a structural discontinuity over this island.
Based on the results, several possible advanced studies involving geomagnetic data and anomaly investigations over the Lombok Island region can be suggested for the future:
i. Monitoring the subduction activity of the back arc region (the Flores Thrust zone) and the accumulated stress over the LSI, that could contribute to middle term hazard assessment with a special attention to the earthquake occurrence in this region. Continuous geomagnetic field measurements from a geomagnetic observatory which can be established in the northern part of the Lombok Island and systematic measurements at several repeat stations can be useful in this regards.
ii. Investigating the specific electrical characteristic (high conductivity) of the crust, that is probably related to some aquifer layers or metal mineralization. It needs other complementary geophysical methods, such as magnetotelluric (MT) or preferably DC resistivity measurements.
iii. Determining the existence of an active structural fault over the Lombok Island, that could be related to long term hazard assessment over the LSI region. This needs an extension of geomagnetic investigations over the neighbouring islands (the Bali Island in the West and the Sumbawa Island in the East; probably also the Sumba and the Flores islands). This seems possible because the regional magnetic lineations might be used to delineate some structural discontinuities, based on the modelling of contrasts in crustal magnetizations.