Browsing by Subject "Bootstrap"

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    Sediment, carbon and nitrogen capture in mountainous irrigated rice systems
    (2016) Slaets, Johanna I. F.; Cadisch, Georg
    Anthropogenic influences have caused landscapes to change worldwide in the last decades, and changes have been particularly intense in montane Southeast Asia. Traditional swiddening cropping systems with low environmental impacts have been largely replaced by forms of permanent upland cultivation, often with maize. The associated soil fertility loss at the plot scale is well documented. In valley bottoms of these areas, paddies have been cultivated for centuries, and are considered some of the most sustainable production systems in the world – in part maintained by the influx of fertile sediments through irrigation. Altered cropping patterns on the slopes therefore also have potential repercussions on rice production, and hence on food security, but the consequences of shifted sediment and nutrient redistribution at the landscape scale are not well understood. In order to assess these effects, methodologies were developed in this thesis that enable low-cost, continuous monitoring of sediment and nutrient transport in irrigated watersheds (Chapter 2), as well as quantification of the uncertainty on constituent loads (Chapter 3). These methods are applied in a case study to determine sediment, organic carbon and nitrogen trap efficiency of paddy rice fields in a mountainous catchment in Vietnam (Chapters 4 and 5). The upland area had an average erosion rate of 7.5 Mg ha-1 a-1. Sediment inputs to the paddy area consisted of 64 Mg ha-1 a-1, of which irrigation water provided 75% and the remainder came from erosion during rainfall events. Erosion contributed one third of the sand inputs, while sediments from irrigation water were predominantly silty, demonstrating the protective effect of the reservoir which buffered the coarse, unfertile material. Almost half of the total sediment inputs were trapped in the rice area. As all of the sand inputs remained in the rice fields, the upland-lowland linkages could entail a long-term change in topsoil fertility and eventually a rice yield loss. Quantification of nutrient re-allocation in Chapter 5 showed that irrigation was even more important as a driver of sediment-associated organic carbon and nitrogen inputs into the rice fields, contributing 90% of carbon and virtually all nitrogen. Direct contributions from erosion to the nutrient status of the paddies were negligible, again underscoring the protective function of the surface reservoir in buffering irrigated areas from unfertile sediment inputs. 88% of the sediment-associated organic carbon and 93% of the nitrogen were captured by the rice fields. Irrigation water additionally brought in dissolved nitrogen, resulting in a total nitrogen input of 1.11 Mg ha 1 a-1. Of this amount, 24% was determined to be in the plant-available forms of ammonium and nitrate, a contribution equivalent to 66% of the recommended nitrogen application via chemical fertilizer. The dependence of paddy soil fertility on agricultural practices in the uplands illustrates the vulnerability of irrigated rice to unsustainable land use in the surrounding landscape. Unfortunately, alternatives for upland land use that are not detrimental to soil quality are hard to come by, due to the economic reality of high maize prices on the world market. Conservation measures and agroforestry systems offer potential, but without some form of payment for environmental services, adoption rates remain low. Finding sustainable solutions is especially urgent as climate change is likely to increase the number of extreme rainfall events and hence intensify the redistribution processes already taking place. In this light, the role of trapping elements in landscapes such as paddy fields and surface reservoirs becomes more important as well. As these features are widely spread throughout tropical landscapes, their role in global sediment and nutrient cycles must be taken into account. The methodologies developed in this thesis, for sediment and nutrient transport monitoring and for uncertainty assessment, can aid in closing the data gap that currently hinders a reliable assessment of the consequences of anthropogenic and climate change, both on food security and on environmental impacts, locally, regionally and globally.
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    Targeting the poor and smallholder farmers : empirical evidence from Malawi
    (2009) Houssou, Nazaire; Zeller, Manfred
    This paper develops low cost, reasonably accurate, and simple models for improving the targeting efficiency of development policies in Malawi. Using a stepwise logistic regression (weighted) along with other techniques applied in credit scoring, the research identifies a set of easily observable and verifiable indicators for correctly predicting whether a household is poor or not, based on the 2004-05 Malawi Integrated Household Survey data. The predictive power of the models is assessed using out-of-sample validation tests and receiver operating characteristic curves, whereas the model?s robustness is evaluated by bootstrap simulation methods. Finally, sensitivity analyses are performed using the international and extreme poverty lines. The models developed have proven their validity in an independent sample derived from the same population. Findings suggest that the rural model calibrated to the national poverty line correctly predicts the status of about 69% of poor households when applied to an independent subset of surveyed households, whereas the urban model correctly identifies 64% of poor households. Increasing the poverty line improves the model?s targeting performances, while reducing the poverty line does the opposite. In terms of robustness, the rural model yields a more robust result with a prediction margin ±10% points compared to the urban model. While the best indicator sets can potentially yield a sizable impact on poverty if used in combination with a direct transfer program, some non-poor households would also be targeted as the result of model?s leakage. One major feature of the models is that household score can be easily and quickly computed in the field. Overall, the models developed can be potential policy tools for Malawi.