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The long-term hydrology of East Africa’s water tower: statistical change detection in the watersheds of the Abbay Basin


Gebreyohannis Gebrehiwot, Solomon; Gärdenäs, Annemieke I; Bewket, Woldeamlak; Seibert, Jan; Ilstedt, Ulrik; Bishop, Kevin H (2014). The long-term hydrology of East Africa’s water tower: statistical change detection in the watersheds of the Abbay Basin. Regional Environmental Change, 14(1):321-331.

Abstract

Forty-five years (1960–2004) of hydrological data from 12 watersheds in the Abbay Basin, Ethiopia, were tested for possible trends over the entire time series and differences in medians (step-wise changes) between three sub-periods. The classification of the sub-periods was based on the major political changes in 1975 and 1991. Variables investigated were rainfall (P), total flow (Qt), high flow (Qh), low flow (Ql), low flow index (LFI) and run-off coefficient (C). Data were checked for outliers, errors and homogeneity. Trend was tested after serial and cross-correlation tests. The data for each variable were serially uncorrelated from 1 to 10 lag years. There were five globally significant trends out of 50 test cases and 36 significant step-wise changes out of 180 tests. The majority of the significant changes were watershed specific. Run-off coefficient was the single variable showing a consistently increasing trend and stood for ca. 25 % of the total significant trends and step-wise changes. Half of these changes occurred after 1991. We concluded that despite the land use policy changes in 1975 and 1991, as well as the long-term soil degradation, the hydrological regime was quite stable over the 45-year period, with the exception of an increase in the run-off coefficient in the latter part of the run-off record in some watersheds.

Abstract

Forty-five years (1960–2004) of hydrological data from 12 watersheds in the Abbay Basin, Ethiopia, were tested for possible trends over the entire time series and differences in medians (step-wise changes) between three sub-periods. The classification of the sub-periods was based on the major political changes in 1975 and 1991. Variables investigated were rainfall (P), total flow (Qt), high flow (Qh), low flow (Ql), low flow index (LFI) and run-off coefficient (C). Data were checked for outliers, errors and homogeneity. Trend was tested after serial and cross-correlation tests. The data for each variable were serially uncorrelated from 1 to 10 lag years. There were five globally significant trends out of 50 test cases and 36 significant step-wise changes out of 180 tests. The majority of the significant changes were watershed specific. Run-off coefficient was the single variable showing a consistently increasing trend and stood for ca. 25 % of the total significant trends and step-wise changes. Half of these changes occurred after 1991. We concluded that despite the land use policy changes in 1975 and 1991, as well as the long-term soil degradation, the hydrological regime was quite stable over the 45-year period, with the exception of an increase in the run-off coefficient in the latter part of the run-off record in some watersheds.

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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Institute of Geography
Dewey Decimal Classification:910 Geography & travel
Language:English
Date:2014
Deposited On:16 Dec 2013 13:27
Last Modified:08 Dec 2017 00:59
Publisher:Springer
ISSN:1436-3798
Publisher DOI:https://doi.org/10.1007/s10113-013-0491-x

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