Statistical analyses of potential evapotranspiration changes over the period 1930–2012 in the Nile River riparian countries

2017

Hydrology

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Abstract:Some of the problems in drought assessments are that: analyses tend to focus on coarse temporal scales, many of the methods yield skewed indices, a few terminologies are ambiguously used, and analyses comprise an implicit assumption that the observations come from a stationary process. To solve these problems, this paper introduces non-stationary frequency analyses of quantiles. How to use non-parametric rescaling to obtain robust indices that are not (or minimally) skewed is also introduced. To avoid ambiguity, some concepts on, e.g., incidence, extremity, etc., were revisited through shift from monthly to daily time scale. Demonstrations on the introduced methods were made using daily flow and precipitation insufficiency (precipitation minus potential evapotranspiration) from the Blue Nile basin in Africa. Results show that, when a significant trend exists in extreme events, stationarity-based quantiles can be far different from those when non-stationarity is considered. The introduced non-parametric indices were found to closely agree with the well-known standardized precipitation evapotranspiration indices in many aspects but skewness. Apart from revisiting some concepts, the advantages of the use of fine instead of coarse time scales in drought assessment were given. The links for obtaining freely downloadable tools on how to implement the introduced methods were provided.

Section: (I) Trend Magnitudementioning

confidence: 99%

“…The distribution of T CSD is approximately normal with the mean of zero and variance (V 1 ) given by Onyutha [34,40]:…”

Section: (I) Trend Magnitudementioning

confidence: 99%

“…• The CSD [30,34,40] trend test Let the given data be represented by X. Another time series Y can be obtained as the replica of X.…”

Section: (I) Trend Magnitudementioning

confidence: 99%

Section: (I) Trend Magnitudementioning

confidence: 99%

Section: (I) Trend Magnitudementioning

confidence: 99%

See 3 more Smart Citations

On Rigorous Drought Assessment Using Daily Time Scale: Non-Stationary Frequency Analyses, Revisited Concepts, and a New Method to Yield Non-Parametric Indices

2017

Hydrology

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Water availability trends across water management zones in Uganda

Atmospheric Science Letters

Asiimwe

Muhwezi

et al. 2021

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This study assessed trends in gridded (0.25 Â 0.25 ) Climate Forecast System Reanalysis (CFSR) precipitation, potential evapotranspiration (PET), and precipitation minus PET (PMP) across the four water management zones (WMZs) in Uganda including Kyoga, Victoria, Albert, and Upper Nile. The period considered was 1979-2013. Validation of CFSR datasets was conducted using precipitation observed at eight meteorological stations across the country.Observed precipitation trend direction was satisfactorily reproduced by CFSR data extracted at five out of eight stations. Negative (positive) values of longterm PMP mean were considered to indicate areas characterized by water scarcity (surplus). Areas with large positive PMP were confined to Lake Victoria and mountains such as Rwenzori and Elgon. The largest negative PMP values were in the arid and semi-arid areas of north and northeastern Uganda. The null hypothesis H 0 (no trend) was rejected (p < 0.05) for increasing annual precipitation trends across the various WMZs except in the extreme eastern parts of the Upper Nile, Kyoga, and Victoria WMZs (or areas along the boundary of Uganda and Kenya). The H 0 (no trend) was rejected (p < 0.05) for decreasing trends in annual PET over West Nile region of the Upper Nile, western parts of Victoria, and the Albert WMZs. For increasing trend in PMP, the H 0 (no trend) was rejected (p < 0.05) across the various WMZs except around the Mount Elgon area. The study findings are relevant for planning of water resources management across the different WMZs in the country.

Space‐time variability of extreme rainfall in the River Nile basin

Intl Journal of Climatology

Willems

2017

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In this study, spatio-temporal variability in daily rainfall extremes based on 0.5 ∘ × 0.5 ∘ gridded data over the Nile basin was analysed using the quantile perturbation method. The co-occurrence of the extreme rainfall variability with the variation in the large-scale ocean-atmosphere conditions was also investigated. Based on a 15-year moving window, it was found that the extreme rainfall shows oscillatory behaviour over multi-decadal time scales. The latitudinal difference in the multi-decadal extreme rainfall oscillations divides the study area into the Northern, Central, and Southern regions. The variability in the extreme rainfall of the Central region is dominantly driven by the variation in the sea surface temperatures of the Atlantic and Pacific Oceans. For the Southern region, extreme rainfall variability is linked to the anomalies in the sea level pressure of the North Atlantic Ocean and the variation in the sea surface temperature of the Indian Ocean. The variation in the extreme rainfall of the Northern region corresponds to the anomalies in sea surface temperatures of the Indian and Atlantic Oceans as well as from the Pacific Ocean.