Estimating the role of upper Blue Nile basin moisture budget and recycling ratio in spatiotemporal precipitation distributions

https://doi.org/10.1016/j.jastp.2019.105064Get rights and content

Highlights

  • The precipitation pattern over upper Blue Nile basin exhibits a decreasing trend from east to northwest (1979–2017).

  • Recycling ratio for four seasons (summer, autumn, spring and winter) were 9.70%, 16.33%, 19.01% and 35.30% respectively.

  • The maximum amount of precipitation is extracted from the locale moistures during the winter season.

  • Egypt and Sudan have got significance precipitation from the upper Blue Nile basin moisture budgets.

Abstract

Upper Blue Nile basin (UBNB) is the water tower of Ethiopia and downstream countries. It contributes significant moisture to the surrounding atmosphere. However, the contribution of the moisture from the basin to the precipitation in the UBNB is not well documented. Therefore, this paper is aimed to estimate the role of UBNB moisture budget and recycling ratio for spatiotemporal precipitation distributions. To this end, we used the European Center for Medium-range Weather Forecast (ECMWF) data from 1979 to 2017. The derived ECMWF results are correlated with in-situ observations with the correlation coefficient of 0.82. During summer season most of the UBNB moisture is converted to precipitation around the central parts of the study area, while in spring it contributes to the southern parts of the study area. Furthermore, the northwest part of the study area is affected by the basins moisture during the autumn season. The calculated recycling ratios for four seasons (summer, autumn, spring, and winter) are 9.70%, 16.33%, 19.01%, and 35.30% respectively with the annual average value of 20.11%. It is evident that the maximum amount of precipitation is captured from the local moisture during the winter season. Generally, UBNB moisture budget had a lesser contribution of precipitation over the study area. It rather contributed significant precipitation to the neighboring countries. Hence, further studies on moisture budget are required to explain this phenomenon in the context of Ethiopia.

Introduction

Understanding of freshwater availability has immense importance in assessing socio-economic and environmental impacts of climate and demographic change (Ganguly et al., 2015). It is also important to ensure energy and food security for climate adaptation (Elliott et al., 2013; Ganguly et al., 2015). There is a scarcity of fresh water around the countries within Abay basin. For instance, Egypt and Sudan received insufficient precipitation that affects their own ecological processes (Pimentel et al., 2004). Since precipitation is a critical factor for geomorphological, hydrological, and ecological processes (Charles and Patrick, 2015). The upper Blue Nile basin (UBNB) in Ethiopian is a highland area, which receives precipitation up to 2000 mm per year (Johnston and McCartney, 2010). It is an extremely vulnerable part of the country due to the availability of sufficient precipitation, surface water and underground water (Johnston and McCartney, 2010). Conversely, at the present time, the scarcity of precipitation and sometimes floods with an irregular frequency influences the surface and underground water due to different mechanisms (Allam et al., 2016; Abera et al., 2017).

As different researchers (Ozsoy, 1981; Kalim and Gao, 2012) indicated that the estimation of precipitation variability in complex geographical feature area is difficult; since it interacts with different moisture holding air masses. Similarly, UBNB basin in Ethiopia consists of variable topographic features. It captured moisture from external water bodies such as Atlantic Ocean, Indian Ocean, Red Sea, Arabian sea, Congo basin (Korecha and Barnston, 2007; Segele and Lamb, 2009), and the local water bodies (e.g. Lake Tana and Abay basin). Furthermore, water bodies, forests, and soil moisture contributed to high amount of evaporation to the study area (Zangvil et al., 2001; Zengxin et al., 2009; Fengjun et al., 2011; Allam et al., 2016). However, the contribution of the water bodies and the soil moisture evaporation on the spatiotemporal distribution of precipitation over UBNB is not well documented (Allen and Ingram, 2002; Held and Soden, 2006; Solomon et al., 2007; Seager and Vecchi, 2010). Since the interaction between different moisture carrying air masses with this variable topographic features leads to complex to study spatiotemporal precipitation variability on this region. Therefore, moisture budget analysis is a good approach for studying the spatiotemporal variability of precipitation and it is generation (Fengjun et al., 2011; Peter and Diane, 2012; Dawn et al., 2016). This is because the linkages between atmospheric dynamics, water vapor conditions, and precipitation are constrained by the moisture budget equation (Zangvil et al., 2001; Zengxin et al., 2009). The basic elements of moisture budget studies are wind vector, precipitation intensity, evaporation, recycling ratio, and outgoing long-wave radiation. The direction of the wind vector is indicated the sources of the moisture that contributed to the precipitation in the study area. The intensity of precipitation is high and the strength of wind vectors is low and diverges at high precipitation intensity levels are indicated the sources of precipitation is local moisture in addition to large scale moisture. This can be strongly evident from outgoing long-wave radiation (OLR) when the local moisture is high traps more OLR, its intensity is decreased (Liou, 2002). The numerical estimation of precipitation from the study area moisture is evaluated by recycling ratio (Fengjun et al., 2011). Conversely, the intensity of precipitation and the strength of wind velocity are proportional and not shown the sign of divergence or convergence, the study area moisture is coming from large scale moisture. Outgoing long-wave radiation has similar distributions when the precipitation is coming from large scale dynamics.

The precipitation and the moisture budget of east African countries are also affected by Inter-tropical convergence zone (ITCZ) (Sahalu et al., 2017). Since UBNB in Ethiopia is a part of east African country its rainfall variability is influenced by ITCZ (Ogallo and Halpert, 1988). The migration of ITCZ in the atmosphere is varied with season and governed by the tilt of the Earth by 23.5° (Emily et al., 2003). In the winter, when the Earth's southern hemisphere tilts towards the sun, the southern hemisphere captures high amount of radiation (Joanna, 2011). The wind blows from northern hemisphere to the southern hemisphere. Hence, gradually an ITCZ migrated to the southern hemisphere with similar to the direction of the wind velocity. Northerly winds is dry and not to contribute precipitation. Since 75% of the Ocean is located in the southern hemisphere (Emily et al., 2003). As UBNB is located in the northern hemisphere (above ITCZ); it couldn't get sufficient precipitation during the winter season. However, less amount of precipitation is got from the local moisture. During the summer season, the moisture dynamics seem to be reverse of the winter season. The wind blows from southwest to northeast and it is carrying moisture from Atlantic Ocean (Trenberth, 1997; Emily et al., 2003). Most of this moisture is captured by the Ethiopian highlands (UBNB) when compared to the other countries owing to its mountainous features.

During recent decades, numerous studies have been published on moisture budget and precipitation variability by different authors in different countries. For e.g. at China by Sun et al. (2007) and Zengxin et al. (2009), over Mediterranean by Fengjun et al. (2011) and, Jin and Zangvil (2010), over Amazon Basin by Drumond et al. (2014) and in Africa by Catherine et al. (2014). In Ethiopia, large scale moisture dynamics is studied by the number of authors (Korecha and Barnston, 2007; Segele and Lamb, 2009; Ellen and Asgeir, 2013), but they didn't estimate the contribution of the study area moisture for the precipitation system. In the case of UBNB, climate and hydrology relations is addressed by Conway (2000); the spatial and temporal rainfall variability is studied by Charles and Patrick (2015); estimation of evaporation variability is studied by Mariam et al. (2016) and water balance modeling is studied by Abera et al. (2017). According to previous literature survey, none is reported the study area moisture budget to contribute on UBNB precipitation. Therefore, for the first time, we estimated the role of UBNB moisture budget and recycling ratio in spatiotemporal precipitation distribution by investigating the long term data from 1979 to 2017 (39 years). It is also examined the variations of atmospheric moisture fields and moisture transport mechanisms over the UBNB.

UBNB is one of the most data scarce regions due to lack of hydro-meteorological stations (Allam et al., 2016; Abera et al., 2017). To fill in this gap, we employed various reanalysis data such as climatic research unit (CRU) and Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP), Climate Hazards Group Infrared Precipitation combined with Station (CHIRPS), and ECMWF ERA-interim data.

Section snippets

Study area description

The study is conducted in the domain of the UBNB of Ethiopia, it is located between 7.40 and 12.5 N and from 34.25 to 39.49 E with a drainage area of 176, 000 km2 (Fig. 1). Lake Tana and Abby River basin is the main water bodies in the study area. These two water bodies have a significant contribution of atmospheric moisture over the study domain. The basin's climate is varied from humid to semiarid (Abera et al., 2017). The annual precipitation increases from northeast to southwest (Conway,

Result and discussion

Lake Tana, Nile River and other many tributary rivers over the study area can have a definite influence on the local evaporation and boundary moisture.

Conclusions and recommendations

Moisture budget studies are a basic tool to understand dynamical, hydrological and climatological aspects of atmospheric research. This paper is aimed to estimate the role of upper Blue Nile basin moisture budget and recycling ratio in temporal and spatial precipitation distributions. ECMWF data is well agreed to the gauge and weather radar Z-R relation model. From the overall validation at annual temporal scale, ECMWF does have the highest skill (the lowest MRE and better bias ratio) values

Acknowledgments

The authors are thankful for constant financial support by Bahir Dar University, Ethiopia and partial support from Debre Markos University, Ethiopia. Our gratitude goes to Dawit Amogne (PhD), Degenie Sahalu (PhD), and Melessew Nigusie (PhD) for their substantial comments and suggestions for this work.

References (52)

  • W. Abera et al.

    Water budget modeling of the Upper Blue Nile basin using the JGrass-NewAge model system and satellite data

    Hydrol. Earth Syst. Sci. Discuss.

    (2017)
  • M.M. Allam et al.

    Estimation of evaporation over the upper Blue Nile basin by combining observations from satellites and river flow gauges

    AGU, Water Resour. Res.

    (2016)
  • M.R. Allen et al.

    Constraints on future changes in climate and the hydrologic cycle

    Nature

    (2002)
  • G.T. Ayehu et al.

    Validation of new satellite rainfall products over the upper Blue Nile basin, Ethiopia

    Atmos. Meas. Technol.

    (2018)
  • J. Beckers et al.

    EOF calculations and data filling from incomplete oceanographic datasets

    Am. Meteorol. Soc. J. Atmos. Ocean. Technol.

    (2003)
  • P. Berrisford et al.

    Atmospheric conservation properties in ERA-interim

    Quart. J. R. Meteor. Soc.

    (2011)
  • P. Berrisford et al.
    (2011)
  • M. Birhan et al.

    Estimation of rainfall intensity from first observation weather radar reflectivity data over upper Blue Nile basin, Ethiopia

    Trans. Sci. Technol.

    (2018)
  • P. Catherine et al.

    Moisture budget analysis of SST-driven decadal Sahel precipitation variability in the twentieth century

    Clim. Dyn.

    (2014)
  • O. Charles et al.

    Spatial and temporal variability of rainfall in the Nile Basin

    Hydrol. Earth Syst. Sci.

    (2015)
  • D. Conway

    The climate and hydrology of the upper Blue Nile River

    Geogr. J.

    (2000)
  • E.S. Dawn et al.

    Use of atmospheric budget to reduce uncertainty in estimated water availability over south Asia from different reanalysis

    Sci. Rep.

    (2016)
  • D. Dee et al.

    The ERA-Interim reanalysis: configuration and performance of the data assimilation system

    Quart. J. R. Meteor. Soc.

    (2011)
  • T. Dinku et al.

    Validation of the CHIRPS satellite rainfall estimates over eastern Africa

    J. R. Meteorol. Soc.

    (2018)
  • G.I. Diro et al.

    Seasonal forecasting of Ethiopian spring rains

    Meteorol. Appl.

    (2008)
  • A. Drumond et al.

    The role of the Amazon Basin moisture in the atmospheric branch of the hydrological cycle: a Lagrangian analysis

    Hydrol. Earth Syst. Sci.

    (2014)
  • V. Ellen et al.

    The effect of moisture transport variability on Ethiopian summer precipitation

    Int. J. Climatol.

    (2013)
  • J. Elliott

    Constraints and potentials of future irrigation water availability on agricultural production under climate change

    Proc. Natl. Acad. Sci. U.S.A.

    (2013)
  • B. Emily et al.

    An observational study of the relationship between excessively strong short rains in coastal east Africa and Indian ocean SST

    Monthly Weather Review, AMS

    (2003)
  • J. Fengjun et al.

    Climatological relationships among the moisture budget components and rainfall amounts over the Mediterranean based on a super-high-resolution climate model

    J. Geophys. Res.

    (2011)
  • C. Funk et al.

    The climate hazards infrared precipitation with stations a new environmental record for monitoring extremes

    Sci. Data

    (2015)
  • A.R. Ganguly et al.

    Water stress on power production. J. Climate adaptation informatics

    Comput. Sci. Eng.

    (2015)
  • C.J. Hahn et al.

    Upper Blue Nile basin water budget from a multi-model perspective

    J. Hydrol.

    (2017)
  • H. Hastuadi et al.

    Fine-tuning satellite-based rainfall estimates

    Earth Environ. Sci.

    (2018)
  • I.M. Held et al.

    Robust responses of the hydrological cycle to global warming

    J. Clim.

    (2006)
  • F.J. Jin et al.

    Relationship between moisture budget components over the eastern Mediterranean

    Int. J. Climatol.

    (2010)
  • Cited by (5)

    View full text