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International Institute for Geo-Information Science and Earth Observation (ITC) 2013

Assessment of drought early warning in Ethiopia : a comparison of WRSI by surface energy balance and soil water balance

Abraha, M.E. (Melaku Estifanos)

Titre : Assessment of drought early warning in Ethiopia : a comparison of WRSI by surface energy balance and soil water balance

Auteur : Abraha, M.E. (Melaku Estifanos)

Etablissement de soutenance : University of Twente International Institute for Geo-Information Science and Earth Observation (ITC)

Grade : Master of Science in Geo-Information Science and Earth Observation 2013

Résumé
Drought is a natural disaster causing adverse impacts on vegetation, animals, and people. Drought is generally considered as a deficiency of rainfall or increase in evapotranspiration over a prolonged period of time. Although it is equally severe in all parts of the world, its impact is more pronounced when it occurs in developing countries where rain-fed agriculture is the major economic base. In Ethiopia, drought occurs frequently and forms the major natural hazard. In many parts of the country, agricultural productions are characterized by significant fluctuations due to variation in moisture availability and this in turn lead to crop failure and widespread drought. This research entails to study the water requirement of three major crops and to evaluate the occurrence of drought in a semi-arid region of northern Ethiopi a, where rainfall is unreliable and unpredictable. Different meteorological and satellite based drought in dices are currently available for monitoring and analysis of drought over a specified period of time. In this research, water requirement satisfaction index (WRSI) is used to examine the spatial distribution of drought and crop performance based on the availability of water during a growing season. In Ethiopia, LEAP has been implemented to estimate WRSI for assessing drought and early warning. However, its standard output lacks temporal and spatial details. As a result, accurate assessment of drought remains a difficult task. This study therefore aims to improve the accuracy of drought assessment by comparing WR SI estimates from soil water balance and energy balance approaches. The study was conducted in Tigray, northern Ethiopia, where drought is a recurrent phenomenon. Evapotranspiration was estimated based on soil water balance and energy balance approaches to understand the occurrence and extent of drought. Geostationary satellite products and ground based meteorological data were employed to estimate WRSI for the three major rain-fed crops in the study area. The findings of the study reveal that water balance method can provide reasonable estimates of actual crop evapotranspiration distribution for rain-fed crops, which aids accurate drought assessment. The findings on comparison between SEBS AET and LSA SAF AET indicated a similar spatial and temporal distribution with a good correlation (> 0.5) for the three major crops studied. Thus it can be inferred that LSA SAF AET can provide a reliable AET estimate for proper assessment of drought in near real time. The WRSI determined using GEONETcast and in-situ data at station level clearly indicated that water balance model is strongly dependent on rainfall and higher WRSI values were observed in areas with high rainfall distribution. The findings further revealed that WRSI estimates based on the energy balance approach can provide appropriate estimates of WRSI for proper monitoring of drought early warning during cloud free days. However, during existence of cloud cover, water balance method is an appropriate alternative for drought monitoring.

Mots clés : Drought assessment, Water Requirement satisfaction index, Water balance and energy balance Crop evapotranspiration, GEONETcast , Reference evapotranspiration ]

Version intégrale (ITC)

Page publiée le 2 avril 2018