Hydrogeological and Hydrological Characterizations and Classification using Remote Sensing Datasets and Models
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Date
2013-06
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Addis Ababa University
Abstract
Ethiopian water resource is facing a range of challenges in water management caused by several natural and man-made impacts, namely over exploitation, deforestation, land degradation and climate variability. Understanding the spatio-temporal characteristics of water storages is therefore crucial for the country, since the welfare of the society depends on the availability of water. The prime objective of this study is to apply remotely sensed and model data to Ethiopia in order to i) analyze the inter-annual, intra-annual and seasonal variabilities of Total Water Storage (TWS), ii) understand the relationship between TWS variations, rainfall and soil moisture anomaly, and iii) study the relationship between the characteristics of aquifers’ and TWS anomalies. The data used in this study includes; monthly gravity field data from the Gravity Recovery And Climate Experiment (GRACE) mission to assess the variation in TWS; monthly rainfall data from the Tropical Rainfall Measuring Mission (TRMM) to assess the response of water storages to the incoming water mass and the effect of rainfall on TWS, and the hydrological model data of Global Land Data Assimilation System (GLDAS) to obtain soil moisture. Our investigation covers a period of 8 years from 2003 to 2011. The results of the study shows that the western part and the north-eastern lowland of the country lost water at a rate of not less than 0.108 cm/year, whereas all the other regions gained water mass at a rate of more than 0.60 cm/year. The impact of rainfall seasonality was also seen on TWS changes, with losing seasons being summer and autumn, and gaining seasons being spring and winter for majority of the regions. From the TWS duration curve (TDC), the percentage time of water mass loss in all regions is also observed as 55% of the study period. Applying the statistical method of Principle Component Analysis (PCA) on TWS, soil moisture, and rainfall variations shows the dominant annual water variability in the western, north-western, northern and central regions, and the dominant seasonal variability in the western, north western and the eastern regions. A correlation analysis between TWS and rainfall indicate a minimum time lag of zero and maximum of six months, whereas no lag has been seen between soil moisture anomaly and TWS. A significant increase in TWS deficit has also been seen in the central highlands of the country over the period of three years (2003 to 2006) with Total Storage Deficit Index (TSDI) ranging from nearly zero in 2003 to -500 % in western region and -800 % in the central and northern regions of the country in 2006. A classification of the country based on TWS Duration Curve shows that TWS variation is high in the western and low in the north eastern parts. The delay response and correlation coefficient between rainfall and TWS is also related to recharge mechanism and revealed that most regions of the country receive indirect recharge. The hydrological characterizations and classification has been carried out as Part-II, separately from the hydrogeologic characterization section only for watersheds in one selected basin which is located in region 2.
The Part-II finding shows the possibility of characterizing and classifying watersheds in a data limited regions through the help of remote sensing instruments and models. Here, the characterization and classification has been made using the catchment climate, catchment function and catchment structure descriptors. The detailed findings of this section have been described in a separate “Abstract” provided at the beginning of Part-II.
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Keywords
Ethiopia, GRACE-TWS, TRMM, Duration Curve, Hydrogeological Characterization