Analysis of the spatial and temporal variability of direct rainfall in Lake Tana, Ethiopia

The Blue Nile's source, Lake Tana, is the biggest lake in Ethiopia and offers multiple services to local, regional, and international communities. The first step to effectively, efficiently, and sustainably utilizing the services that the lake may provide is to understand its water balance using accurate estimations of its water balance parameters, including direct rainfall. Direct rainfall to Lake Tana is one of the most important water balance terms of the Lake that needs a precise estimate. This study tries to analyze the spatial and temporal variability of direct rainfall to the Lake and estimate the mean annual and mean monthly direct rainfall amount to the lake using sufficient data and appropriate methodologies. 30 years (1986-2015) monthly and mean annual data from 13 meteorological stations were collected and used to analyses the spatial and temporal variability. Spatial and statistical tools were used for data processing, analysis, and presentation. Five interpolation techniques:- Thiessen polygon, spline, isohyetal, inverse distance weighting, and Kriging were considered and their performances were assessed and compared using evaluation criteria. The results indicate that the Isohytal method is slightly better than the other four methods and it is also easy to implement in a geographic information system (GIS) with Geostatic Analysis in ArcGIS. Further, the analysis has shown that the mean annual direct rainfall to Lake Tana is 1313.43 mm with a standard deviation of 259.01. Besides, it has been seen that there is a significant spatial and temporal variability of direct rainfall on Lake Tana. In terms of spatial variability, the Lake gets maximum direct rainfall in its southeastern part with an annual mean value of 1720 mm and a minimum value in its northwest parts with an annual mean value of 860 mm. In terms of Temporal variability, maximum direct rainfall is estimated in July as 374.11 mm in the Summer season and the minimum is less than 12.3 mm depth in December, January, February, and March in Winter.