Assessment of irrigation expansion and implications for water resources by using RS and GIS techniques in the Lake Tana Basin of Ethiopia.

Understanding irrigation expansion and its implications on water availability is critical for development of decision support tools for sustainable water resources planning. Irrigation expansion in the Lake Tana Basin of Ethiopia from 1986 to 2016 was examined using image classification from thematic maps and remote sensing imagery (Landsat5-8 TM/ETM+, OLI). The soil-adjusted vegetation index (SAVI) imagery was used to identify irrigated cropland based on greenness. Irrigation water consumption was determined by calculating irrigation water requirements of the major crops grown with irrigation and using the estimated irrigated land size of the basin. Irrigated croplands have increased in area from 55 to 65 % in the last past 30 years (from 1986 to 2016). The classification obtained is accurate to the level of 89.6% and Kappa coefficient of 0.83. The Landsat imagery is useful in tracking of the spatiotemporal patterns of irrigated croplands for water management purposes, especially in data deficient areas dotted sparsely by small holder irrigated farms. The results indicated that irrigation consumption in the basin increased from 0.380 km year in 1986 to 0.798 km year in 2016. It is found that the available water cannot sustain the projected irrigation expansion unless alternative water sources are found. Also, more precise irrigation technologies have to be implemented to minimize water losses.