IRRIGATION WATER REQUIREMENTS UNDER CHANGING CLIMATE IN TANA BASIN, ETHIOPIA

Abstract

Global warming and climate variability are emerging as foremost environmental problems in the 21st century, particularly in developing countries. It is accountable for numerous effects on hydrological cycle, which is significantly influenced by the climatic parameters. Changes in rainfall and temperature patterns lead to floods and droughts. Climate change is also one of the most sensitive issues for evapotranspiration and water requirements of different crops. Specifically, availability of water will be influenced by the ongoing climate changes. Therefore, assessment of spatiotemporal climatic variability and overall hydrological trends of the basin are vital for planning of irrigation and other water resources development and management activities. This study has been undertaken to evaluate the trend and the variability of climatic parameters, and assess their effects on crop water requirement and irrigation water requirement in Tana basin located in Amhara Region of Ethiopia. The meteorological data of 10 stations for the period of 1980-2015 and 2011–2100 were considered for historical trend analysis and future projection analysis, respectively. Mann–Kendall (MK) test and Sen’s slope estimator were used to assess trends and spatiotemporal variability of climatic variables in Tana basin for 36 years (1980 to 2015). The inverse distance weighted (IDW) technique was used to determine the spatial distribution of rainfall and temperature. Both increasing and decreasing trends in the annual mean rainfall were observed. A significantly decreasing mean annual rainfall has been found at Debretabor, Injibara and Maksegnet stations with a magnitude of −2.07 mm/year, −2.92 mm/yr, and −4.62 mm/yr, and corresponding percentage change of 5.32%, 5.5%, and 15.13%, respectively. In contrast, a significantly increasing trend of mean annual rainfall was observed at Addis Zemen (0.81 mm/yr) with a percentage change of 2.46%. During kiremt season (major rainy season), significantly decreasing trends were recorded at Kemer Dengay (0.81 mm/yr) and Injibara (2.1mm/yr) stations with a percentage change of 1.94% and 2.1%, respectively. In contrast, the significantly increasing trend was observed at Addis Zemen station (0.97 mm/yr). During belg season (small rainy season), a significantly decreasing trend was observed at Gonder (2.63mm/yr) and Maksegnet (0.97mm/yr) whereas a significantly increasing trend was observed at Debretabor station (1.39 mm/yr). A significantly decreasing trend of bega season rainfall was observed at three stations. The study revealed a significant rise in minimum, maximum, and mean temperatures for nearly 90% of stations. A rising trend of annual maximum temperature was obtained between 1980 and 2015, with an increase of 1.08 °C. Trends of relative humidity and wind speed were also evaluated for nine stations in the basin. It can be observed from MK and Sen’s slope estimator that about 66% of the stations exhibited an increasing trend in wind speed at annual time scale. However, only one station for annual scale and two stations for bega season showed increasing trends at a 10% significance level. In contrast, mean annual relative humidity showed a decreasing trend at about 80% of the stations, varying from 0.03% /year to 0.66% /year; however, only three stations out of them exhibited significantly decreasing trends at annual time scale at 5% significant level.