EVALUATING CLIMATE CHANGE IMPACTS on CROP WATER REQUIREMENTS in GOJEB RIVER CATCHMENT, ETHIOPIA

Abstract

Climate change primarily affects crops by increasing potential evapotranspiration, which puts pressure on local agricultural water availability. This poses a significant threat to agricultural sustainability, particularly in agricultural water management. To manage water effectively amid climate change, it's essential to understand current and future aspects of the water cycle, including evapotranspiration, as they directly affect irrigation needs. This study aims to investigate the impacts of climate change on crop and irrigation water requirements in the Gojeb River catchment. Multiple RCMs were used, considering both the RCP4.5 and RCP8.5 emission scenarios. The RCMs data were bias corrected using CMhyd tool. Two future periods were chosen for analysis: 2025-2060 (near-future) and 2061-2096 (far-future). These periods were compared to a reference period (1985-2019) to assess the impacts of climate change on potential evapotranspiration and crop water requirements. The results indicate that, under both emission scenarios, the average annual precipitation is projected to increase with the highest levels recorded at the Bonga station. The projected mean annual precipitation is expected to increase in Shabe, Jimma, and Shabe stations under RCP4.5. For the Shabe station, the range will rise from 20.20-102.94 mm/year to 130-300.67 mm/year. For the Jimma station, it will rise from 15.77-51.98 mm/year to 136.23-208.67 mm/year. And for the Shabe station again, it increased from 124.88-291.60 mm/year to 147.48-376.73 mm/year. However, Bonga station's precipitation is projected to decrease from -101.88 to -511.98 mm/year to 114.48 to -207.78 mm/year under the same scenarios. Similar to the mean annual precipitation projection results of the RCP4.5 scenarios, the mean annual precipitation is projected to increase under RCP8.5. Besides, the mean annual maximum and minimum temperatures shows significant shifts and increase at all stations for near and far-future under RCP4.5. More moderately increasing maximum and minimum temperature trends were observed for all stations under the RCP8.5 scenarios during the same periods. Additionally, the results of the analysis clearly indicate a consistent upward trend in potential evapotranspiration (ET₀) under both RCP4.5 and RCP8.5 scenarios when compared to the baseline period. At the Bonga station, the projected increases in ET₀ are quite significant: 0.7 mm/day for RCP4.5 in the near future (2025-2060), 1.2 mm/day for RCP8.5 in the near future, 0.9 mm/day for RCP4.5 in the far future (2061-2096), and 1.83 mm/day for RCP8.5 in the far-future. At the Dedo station, the projected increases in ET₀ are 0.42 mm/day for RCP4.5 (2025-2060), 0.58 mm/day for RCP8.5 (2025-2060), 0.3 mm/day for RCP4.5 (2061-2096), and 0.49 mm/day for RCP8.5 (2061-2096), indicating an increasing water stress for crops, particularly during the dry season. Similar trends of slight increases in ETO were also observed at the Jimma and Shabe stations under the different RCP scenarios. These increases highlight the substantial rise in crop water demand. During the base period and future projections under RCP 4.5, maize consistently has the highest crop water requirements (CWRs) at all stations, while barley requires less water. Specifically, at the Shabe station, the CWRs for maize are projected to increase from (24.09-86.85) to (74.61-163.40) mm/year. The CWRs for sorghum, barley, and wheat changed from (42.25-89.05) to (44.75-129.05), (63.35-88.02) to (72.85-140.19), and (14.25-108.20) to (31.75-119.09) mm/year, respectively. At the Jimma station, the CWRs for maize increased range from (53.36-78.95) to (63.51 137.13) mm/year. For sorghum, barley, and wheat, the CWRs changed from (25.70-56.49) to (26.36 76.21), (14.80-55.16) to (17.78-83.82), and (14.13-63.40) to (19.35-112.41) mm/year, respectively. At the Dado station, the CWRs for maize are projected to increase from (41.05-94.71) to (65.68-135.51) mm/year. The CWRs for sorghum, barley, and wheat changed from (39.34-85.83) to (47.27-98.62), (34.00-173.12) to (55.94-153.28), and (57.34-162.46) to (85.27-173.62) mm/year, respectively. Lastly, at the Bonga station, the CWRs for maize changed from (3.71-83.96) to (16.97-109.92) mm/year. For sorghum, barley, and wheat, the CWRs can shift from (5.21-151.61) to (22.98-154.70), (14.21-84.46) to (20.02-107.00), and (10.62-82.71) to (14.98-98.98) mm/year for the near-future and far-future periods under RCP4.5. Furthermore, the study calculated irrigation water requirements for the four crops under RCP8.5 scenarios, showing an increase in water needs for all crops in both the near and far future at all stations. In conclusion, the study it can serve as benchmark information for future research and guide watershed managers and environmentalists in considering the impacts of climate change on surface water availability in the Gojeb River catchment.