Sensitivity of Reference Evapotranspiration and Soil Evaporation to Climate Change in the Eastern Mediterranean Region

Abstract

Climate data generated by a regional climate model (RCM) under the A2 scenario were used to quantify the evaporative demand of the atmosphere in the Mediterranean region of Turkey in a baseline period (1994–2003) and the future (2070–2079). The daily reference evapotranspiration and bare soil evaporation were computed using the FAO-56 Penman-Monteith and E-DiGOR models, respectively, for both periods. The sensitivity of Penman-Monteith type equations to the major climatic variables was determined. Based on decadal averages, solar radiation, air temperature, and wind-speed were projected to increase from 16.084 to 16.324 MJ m⁻² day⁻¹, from 19.3 °C to 20.7 °C, and from 0.75 to 0.77 m s⁻¹ respectively, by the period of 2070–2079 compared with the baseline period. By contrast, the relative humidity is expected to decrease from 68.1 to 67.5% (equivalent to a 0.9% reduction). The reference evapotranspiration (Eto) and potential soil evaporation (Ep) are projected to increase by 92.0 mm year⁻¹ and 68.6 mm year⁻¹ respectively by the 2070s. Conversely, the actual soil evaporation (Ea) is expected to decrease by 49.6 mm year⁻¹ by the same period due to the decrease in rainfall and soil wetness.

The reference evapotranspiration was more sensitive to the net radiation in all seasons; followed by the air temperature in the summer months, and by the relative humidity in the winter months under both the present and future conditions. In terms of the sensitivity coefficients, the Ep responded better to the changes in climatic variables than the Eto. The sensitivity of Ep to the key climatic elements varied with the seasons: the net radiation was the most causative variable in the summer, whereas the air temperature and relative humidity were the most influential variables in the winter. The mean sensitivity coefficients for air temperature and wind-speed are projected to increase from 0.40 to 0.45 and from 0.15 to 0.19 respectively by the period of 2070–2079. A slight change in the sensitivity coefficient for relative humidity is projected. This could be explained by the expected air temperature rise and the increase in wind-speed, followed by a negligible decrease in humidity, which can increase the Ep rate. By contrast, the relative contribution of the net radiation to Ep would decrease in the future with a coefficient decreasing from 0.84 to 0.80. This outcome can be attributed to the proportionally higher increases in air temperature and wind-speed in the future, which would reduce the relative portion of the net radiation.

M. Aydın, Retired Professor, Mustafa Kemal University, Department of Soil Science and Plant Nutrition, Antakya, Turkey; e-mail: maydin08@yahoo.com.

T. Watanabe, Professor, Kyoto University, Regional Planning Graduate School of Global Environmental Studies, Kyoto, Japan; e-mail: nabe@kais.kyoto-u.ac.jp.

S. Kapur, Retired Professor, Çukurova University, Department of Soil Science and Plant Nutrition, Adana, Turkey; e-mail: kapurs@cu.edu.tr.