Impact of Forestation and Land-use Changes on Desert Climate

Growing forests is an effective way of removing CO2 from the atmosphere. Forestation projects were started in China, Germany, and the Middle East. Saudi Arabia announced its ambitious “Saudi Green Initiative,” intending to plant ten billion trees. Given the insufficient rainfall to support the initiative, vegetated areas will require irrigation, effectively increasing evaporation. In addition, those areas have a lower albedo than bare land, absorbing more solar radiation. Enhancing precipitation due to the recycling of evaporated water is important as it reduces the amount of freshwater required for irrigation.

In this study, we focus on the regional climate impact of irrigated forested or vegetated areas on temperature and precipitation over the Arabian Peninsula to quantify their effect on livability and evaluate the water recycling potential. First, we studied the climate effect of irrigated farming developing over vast areas in Saudi Arabia since the 1980s. The agricultural areas were mapped using available satellite-based observations from the Landsat platforms, which capture optical and thermal data every 16 days at a resolution of 30 m to 100 m. Second, we projected the climate impact of widespread forestation over the Arabian Peninsula.

The analysis of the long-term precipitation changes caused by irrigated farming is hindered by the lack of in situ observations and the limitations of global-scale observation data sets. Most reanalysis products have contradictory evaporation trends and indicate an overall reduction in rainfall since the 1980s. The recycled precipitation cannot be estimated reliably because of reanalysis increments and background rainfall variability. Presumably, the local increase in rains occurs downstream of the irrigated areas rather than over them. Along with the analysis of observations, we conducted numerical experiments mimicking the effect of irrigated agricultural fields using a non-hydrostatic regional meteorological model (WRF), covering the whole Arabian Peninsula by a 9x9 km2 grid, with 3x3 km2 nesting over the irrigated areas. Irrigation water is accounted for by tagging moisture evaporated from agricultural regions. The amount of tagged water vapor falling as rain represents recycled precipitation. The simulated evaporation and local temperature response strongly depends on the level of irrigation. Large-scale subsidence suppresses the local deep convection over most parts of the Arabian Peninsula. Strong turbulence quickly mixes evaporated water vapor within a six km thick atmospheric boundary layer, preventing precipitation in shallow convection so that the fraction of recycled rainfall appears to be low.