Modeling of Regional Hydroclimate Change over the Indian Subcontinent: Impact of the Expanding Thar Desert

The Thar Desert between northwestern India and Pakistan is the most densely populated desert region in the world. As a result of population growth and extensive agricultural practices, groundwater over northwestern India is being progressively depleted and the region is under the threat of future desertification. Indeed, vast areas surrounding the current region of the Thar desert are affected by rapid soil degradation and vegetation loss.

The impact of a realistically expanded desert area on the South Asian summer monsoon hydroclimate is investigated by means of 7-month ensemble sensitivity experiments with the Weather Research and Forecasting model. The scenario is implemented in the model by changing vegetation types and related parameters (greenness fraction, albedo, surface roughness length, emissivity, etc.).

It is found that extended desertification significantly affects the monsoon at local and large-scale. Locally, the atmospheric water cycle weakens, since precipitation, evaporation, and atmospheric moisture convergence all decrease; soil moisture and runoff reduce too. Air temperature cools due to albedo increase and reduction of surface turbulent fluxes; the cooling is partially offset by adiabatic descent generated to maintain thermodynamic balance and originating at the northern flank of the low-level anticyclone forced by desert subsidence. Regionally, an anomalous northwesterly flow over the Indo-Gangetic Plain weakens the monsoon circulation over northeastern India, causing precipitation to decrease and the formation of an anomalous anticyclone in the region. As a result, the middle Troposphere cools due to decreased latent-heat release but the ground heats up due to cloudiness reduction. At larger scale, interaction between the anomalous circulation and the mountains leads to precipitation increase over the Eastern Himalayas and Indochina.

The findings of this pilot study reveal that the expansion of the Thar Desert can lead to a pronounced and large-scale impact on summer monsoon hydroclimate, with the potential to redistribute precious water over South Asia.