Irrigation and the Effect of Cultivation Upon Dust Emission Trends During the Historical Period

Global dust emission decreases in simulations of the CMIP6 historical period (1850-2014) by the NASA Goddard Institute for Space Studies ModelE2.1, despite dust source strength that is invariant with time. This trend is dominated by a reduction in emission within the Thar Desert in all model versions. The decrease is coincident with an expansion of irrigation near the Thar Desert over the historical period that increases soil moisture and the wind speed threshold for dust emission. Wind speed itself decreases over the Thar Desert through a reduction in onshore flow during the Indian summer monsoon, consistent with the impact of irrigation reported by previous studies. Here we examine single-forcing experiments and their effect upon the monsoon circulation to identify the forcing responsible for the wind speed trend. Cultivation is typically assumed to increase source extent and dust emission. We show that irrigation has the opposite effect, demonstrating that the precise impact of cultivation upon the aerosol burden depends upon the specific agricultural practices. We consider what processes must be represented to simulate the impact of irrigation.