Multiple equilibria on planet Dune: Climate-vegetation dynamics on a sandy planet

We study the interaction between climate and vegetation on an ideal water-limited planet, focussing on theinfluence of vegetation on the global water cycle. We introduce a simple mechanistic box model consisting in atwo-layer representation of the atmosphere and a two-layer soil scheme. The model includes the dynamics ofvegetation cover, and the main physical processes of energy and water exchange among the differentcomponents. With a realistic choice of parameters, this model displays three stable equilibria, depending on theinitial conditions of soil water and vegetation cover. The system reaches a hot and dry state for low values ofinitial water content and/or vegetation cover, while we observe a wet, vegetated state with mild surfacetemperature when the system starts from larger initial values of both variables. The third state is a cold desert,where plants transfer enough water to the atmosphere to start a weaker, evaporation-dominated water cyclebefore they wilt. These results indicate that in this system vegetation plays a central role in transferring waterfrom the soil to the atmosphere and trigger a hydrologic cycle. The model adopted here can also be used toconceptually illustrate processes and feedbacks affecting the water cycle in water-limited continental areas onEarth.