Irrigation in JULES land-only simulations over South and East Asia

TODT, MARKUS; MCGUIRE, PATRICK; MÜLLER, OMAR V.; VIDALE, PIER LUIGI

Workshop; JULES Remote Open Science Meeting; 2020

Uk Centre for Ecology & Hydrology

Capturing soil moisture-atmosphere feedbacks in a weather or climate model requires realistic simulation of various land surface processes. However, irrigation and other water management methods are still missing in most global climate models today, despite irrigated agriculture being the dominant land use in parts of Asia. In this study, in preparation for coupled land-atmosphere simulations, we test the irrigation scheme available in JULES (Joint UK Land Environment Simulator) by running land-only simulations over South and East Asia driven by WFDEI (WATCH Forcing Data ERA-Interim) forcing data. We use a water-limiting version of the irrigation scheme, which extracts water for irrigation from groundwater and rivers, and we prescribe irrigation for C3 grasses in order to simulate the effects of agriculture, albeit retaining the simpler 5-PFT (plant functional types) configuration in JULES. Irrigation generally increases soil moisture and thus evapotranspiration, which results in increasing latent heat fluxes and decreasing sensible heat fluxes. Irrigation also affects water fluxes within the soil, e.g. runoff and drainage into the groundwater level, as well as soil moisture outside of the irrigation season. We find that the irrigation scheme, at least in the uncoupled land-atmosphere setting, can rapidly deplete groundwater to the point that river flow becomes the main source of irrigation (over the North China Plain and the Indus region) and can have the counterintuitive effect of decreasing annual average soil moisture (Ganges plain). Comparison with combined observational/machine-learning products for turbulent fluxes shows that while irrigation can reduce biases, especially over the North China Plain, other biases in JULES, unrelated to irrigation, are larger than any improvements due to the inclusion of irrigation.