Impacts of Farmland Irrigation on Land Surface Water and Energy Balance over a Humid Region: Development and Benefits of a Crop-Classified Dynamic Irrigation Scheme in Noah-HMS

Irrigation practice has impacts on the natural environment by changing the water and energy balance at the land surface and thereby interacting with the atmosphere. To quantify such impacts and estimate irrigation water demand, process-based hydrological models with a representation of irrigation practice are often used. However, the applicability of existing irrigation schemes is limited to arid and semi-arid regions. Likewise, it is still lack of more sophisticated irrigation schemes that can be particularly applicable to humid regions. This study presents the newly developed Crop-classified Dynamic Irrigation (CDI) scheme that has been two-way coupled into the land surface-hydrologic model Noah-HMS. Such development allows to distinguish the different irrigation practices for "rice" and "non-rice" crops and to estimate irrigation water demand. We have applied the newly developed model to an important grain and industrial crop production base in southern China, namely, Poyang Lake Basin (PLB), where the sown area of rice accounts for more than 60% of the sown area of all crops. As compared to the widely used, traditional Dynamic Irrigation (DI) scheme, the CDI-incorporated Noah-HMS improves the simulations of water and energy balance over the PLB from 2007 to 2015, especially irrigation water amount simulation. The relative error for irrigation water amount of CDI (DI) is -18.1% (-56.8%). In terms of surface water balance, the inclusion of irrigation practice has larger impacts on the simulated soil moisture (+1.7%) during dry years than that (+0.9%) during wet years, while has larger impacts on the simulated surface runoff (4.6%) in wet years than that (2.4%) in dry years. In terms of surface energy balance, irrigation practice leads to increased latent heat flux by 0.9 W/m² (1.4%), decreased sensible heat flux by 0.5 W/m² (1.3%), decreased ground heat flux by 0.02W/m² (5.0%), and increased net radiation by 0.09 W/m² (0.1%). Such impacts on the surface water and energy balance become more pronouncing at local scale especially over the intensively irrigated areas, for example the Nanchang city region. We conclude that our Crop-classified Dynamic Irrigation scheme is especially beneficial for applications in multiple cropping humid regions. Furthermore, our modeling development has the potential to be further extended into the fully coupled atmospheric-hydrologic modeling systems with a more holistic representation of human activities.