Effect of an irrigation scheme implemented in a regional climate model over Central Asia

Since irrigation plays an important role in Central Asia, this study focuses on the effects of irrigation within the regional climate model REMO (version 2015) and variants of REMO on the simulation of Central Asia in a resolution of 0.11°. Besides the standard version, a coupling with an interactive vegetation scheme (iMOVE) and a multilayer soil hydrological scheme (REMO-5L) as well as the combination of these two modules (iMOVE-5L) is examined. The usage of the multilayer soil scheme enabled the introduction of a simple flooding irrigation scheme. Consequently, these two model versions using the multilayer soil scheme are run with this irrigation scheme as well. Generally, irrigation is applied once per day between April and September.

We observe a cooling effect on the 2m temperature in the irrigation simulations compared to the non-irrigated locations and simulations which is caused by the partitioning of surface energy fluxes more towards latent heat in comparison to sensible heat and, thus, an increase in evapotranspiration. Furthermore, the combination of interactive vegetation and multilayer soil scheme (iMOVE-5L) leads to warm biases in the southwestern part which were present earlier as a cold bias in the standalone iMOVE version. The extreme temperatures i.e. maximum and minimum are simulated better in the case of a multilayer soil scheme in comparison to other experiments. For precipitation, irrigation leads to an overestimation caused by higher evapotranspiration. The wet biases present over northern and orographic regions in standard REMO and iMOVE get reduced in REMO-5L and iMOVE-5L with slightly better representation in REMO-5L.