Irrigation scheme

In each grid cells, the land use is partitioned into an irrigation part and non-irrigation part, based on the Global Map of Irrigated Areas (Siebert et al., 2005) dataset. The irrigation part land use is set to SiB2 land use "Agriculture or C3 Grassland". The non-irrigation part land use is obtained from the Global Land Cover Characterization dataset (http://LPDAAC.usgs.gov/glcc/glcc.html). For the calculation of water and energy flux between atmosphere and land surface, the mosaic strategy is used. The SiB2 model is performed at irrigation and non-irrigation tiles, respectively, and each tile interacts with the atmosphere independently. The runoff from irrigation and non-irrigation tiles is mixed homogeneously throughout the grid square and routed to the river channel.

The irrigation scheme is based on simulated soil moisture in the irrigation tiles and available water for irrigation. The main purpose of irrigation is to keep the soil moisture in the irrigation tiles above the wilding point level. The SiB2 model is modified to consider irrigation water use, based on the predicted soil moisture. Irrigation starts when soil moisture below the wilding point level, and continues until soil moisture reaches field capacity level. During the irrigation time, if precipitation charges soil water and makes the soil moisture reaches field capacity level, irrigation will stop. The water loss in irrigation channel is not considered in this approach, so the simulated irrigation requirement is the net irrigation consumption. The available water for irrigation is estimated based on the predicted river flow in the river routing module. Irrigation water can be extracted from two possible sources, the local river runoff or the river runoff at assigned river channel. Basically, the irrigation water is extracted from river runoff locally. For the irrigation centralized area, the grid clusters are recognized as irrigation districts. The irrigation districts usually extract irrigation water from specific river channels. If the irrigation distracts is outside of the river basin, the irrigation water is taken from specific river channels. Usually, the specific river channels are assigned to the nearest main stem of the river network to get as steady as possible water supply. No reservoir operation is taken into account in this study although the irrigation water availability might be affected by reservoir management. Considering the realistic situation, speed limitation is set for the irrigation water withdrawals. The irrigation water withdrawals capacity is 500 m³/s in the study area.