PUBLICATIONS Journal of Advances in Modeling Earth Systems RESEARCH ARTICLE Modeling irrigation-based climate change adaptation in 10.1002/2014MS000402 agriculture: Model development and evaluation in Northeast Key Points: China This study developed a coupled crop production and river circulation Masashi Okada1, Toshichika Iizumi1, Gen Sakurai1, Naota Hanasaki2, Toru Sakai1, Katsuo Okamoto1, model and Masayuki Yokozawa3 The model can accurately capture the major features of hydrology and 1National Institute for Agro-Environmental Sciences, Tsukuba, Japan, 2National Institute for Environmental Studies, crop yield 3 The model is useful in assessing Tsukuba, Japan, Graduate School of Engineering, Shizuoka University, Hamamatsu, Japan climate change adaptation based on irrigation Abstract Replacing a rainfed cropping system with an irrigated one is widely assumed to be an effective Supporting Information: measure for climate change adaptation. However, many agricultural impact studies have not necessarily Supporting Information S1 accounted for the space-time variations in the water availability under changing climate and land use. Figure S1 Moreover, many hydrologic and agricultural assessments of climate change impacts are not fully integrated. Figure S2 Figure S3 To overcome this shortcoming, a tool that can simultaneously simulate the dynamic interactions between crop production and water resources in a watershed is essential. Here we propose the regional production Correspondence to: and circulation coupled model (CROVER) by embedding the PRYSBI-2 (Process-based Regional Yield Simula- M. Yokozawa, tor with Bayesian Inference version 2) large-area crop model into the global water resources model (called [email protected] H08), and apply this model to the Songhua River watershed in Northeast China. The evaluation reveals that the model’s performance in capturing the major characteristics of historical change in surface soil moisture, Citation: river discharge, actual crop evapotranspiration, and soybean yield relative to the reference data during the Okada, M., T. Iizumi, G. Sakurai, N. Hanasaki, T. Sakai, K. Okamoto, and interval 1979–2010 is satisfactory accurate. The simulation experiments using the model demonstrated that M. Yokozawa (2015), Modeling subregional irrigation management, such as designating the area to which irrigation is primarily applied, irrigation-based climate change has measurable influences on the regional crop production in a drought year. This finding suggests that adaptation in agriculture: Model development and evaluation in reassessing climate change risk in agriculture using this type of modeling is crucial not to overestimate Northeast China, J. Adv. Model. Earth potential of irrigation-based adaptation. Syst., 7, 1409–1424, doi:10.1002/ 2014MS000402. Received 6 NOV 2014 Accepted 28 AUG 2015 1. Introduction Accepted article online 1 SEP 2015 Published online 24 SEP 2015 Many studies have widely deemed that replacing a rainfed cropping system with an irrigated one can be an effective measure for climate change adaptation [Porter et al., 2014; Challinor et al., 2014]. This is true as long as this replacement does not lead to unsustainable water extraction and/or as long as there is still suffi- cient irrigation water available [Elliott et al., 2014]. However, many agricultural impact assessments have used time-constant future irrigation scenarios derived from statistical data or other independent studies (some exceptional integrated studies are seen in Deryng et al. [2011], Biemans et al. [2013], and Kummu et al. [2014]). General hydrologic models or land-surface models have been used to assess the hydrologic impacts due to climate change [Doll€ and Schmied, 2012; Hagemann et al., 2013; Hanasaki et al., 2013]. These hydro- logic models have been intensively tested for their ability in capturing the major components of terrestrial water cycling, including river discharge, actual evapotranspiration, and soil moisture [Alcamo et al., 2003; Hanasaki et al., 2008; Biemans et al., 2009], but have not necessarily been evaluated for their ability in simu- lating crop growth and yield (a few exceptions are seen in Bondeau et al. [2007] and Biemans et al. [2013]). VC 2015. The Authors. Although a recent study combines the outputs of the hydrologic models and crop models to evaluate the This is an open access article under the potential impacts of water availability on future crop productivity [Elliott et al., 2014], the hydrologic models terms of the Creative Commons and crop models are separately simulated and thus still has not yet fully integrated. These facts necessitate Attribution-NonCommercial-NoDerivs License, which permits use and an improved assessment of potential crop production under changing climate, varying water availability, distribution in any medium, provided and expanding irrigated cropland areas, using a tool that can simultaneously simulate the dynamic interac- the original work is properly cited, the tions between crop production and water resources (e.g., river water) in a watershed. use is non-commercial and no modifications or adaptations are China is the leading crop-producing country worldwide and accounted for 20% of global cereal production made. in 2011 [Food and Agriculture Organization of the United Nations, 2014]. The Northeast China Plain is a major OKADA ET AL. MODELING IRRIGATION-BASED ADAPTATION 1409 Journal of Advances in Modeling Earth Systems 10.1002/2014MS000402 crop-producing region and important for maintaining China’s food balance. However, the recent warming and drying trends in Northeast China are evident [Tao et al., 2003; Piao et al., 2010; Yu et al., 2014]. These changes in climate have likely led to an increase in the extent of drought-damage cropland area and a decrease in the area suitable for cultivation of some crops, such as spring wheat [Jiao et al., 2007, 2008]. A decreasing trend in water resources, including the river discharge of the Songhua and Liao Rivers, which are major river channels in this region, has accompanied these changes, increasing concerns about food supply in the coming decades [Piao et al., 2010]. Hence, it is valuable to revisit the following questions that are posed in a series of previous studies [Jimenez Cisneros et al., 2014; Porter et al., 2014], using North- east China as the example: Is the available water resources sufficient for crop production under changing climate and expanding irrigated area? Given the pressure to maintain the increasing food demand in China, how much can subregional irrigation management increase the total crop production in a given watershed? To address these issues, this study (1) developed the regional production and circulation coupled model (CROVER) that can simultaneously simulate crop growth and yield, river discharge and their dynamic inter- actions by embedding the PRYSBI-2 large-area crop model [Sakurai et al., 2014] into the H08 global water resources model [Hanasaki et al., 2008]; (2) applied this coupled model to the Songhua River watershed in Northeast China and evaluated the model’s performance by comparing the historical model simulation, including soybean yield, river discharge, surface soil moisture, actual crop evapotranspiration, with the ref- erence data; (3) performed simulation experiments using the coupled model to quantify the possible influ- ence of subregional irrigation management in a drought year on regional crop production. 2. Study Area Our study area includes the Northeast China Plain, the main portion of which is located in the cool- temperate and partially semiarid zones and is surrounded by three mountain ranges: the Daxing’anling, Xiaoxing’anling, and Chanbai Mountains (Figure 1a). The main watersheds consist of the Songhua and Liao Rivers. The Songhua River flows across the Northeast China Plain from west to east after its tribu- taries join at its upper stream (Figure 1b). The riverhead of the tributary flowing from the north side (called the Nen River) is located in the Daxing’anling and Xiaoxing’anling Mountains, whereas that of the tributary flowing from the south side (called the Second Songhua River) is located in the Chanbai Mountains. The areal extent of the Songhua River watershed reaches 545,000 km2 (supporting informa- tion Figure S1a), and the total river length is 1927 km. The river flow is mainly sourced from precipita- tion in summer (which accounts for 60–80% of the annual runoff [Asian Development Bank, 2005]) and snowmelt from March to May. The mean quantity of water resources in the Songhua River watershed and its surrounding small watersheds in 2002–2011 is approximately 130 3 109 m3 [Ministry of Water Resources of the People’s Republic of China (MWR), 2015]. Twenty-four percent of the water resources (31 3 109 m3) are consumed by agriculture [MWR, 2015]. Two reservoirs with the storage capacity of >109 m3, the Baishan and Fengman reservoirs, are located in the upper stream of the watershed (Fig- ure 1b). The Northeast China Plain is the largest crop-producing region in China. Among other crops, soybean pro- duced in this region accounted for approximately 40% of the national production in 2011 [National Bureau of Statistics of China (NBSC), 2014]. Therefore, soybean was selected for this study. Soybean is mainly culti- vated in the northern part of the watershed (Figure 1a). The Liao River and its watershed were thus excluded from the analysis. 3. Data and Model 3.1. General Description of the CROVER Model As schematically