DEVELOPMENT OF INTEGRATED WATER RESOURCES MODELTO ESTIMATE ANTHROPOGENIC IMPACTS
ON TERRESTRIAL WATER CYCLEShunji KOTSUKI1 and Kenji TANAKA2Graduate School of Engineering Kyoto University, Kyoto, Japan
Disaster Prevention Research Institute, Kyoto University, Uji, Japan
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In recent years, rapid growths of the world population and economy have increased anthropogenic
impacts on global water cycles. The aim of this study is to develop an integrated water resources model toestimate anthropogenic impacts on terrestrial water cycle. The authors have developed water resources
model, which comprises a hydrological land surface, river routing, irrigation, and reservoir operationmodels. A land surface model (Simple biosphere model including urban canopy [1]) calculates thehydrological land surface and irrigation processes. River routing model uses kinematic equations to
calculate river discharge with intakes and drainages in channels. The reservoir operation model [2] hasbeen integrated into river routing process. Terrestrial impacts of irrigations and reservoir operations havebeen estimated using the model in this study. The authors have performed terrestrial water cycle simulationfrom 1994 through 2003 using reanalyzed meteorological forcing and land surface parameters.
Irrigation maintains water levels in paddy fields and soil moisture on croplands within the appropriate
ranges for each crop defined by crop calendars. The crop calendars, which are used in hydrological land
surface analysis, have been generated by phonological analysis of normalized difference vegetation index(NDVI). Our crop calendars generated for six crops (rice, spring wheat, winter wheat, maize, cotton, andsoybean) agree well with crop calendars commonly used in many countries. To estimate vapor supply from
irrigated croplands, the authors have performed global land surface simulations with and without irrigation
using the crop calendars. The vapor supply from the land surface has been also estimated by theatmospheric water balance method using reanalyzed climate conditions. The estimated vapor supply differs
by 40% between estimation with and without irrigations on large scale irrigation fields. The vapor supply
estimated with irrigation agrees well with estimated vapor supply from atmospheric water balance on
irrigated fields in north China and United States, suggesting that land surface models including irrigation
reproduce vapor supply better than with the models including no irrigation. Our study demonstrates that
atmospheric conditions in global and regional climate models can be improved by adding irrigation process
into their land surface modeling.
Reservoirs regulate river discharge in order to satisfy water demands in downstream, to prevent floods
and inundations, and to generate electric powers. A global river channel networks have been scaled up
from 1 km resolution flow direction map to the model mesh size of 20 km resolution, which agree wellwith statistical catchment areas. Large reservoirs, whose locations are obtained from a global database [3],have been allocated on the global river channel networks. To estimated impacts of reservoir operations, theauthors have performed global river discharge simulations with and without reservoirs and irrigations. The
simulation with reservoir operation model agrees recorded river discharge better in many stations than themodel without the operations. Reservoir operations reduce peak river discharge and increase low riverdischarge. Reservoir operations have a strong impact on river discharge compared to intakes for irrigatedcroplands.
[1] Tanaka K. 2004. Development of the New Land Surface Scheme SiBUC Commonly Applicable toBasin Water Management and Numerical Weather Prediction Model. Doctoral Dissertation, Graduate
School of Engineering, Kyoto University: Kyoto; 289.
[2] Hanasaki N, Kanae S, Oki T. 2006. A reservoir operation scheme for global river routing models.
Journal of Hydrology 327: 22-41. DOI:10.1016/j.jhydrol.2005.11.011.
[3] Lehner B, Doll P. 2004. Development and validation of a global database of lakes, reservoirs andwetlands. Journal of Hydrology 296: 1-22. DOI:10.1016/j.jhydrol.2004.03.028.