ESTIMATING IMPACTS ON TERRESTRIAL WATER CYCLES BY
INTEGRATING RESERVOIR OPERATION AND IRRIGATION PROCESSESTO HYDROLOGICAL SIMULATIONShunji KOTSUKI1 and Kenji TANAKA2Graduate School of Engineering Kyoto University, Kyoto, Japan
Disaster Prevention Research Institute, Kyoto University, Uji, Japan
121000 words
Estimating Impacts on Terrestrial Water Cycles by Integrating Reservoir Operation and Irrigation
Processes to Hydrological Simulation

Rapid growths of the world population and economy have increased anthropogenic impacts onthe terrestrial water cycles. It is important to integrate major anthropogenic activities tohydrological simulations for understanding terrestrial water cycles properly. The authors havedeveloped an integrated hydrological model that comprises four sub-models: land surface, river
routing, irrigation, and reservoir operation models. The integrated hydrological model simulates
terrestrial water cycles as interactions between these four sub-models. The model benefits on not
only simulating in-land natural water cycles, but also considering anthropogenic activities such as
reservoir operations and irrigation. Additionally, simulations can be performed with and without
the irrigation and reservoir operation sub-models to estimate those impacts on water cycles.
Terrestrial water cycles simulations have been performed from 1994 through 2003 using
reanalyzed meteorological forcing and land surface parameters. This study integrates reservoir
operations and irrigation processes to hydrological simulations in order to estimate their impacts
on terrestrial water cycles.
The integrated hydrological model is based on a land surface model. For land surface analysis,
the simple biosphere model including urban canopy (SiBUC; Tanaka, 2004), which wasdeveloped based on the simple biosphere model (Sellers et al., 1996), is used. The river routing
model uses kinematic wave equations to calculate river discharge with simulated runoff from theland surface model. The river routing model also considers intake and drainage for irrigation
fields from river channels. The irrigation model is coupled with the land surface model. Soil
moisture and water level on croplands are interchanged between the land surface and irrigation
models. The reservoir operation model regulates river discharge with considering irrigation waterrequirements from downstream areas. The reservoir operation model (Hanasaki et al., 2006) isalso coupled with the river routing model.
The irrigation model maintains water level or soil moisture on croplands within theappropriate ranges for each crop defined by crop calendars. The crop calendars, used in the land
surface analysis, have been generated by time-series analysis of normalized difference vegetationindex (NDVI) provided by the Spot-Vegetation satellite images. The crop calendars generated forsix crops (rice, spring wheat, winter wheat, maize, cotton, and soybean) agree well with cropcalendars commonly used in many countries. Estimated annual irrigation water demands
correspond to statistical data in many countries, suggesting that irrigation water requirements are
simulated properly.
Impact of irrigation on vapor supply from land surface was performed by the land surface
simulations with and without irrigation model globally. The vapor supply from the land surface

has been also estimated by the atmospheric water balance method using reanalyzed climateconditions. The atmospheric water balance method calculates vapor supply from the land surfaceproperly when atmospheric conditions were reproduced well in reanalysis data. The estimatedvapor supply differs by 40% between land surface simulations with and without the irrigation
model on the large scale irrigation fields. The vapor supply simulated by the land surface analysis
with the irrigation model agrees well with estimated vapor supply calculated by atmosphericwater balance conditions on irrigated fields in north China and United States. It suggests that land
surface model including irrigation reproduce vapor supply better than the model including noirrigation. Our study demonstrates that atmospheric conditions in global or regional climate
models can be improved by adding irrigation into their land surface modeling.
The reservoir operation model uses irrigation water requirements calculated by the irrigation
model. The model regulates river discharge in order to satisfy water demands in downstream areas
and to prevent floods and inundations. The reservoir operation model reduces peak river discharge
and increases low river discharge. Global river channel networks have been scaled up from a 1 km
resolution flow direction map to the model mesh size of 20 km resolution. Large reservoirs,
whose locations are obtained from the global lakes and wetlands database (Lehner et al., 2004),
have been allocated on the global river channel networks data.
Assessment of the impact on river discharge from reservoir operations, intakes and drainages
for irrigation fields was performed by simulations of river discharge globally with and without thereservoir operation and the irrigation models. River discharge simulated monthly is comparedwith recorded data provided by the global river discharge center. The simulation with thereservoir operation model agrees the recorded data in many stations better than without thereservoir operation model. On the other hand, intake and drainage for irrigation fields affect riverdischarge in small rates. Reservoir operations have a strong impact on seasonal river discharge
compared to intake and drainage for irrigation fields.
The integrated hydrological model has been used to investigate impacts of irrigation andreservoir operations on terrestrial water cycle. The model is applicable to data assimilation
systems using observations from several satellites, which would be performed in our future study.