New Approaches to Hydrological Prediction in Data-sparse Regions (Proc. of Symposium HS.2 at the Joint IAHS & IAH Convention, Hyderabad, India, September 2009). IAHS Publ. 333, 2009. 52 Calibration of a semi-distributed hydrological model using discharge and remote sensing data LAL P. MUTHUWATTA 1 , MARTIJN J. BOOIJ 2 , TOM H. M. RIENTJES 3 , M. G. BOS 3 , A. S. M. GIESKE 3 & MOBIN-UD-DIN AHMAD 4 1 International Water Management Institute, PO Box 2075, Colombo, Sri Lanka 2 Water Engineering and Management, Faculty of Engineering Technology, University of Twente, PO Box 217, 7500 AE Enschede, The Netherlands m.j.booij@utwente.nl 3 International Institute for Geo-Information Science and Earth Observation (ITC), PO Box 6, 7500 AA Enschede, The Netherlands 4 CSIRO Land and Water, GPO Box 1666, Canberra ACT2601, Australia Abstract The objective of this study is to present an approach to calibrate a semi-distributed hydrological model using observed streamflow data and actual evapotranspiration time series estimates based on remote sensing data. First, daily actual evapotranspiration is estimated using available MODIS satellite data, routinely collected meteorological data, and applying the SEBS algorithm. Second, the semi-distributed hydrological model HBV is calibrated and validated using the estimated evapotranspiration and observed discharge. This is done for multiple sub-basins of the Karkheh River basin in Iran. The Nash-Sutcliffe coefficient (NS) is calculated for each sub-basin. Maximum and minimum NS values for the calibration using observed discharge are 0.81 and 0.23, respectively, and using estimated evapotranspiration 0.61 and 0.46, respectively. The comparison of model simulations with multiple observed variables increases the probability of selecting a parameter set that represents the actual hydrological situation of the basin. The new calibration approach can be useful for further applications, especially in data-sparse river basins. Key words hydrological modelling; SEBS; remote sensing; MODIS; HBV model; actual evapotranspiration; Monte Carlo simulation; Karkheh River basin, Iran INTRODUCTION Hydrological models are usually calibrated by using discharge time series observed at one or a few locations in the river basin. However, simulating one variable (e.g. discharge) close to the observed one does not guarantee the satisfactory simulation of other model output such as actual evapotranspiration (Mroczkowski et al., 1997; Seibert, 2002). Furthermore, river basins are subject to several activities, one of which is agriculture that uses water from the natural system by means of surface water or groundwater diversions. Part of this diverted water is transferred to the atmosphere by evaporation and transpiration. The rest remains in the system and contributes to the groundwater aquifer and to the river discharge through various flow processes. Therefore, in modelling studies it is crucial to consider the effects of human activities on observed discharge time series. This would increase the trustworthiness of the calibration and eventually help to estimate reliable parameters that represent the catchments under scrutiny. But, acquiring data on water diversions and consumption is often not feasible due to technical, legislative and adminis- trative constraints. In that respect, using remote sensing (RS) information could be a useful addition to hydrological modelling, since it provides information on ground conditions that could be converted into hydrological variables such as actual evapotranspiration (Bastiaanssen et al., 1998; Su, 2002). This provides the volumes of water consumed by different land-use classes both spatially and temporally. Incorporating these spatial and temporal estimates of actual evapotrans- piration with hydrological model calibration indirectly incorporates the effects of diversions. The objective of this study is to present an approach to calibrate a semi-distributed hydrological model using observed streamflow data and actual evapotranspiration time series estimates based on RS data. The study area is the Karkheh River basin in Iran, which is described first. The approach to estimate daily actual evapotranspiration time series using available RS data and routinely collected meteorological data, and the hydrological model HBV to simulate river discharge are then detailed. Finally, the results and discussion are presented. Copyright © 2009 IAHS Press