Forcing Data Organization for the Lesser Zab River Basin in Iraq to Build a Coherent Hydrological Model

Abstract

Two of the most challenging issues in building robust hydrological models are determining model parameter uncertainty and examining parameter sensitivity to the system forcing data. These are especially important in dry regions with scarce data. Addressing them requires accurate forcing and basin data, usually from several different sources. The main goal of this research was to show how to construct a coherent hydrological model by (1) quantifying the sensitivity of the variable infiltration capacity (VIC) model parameters in the Lesser Zab River Basin in Iraq; and (2) implementing a complete and accurate set of forcing data by testing the validity of two sources, the National Centers for Environmental Prediction (NCEP) and Global Implemented Data (GIDAL). Although results are for a case in Iraq, the approach is valid in other regions with similar conditions and challenges. A predeveloped technique was applied to adjust the forcing data to have properties similar to the in situ observations. The parameter sensitivity was evaluated using generalized likelihood uncertainty estimation (GLUE) for seven candidate parameters of the VIC random sets for the year 2000. The VIC model was calibrated using the Kling–Gupta efficiency (KGE) for both daily and monthly time scale. Results indicated that on the average NCEP outperformed GIDAL, implying and that these data sources should not be utilized without thorough evaluation. The uncertainty analysis demonstrated the VIC model’s validity in producing realistic forecasts. Calibration of the VIC model’s daily outputs using the KGE was 0.49; this value was 0.63 for monthly runs. The calibrated model residuals lacked nonnormality, heteroscedasticity, and autocorrelation, indicating that the outcomes are reliable outside the calibrated time frame. The practical applications of this work stem from the fact that many regions of the world have scarce data but require accurate hydrological models for diverse management purposes. By showing how to augment scarce data in Iraq, new methods will be made available to others. The study did not aim to provide a data set framework, but to pave the road for future data implementation in the region and in similar regions. This study can be considered as a reference for future comparison by using similar approach to overcome data scarcity. The study is part of collaborative work by IHE-Delft and the Iraqi Ministry of Water Resources (MoWR) to examine the impact of climate change on water availability. It will be followed by a study to analyze the climate change impact using a bottom-up approach for the study area. The outcomes will be beneficial to decision makers to understand undesirable future impacts and select the most promising adaptation strategies. This shared problem requires cooperation among researchers to develop improved data preparation procedures such as those presented in this study.