Document Type : Research Paper

• هیدرولوژی

Simulation of water balance components using TOPKAPI-X distributed hydrological model (Case study: Kashkan basin)

EXTENDED ABSTRACT

Introduction

Hydrological models are a simplified representation of the real hydrological system, which help to study the functioning of the basin in response to various inputs and to better understand the hydrological processes. The use of models to estimate the annual runoff of watersheds in arid and semi-arid areas without hydrometry stations has been of interest to hydrology researchers for a long time. For this purpose, in this research, after introducing the capabilities of TOPKAPI-X as a hydraulic-hydrological model (which has not been applied by the researchers in Iran so far), the precipitation-runoff processes of Kashkan basin in the environment TOPKAPI-X has been modeled to simulate the flood flow process. According to the review of scientific sources, it was found that the TOPKAPI-X is a model with a high capability for simulating the flow rate and water balance, and the efficiency of this model has not been evaluated in the Kashkan basin.

Material and methods

The studied area of ​​this research is the Kashkan subbasin of the Karkhe river basin. The TOPKAPI-X model is a type of continuous and distributed runoff precipitation model that has been successfully implemented as a research and operational hydrological model in many basins in the world (Liu and Todini, 2002). This model consists of five main modules that simulate hydrological processes including subsurface flow, underground flow, channel flow, evaporation and transpiration, and snow. This model can simulate in minute, hourly, or daily time steps.

In this model, continuous time series data were considered in daily time step. The time series of daily precipitation during the statistical period, including the daily rainfall of 19 stations, during the statistical period of 1999 to 2020 was used to simulate the flow. After running the model several times, the general parameters of the model were manually changed each time, until their optimal values were obtained by considering the appropriate values of the evaluation criteria (NS and Bias). Finally, after the calibration of the model, a six-year period (2014-2020) has been considered as the model validation.

Results

In this research, to simulate the daily flow and determine the water balance of the basin, all the components of the water balance such as precipitation, snowmelt, surface runoff, infiltration, evapotranspiration, deep infiltration, and subsurface flow were simulated.

The results of the comparison of hydrographs during the peak flows show the good efficiency of the model. Also, visual comparison of the observed and simulated hydrographs show that the time to peak in two hydrographs is the same, and have occurred in one day. According to the Nash-Sutcliffe criterion, the efficiency of the model in estimating the flow rate in the two periods of calibration and validation is 61.9% and 61.7%, respectively, in the Kashkan basin. The results of calibration and validation showed that the validation results of the model were somewhat weaker than the calibration results, which are consistent with the findings of Crook et al. (2005) and Ravasuka et al. (2014). In general, in some parts of the statistical period, the results of the model were satisfactory, and in some periods, the simulation process was weak. This issue can be related to the sensitivity of the model to the length of the calibration period (Zarei et al. 2012; Crook et al. 2005) and the error related to the mathematical structure of the model (Molehi et al. 2006). The effectiveness of this model in simulating the flow is consistent with the results of Liu et al. (2009), Vischel et al. (2008), Kasia et al. (2009), Sinclari et al. (2013), and Janabi et al. (2015).

Conclusion

Today, it is possible to estimate the various components of the water balance using distributed hydrological models. In this research, the surface runoff and water balance components of the basin were obtained based on model effective parameters in the daily time step with appropriate accuracy, and the initial hydrograph of the runoff was extracted. In the calibration stage, to improve the simulation and better match between the observed and simulated discharges, the model's effective parameters were calibrated. The studied area had different land use and soil types. In general, the TOPKAPI-X model showed relatively good performance for the studied basin. Also, the results of this research can be used for studies, especially hydrology studies, natural resources management and planning, environment and water resources.