Simulation of the Climate Change Impact on Hydrologic Response of Forested Catchments (Case study: Kasilian Representative Basin)

Introduction There is a wide agreement in the international scientific society that climate change will modify climatic variables and hydrological extremes. Increasing greenhouse gases in the atmosphere leads to change air temperature and precipitation. Changes in air temperatures and precipitation have significant effects on the hydrological cycle. Today, general circulation models (GCM) are the most powerful tools for evaluating the effects of climate change. The outputs of this model are presented as inputs of hydrological models. Hydrological models act as a valuable tool for assessing the hydrologic characteristics of diverse catchments and effective evaluation of the hydrologic consequences of climatic change. Amidst hydrological models, there is a physically based model that does not require long-term data, and in small catchment areas, which do not have long recorded data, they can be used. No significant work has been done to Simulation of Climate change impact on water balance component of Kasilian representative basin. The Kasilian Representative catchment is a part of Kasilian basin with an area of 67 km2. Kasilian River is considered as one of the headwaters of Talar River that eventually flows into the Caspian Sea. The geology of the catchment is dominated by sedimentary rocks. The aim of this study was to simulate the role climate change impacts on stream flow and water balance components in Kasilian representative basin as a small and forested watershed. Methods In order to find out the relationship between the rainfall-runoff process, basin characteristics, and the parameters of a water balance model, the BROOK90 model, has been implemented. BROOK90 model is a physically-based, parameter-rich, hydrologic model written and supported by Anthony C. Federer. Below the ground, the model includes many soil layers ranging from 1 to 25, each with its own thickness and having different physical properties. The Penman-Monteith equation is used to estimate the rate of evapotranspiration. The model uses the Shuttleworth and Wallace (1985) method to separate transpiration and soil evaporation from sparse canopies. The soil water characteristics are defined using a modified approach of the Brooks and Corey (1964), and Saxton et al. (1986) from 11and 10 classified textural classes, respectively. The water movement through the soil is simulated using the Darcy–Richards equation. It considers water stored as intercepted rain, intercepted snow, snow on the ground, soil water in from one to many layers, and groundwater. Snow accumulation and melt are controlled by a degree-day method with cold content. Evaporation is the sum of five components: evaporation of intercepted rain and snow, snow and soil evaporation, and transpiration. Stream flow is generated using the following simplified processes: stream flows by source area flow or subsurface pipe-flow and delayed flow from vertical or downslope soil drainage and first-order groundwater storage. Further details are provided in the BROOK90 documentation manual (Federer, 2015). Twenty years of hydro-climatology observation data (1992–2011) were used for setting Brook90 for the basin. Data from the period 1992–1997 was used for calibration, and the interval 1998–2000 was considered as the validation period. The calibration of Brook90 and validation of model performance were based on daily discharge data from the catchment outlets and was done by trial and error. Visual inspection of the measured and simulated discharge curves mean bias error (MBE), Correlation Coefficient, Coefficient of Determination, and Nash–Sutcliffe model efficiency coefficient were the indicators for model performance. Statistically downscaled GCM data were used to show hypothetical climate change scenarios. The hydrological responses of the catchment were simulated for several hypothetical climate change scenarios. The results were compared with the reference or base case (present climate conditions). Results Results of the simulation showed good accordance between the observed and simulated values with the final parameter sets using the BROOK90 model. The simulation results demonstrate that the model can give a fair estimation of the water balance components of this basin. The estimated increase of precipitation causes an increase in all water balance components, especially in runoff components. The estimated variation of precipitation (Sc1-Sc4) will considerably affect Annual runoff in the future period. The increase in annual runoff based on model predictions was estimated to be 53.3% for the Sc3 scenario at the catchment. Between all runoff components, the SRFL component shows the most sensitivity to increasing precipitation. Evapotranspiration components do not show significant sensitivity to estimated variation of the precipitation. The estimated increase of temperature (Sc5-Sc7) will significantly affect evapotranspiration rates and runoff in the future period. The increase in annual evapotranspiration based on model predictions was estimated to be 13.14% for the Sc7 scenario at the catchment. This would be a change from 654.2 mm yr-1 in the control period (base run) to 730 mm yr-1 in the future (Sc7). Annual runoff at Kassilian was predicted to decrease from 363.6 mm yr-1 in the control period to 238.5 mm yr-1 or 34.4% for Sc7 scenario in the future. These increases in winter minimum temperatures above the freezing point would be reflected in changes to the period of snow cover and mean lengths of snow cover. Based on the results, the BROOK90 featured its simplistic approach to simulate the role climate change impacts on stream flow and water balance components in a small and forested watershed. With an estimated increase in temperature, the annual runoff is expected to decrease, and the annual cycle will change significantly. Winter runoff is expected to increase, the runoff maximum will shift, and runoff in spring and summer will decrease notably. This condition plays an important role in increasing the potential of flooding and a decrease in groundwater storage in the basin. The estimated increase of precipitation causes an increase in all water balance components, especially in runoff components. Therefore, it is possible to expect more flood and water shortages event in the future.